There is a growing interest in the use of wood in new building, not least because it has low embodied energy and it is an infinitely renewable resource. Despite a great deal of innovation in the use of wood in construction in recent years, the fundamentals of using this natural material have not really changed: the different types of wood have different properties and differing responses to the environment in which they are used. When used correctly, wood is an excellent building material but when inappropriately specified or used, it may cause problems. Poor understanding of the properties of wood and the many species and grades that are commercially available can result in this versatile material performing below expectation, and certainly less well than could have been achieved with greater understanding about how best to use it. How Wood Works is a combination of the author's two previous books, into one comprehensive volume. Revised and updated material to deal with the essentials of structural design and building in timber, in a sustainable manner while reflecting on changes in Standards and other Regulations and expanding on certain technical areas - such as more detailed wood science and wood structure.
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Part One How Timber Works: Wood as a Material — Its Main Processes and Uses
1 Wood as a Material
1.1 Tree growth and wood formation
1.2 Cellulose, carbon dioxide, and oxygen
1.3 The essential cell structure of wood
1.4 Wood grain
1.5 Dimensional changes in wood
1.6 Cambium, pith, heartwood, and sapwood
1.7 Natural durability
1.8 Permeability in timber
1.10 Chapter summary
2 More on Wood – With Some Comments about Timber Trading
2.1 Should we call it ‘wood’ or ‘timber’ (or even ‘lumber’)?
2.2 Wood species and timber trading
2.3 Softwoods and hardwoods
2.4 Some more information on wood's cell structure
2.5 The significance of ‘trade names’ versus ‘scientific names’
2.6 Growth rings
2.7 Earlywood and latewood
2.8 Rate of growth in softwoods
2.9 Rate of growth in hardwoods
2.10 Chapter summary
3 Two Fundamental Factors in Using Wood: Fire and Water
3.1 Wood and fire
3.2 Wood and moisture
3.3 Kiln drying
3.4 Air drying
3.5 Problems with timber as it dries
3.6 Timescales for drying timber
3.7 Chapter summary
4 Specifying Timber: For Indoor or Outdoor Uses – With Some Information on the Biological Attack of Wood
4.1 British and European standards
4.2 Durability and treatability of different wood species
4.3 Use classes
4.4 Examples of the use of timbers in different use class situations
4.5 Hazard and risk – and their relative importance
4.6 Use class 1: examples
4.7 Insects that can attack wood
4.8 Treatment against insect attack
4.9 Use class 2: examples
4.10 Use class 3: examples
4.11 Use class 4: examples
4.12 Wet rot and ‘dry rot’
4.13 Modified wood
4.14 Use class 5: examples – plus two marine organisms which can eat wood
4.15 Chapter summary
5 Wood Preservatives and Wood Finishes
5.1 Rule number one: treat the timber last!
5.2 Wood preservative types
5.3 ‘Old’ and ‘new’ treatments
5.4 Basic methods of timber treatment
5.5 Preservative chemicals
5.6 ‘Treated’ timber
5.7 Chapter summary
6 Timber Quality: Defects in Wood and Grading for Appearance
6.1 The need for grading
6.2 ‘Quality’ or ‘grade’?
6.5 Different types of grading
6.6 ‘Appearance’ grading
6.7 Appearance grading based on selection by inherent defects
6.8 Scandinavian appearance grades
6.9 Unsorted, fifths, and sixths
6.10 Russian appearance grades
6.11 Saw falling
6.12 European appearance grades
6.13 North American appearance grades
6.14 Clears, merchantable, and commons
6.15 A comparison of Scandinavian and North American grades
6.16 Appearance grading based on ‘cuttings’
6.17 NHLA grades
6.18 Malaysian grades
6.19 Rules are made to be bent (within reason!)
6.20 Shipper's usual
6.21 J classes
6.22 Exposed face
6.23 Chapter summary
7 Strength Grading and Strength Classes
7.1 Appearance versus strength
7.2 Visual strength grades
7.3 GS and SS strength grades
7.4 Strength classes for softwoods
7.5 BS EN 1912
7.6 SC3, SC4: C16 and C24
7.7 Machine grading
7.8 Other strength grades: Europe and North America
7.9 Select structural, no. 1 and no. 2 structural, and stud grades
7.12 Specifying a strength class or wood species: things to think about
7.13 Hardwood strength grading
7.14 Tropical hardwoods
7.15 Temperate hardwoods
7.16 The ‘size effect’
7.17 Hardwood strength classes
7.18 Marking of strength‐graded timber
7.19 Chapter summary
8 Wood‐Based Sheet Materials
8.1 Plywood construction
8.2 Two fundamental properties of plywood
8.3 The basic types of plywood
8.4 Problems with veneer ‘layup’
8.7 EN 636 plywood types
8.8 Adhesives used in plywood
8.9 BS 1088 marine plywood
8.10 Plywood glue bond testing
8.11 Plywood face quality
8.12 Appearance grading of face veneers
8.13 Plywood certification
8.16 Chapter summary
9 Principles of Timber Engineering (by Iain Thew)
9.1 Timber as an ‘engineering material’
9.2 Loads: their actions on structures
9.3 Load transfer
9.4 Bending, compression, and tension stresses
9.5 The use of strength classes
9.6 Load duration and its significance
9.7 Effects of timber moisture content on engineering properties
9.8 Load sharing
9.9 Deflection and ‘creep’
9.10 Trussed rafters
9.11 ‘Engineered timber’ joists
9.12 Glulam and LVL
9.13 Cross‐laminated timber
9.14 Chapter summary
Part Two Using Timber and Wood-Based Products in Legal and Sustainable Way
10 Some Things You Should Know About Wood, Trees, and Forests
10.1 Some very basic comments on how trees grow
10.2 How long can trees live for – and how ‘old’ is an old tree?
10.3 The properties of different timbers
10.4 Distribution of tree types
10.5 Natural forests
10.6 Managed forests: conifers
10.7 Managed forests: broadleaved trees
10.8 Plantations: both softwoods and hardwoods
10.9 Planting trees to help with climate change
12 Voluntary Timber Certification Schemes
12.1 Some more details about the RPP
12.2 Checking legality I: the Corruption Perceptions Index
12.3 Checking legality II: FLEGT
12.4 Checking legality III: MYTLAS
12.5 Checking legality IV: other ‘legality’ certification schemes
12.6 Checking sustainability: chain‐of‐custody certification
12.8 Third‐party assurance
12.9 How chain‐of‐custody schemes operate
13 UK Government, EU, and Other Countries' Regulations: Legally Trading in World Timbers
13.1 Checking up on the checkers: CPET and beyond
13.2 Help with legality and sustainability requirements in a UK context
13.3 The UK government's requirements for TPP
13.4 Category A and category B: ‘proof of compliance’
13.5 Current and future supplies of certified timber
13.6 The EUTR: Europe's compulsory ‘timber legality scheme’
13.7 Due Diligence
13.8 How to Satisfy the EUTR (or now, in the UK, the UKTR)
13.9 Putting together a due diligence system
13.10 Maintaining a due diligence system
13.11 ‘First placer on the market’
13.12 CE marking and the CPR
13.13 CITES: what exactly is It?
14 Softwoods Used in Construction – With Their Main Properties and Sustainability Credentials
14.1 European redwood, or scots pine (Pinus sylvestris)
14.2 European whitewood (principally Picea abies)
14.3 Sitka spruce (Picea sitchensis)
14.4 Western hemlock (Tsuga heterophylla)
14.5 ‘Douglas fir’ (Pseudotsuga menziesii)
14.6 Larch (mainly Larix decidua and Larix kaempferi/Larix leptolepis)
14.7 ‘Western red cedar’ (Thuja plicata)
14.8 Southern pine (Pinus spp., principally Pinus elliottii, Pinus echinata, Pinus palustris, and Pinus taeda)
14.9 Yellow pine (Pinus strobus)
14.10 Species groups
15 Some Hardwoods Used in Construction – With Their Main Properties and Sustainability Credentials
15.1 ‘Vulnerable’ timbers
15.2 Timber names: a bit more information
15.3 Ash, American (Fraxinus spp.)
15.4 Ash, European (Fraxinus excelsior)
15.5 Ayan (Distemonanthus benthamianus)
15.6 Basralocus (Dicorynia guianensis or Dicorynia paraensis)
15.7 Beech, European (Fagus sylvatica)
15.9 Birch, European (mainly Betula pubescens, sometimes Betula pendula)
15.10 Cherry, American (Prunus serotina)
15.11 Chestnut, sweet (Castanea sativa)
15.13 Ekki (Lophira alata)
15.15 Eveuss (Klainedoxa gabonensis)
15.16 Gedu nohor (Entandrophragma angolense)
15.17 Greenheart (Chlorocardium rodiei; formerly Ocotea rodaiei)
15.18 Guariuba (Clarisia racemosa)
15.19 Idigbo (Terminalia ivorensis)
15.20 Iroko (Milicia excelsa)
15.21 Kabukalli (Goupia glabra)
15.22 Kapur (Dryobalanops spp.)
15.23 Keruing (Dipterocarpus spp.)
15.24 Kosipo (Entandrophragma candollei)
15.25 Mahogany, African (principally Khaya ivorensis and Khaya anthotheca)
15.26 Mahogany, Central American (Swietenia macrophylla)
15.27 Maple (Acer saccharum)
15.28 Majau (Shorea spp.)
15.29 Massaranduba (Manilkara spp.)
15.30 Meranti (Shorea spp.)
15.31 Merbau (Intsia bijuga)
15.32 Missanda (Erythrophleum guineense and Erythrophleum ivorense)
15.34 Oak, American red (principally Quercus rubra and Quercus falcata)
15.35 Oak, American white (principally Quercus alba and Quercus prinus, but also Quercus lyrata and Quercus michauxii)
15.36 Oak, European (mainly Quercus robur, but also Quercus petraea)
15.37 Obeche (Triplochiton scleroxylon)
15.39 Opepe (Nauclea diderrichii)
15.40 Padauk (Pterocarpus soyauxii)
15.41 ‘Red grandis’ (really Eucalyptus grandis)
15.42 Sapele (Entandrophragma cylindricum)
15.43 Tatajuba (Bagassa guianensis)
15.44 Teak (Tectona grandis)
15.48 Utile (Entandrophragma utile)
15.49 Walnut, American (Juglans nigra)
15.50 Walnut, European (Juglans regia)
15.51 Whitewood, American or tulipwood (Liriodendron tulipifera)
16 The Use and Reuse of Timber and Wood‐Based Products: The Carbon Cycle, End‐of‐Life Disposal, and Using Wood as Biomass
16.1 Should we ‘save’ all the trees?
16.2 The true ‘carbon cycle’
16.3 End‐of‐life disposal of timber and wood‐based products
16.4 Recycled timber
16.5 Disposal of timber in landfill
16.6 Burning wood: fossil fuels versus biomass
17 Energy Considerations: Other Construction Materials Compared with Wood
17.1 Embodied energy
17.2 Cradle‐to‐grave analysis
17.3 Cradle to cradle: or the ‘circular economy’
17.5 Assessment criteria
17.6 Contribution of timber to ‘sustainable building’
17.7 The overall cost of being ‘sustainable’
Appendix A A Glossary of Wood and Timber Terms Used in the Timber and Construction Industries
Appendix B A Select Bibliography of Some Useful Technical Reference Works, Plus Some Other Information on Timber and Wood‐Based Products
Appendix C Some Helpful Technical, Advisory, and Trade Bodies Concerned with Timber
End User License Agreement
Table 3.1 Euroclass ‘reaction to fire’ classifications.
Table 3.2 Suggested EMC ranges for different end uses.
Table 3.3 Examples of various timbers showing different movement classes.
Table 4.1 Natural durability ratings from BS EN 350: durability of wood and w...
Table 4.2 Examples of timbers within each of the durability classes given in ...
Table 4.3 Simplified version of a table showing Use Classes.
Table 6.1 Comparison of Russian and Scandinavian timber qualities.
Table 6.2 Comparison of Scandinavian and North American softwood qualities.
Table 6.3 Joinery classes derived from BS EN 942: 2007, showing maximum permi...
Table 7.1 UK species/grade combinations and resulting strength classes.
Table 7.2 Relationship between THA, THB, TH1, and TH2.
Table 7.3 UK hardwood species and grade combinations for different strength c...
Table 8.1 Particleboard types.
Table 8.2 OSB types.
Table 8.3 Wood fibreboard types (both wet‐ and dry‐process boards).
Table 9.1 Characteristic values for strength classes.
Table 9.2 Load‐duration classes.
Table 9.3 Limits on net final deflection.
Table 13.1 Timber species on the different CITES appendices (‘lists’) as at 2...
Figure 1.1 Schematic of how photosynthesis works within a living tree.
Figure 1.2 How trees make wood and give us oxygen.
Figure 1.3 ‘Exploded’ view of a typical softwood tracheid, showing the diffe...
Figure 1.4 Example of figure (i.e. the pattern) on the surface of timber.
Figure 1.5 Wood surface showing the wood's grain (its ‘fibres’).
Figure 1.6 Directions: along and across the grain.
Figure 1.7 Two pieces of timber: one straight‐grained, showing a smooth surf...
Figure 1.8 Schematic cross‐section of a log, showing the different zones fro...
Figure 1.9 Rays in timber (highly magnified), viewed end‐on.
Figure 1.10 Schematic section of a log, showing the radial and tangential di...
Figure 1.11 Schematic diagram of ‘simple pits’, as found in the walls betwee...
Figure 1.12 Schematic diagram of ‘bordered pits’, as found in the walls betw...
Figure 2.1 A typical conifer (softwood), showing needle‐like ‘leaves’ and co...
Figure 2.2 A typical broadleaf (hardwood), showing ‘leaf‐shaped’ leaves and ...
Figure 2.3 Approximate distribution of softwoods and hardwoods throughout th...
Figure 2.4 Microscopic cross‐sectional view of softwood tracheids (×200), sh...
Figure 2.5 Cross‐sectional view of a ring‐porous hardwood (×20), with wider ...
Figure 2.6 Cross‐sectional view of a diffuse‐porous hardwood (×50), with sca...
Figure 2.7 Comparison of a fast‐grown softwood (round piece) with a slow‐gro...
Figure 2.8 Fast‐grown ring ‐porous hardwood, showing very wide bands of late...
Figure 2.9 Annual rings in a diffuse‐porous hardwood – formed, in this case,...
Figure 3.1 Archive photograph showing ‘failed’ steel girders being supported...
Figure 3.2 (a) Pair of wooden fire doors at the completion of a successful o...
Figure 3.3 Resistance‐type moisture meter fitted with a hammer probe. (NB: T...
Figure 3.4 Chart showing expected moisture content based on readings of temp...
Figure 3.5 (a) Typical drying kiln.(b) Timber correctly stacked, awaitin...
Figure 3.6 Timber stacked for air drying – but in this example, not very wel...
Figure 3.7 A typical ‘prong test’, with this sample indicating that the timb...
Figure 3.8 ‘Honeycombing’ (internal cracking), seen here in oak.
Figure 3.9 Timber which has suffered from ‘collapse’.
Figure 3.10 Cupping, caused by greater shrinkage of the face farther away fr...
Figure 3.11 Radially‐sawn timber, where both wide faces shrink equally, and ...
Figure 3.12 ‘Diamonding’ (lower right‐hand image), found especially in kiln‐...
Figure 3.13 Round section going ‘oval’ due to greater shrinkage in the tange...
Figure 3.14 (a) Lengthwise ‘bow’ in a timber board. (b) Lengthwise ‘spring’ ...
Figure 4.1 Typical Use Class 1 location: built‐in fitments in a bedroom.
Figure 4.2 Cracking in oak skirting caused by over‐drying in situ
, after ini
Figure 4.3 Life cycle of a typical wood‐destroying insect. Stage 1: eggs lai...
Figure 4.4 (a) Sketch of the adult ‘woodworm’ or common furniture beetle (An...
Figure 4.5 Leak in a roof, causing dampness in the timbers, which may eventu...
Figure 4.6 Studs and rails in a timber‐frame external wall at risk from inte...
Figure 4.7 Typical Use Class 3 locations: (a) fence rails; (b) deck boards....
Figure 4.8 External cladding, showing an excellent profile with thick boards...
Figure 4.9 Western red cedar used as external cladding, without the need for...
Figure 4.10 Typical Use Class 4 locations: (a) fence posts; (b) jetty suppor...
Figure 4.11 Softwood post that has rotted at the ground line.
Figure 4.12 Exhortation from the UK Wood Protection Association's brochure: ...
Figure 4.13 Timber suffering from an advanced stage of wet rot (note the ‘cu...
Figure 4.14 Piles in tidal waters which have been attacked by the gribble (L...
Figure 4.15 ‘Shipworm’ (Teredo
spp.) attack in coastal timber.
Figure 5.1 Treated zones in spruce (top) and pine (bottom). Note the absence...
Figure 5.2 Typical high‐pressure treatment tank.
Figure 5.3 Weathering of exterior wood without any protective coating applie...
Figure 5.4 Failure of a ‘film‐forming’ exterior paint due to peeling and fla...
Figure 5.5 Typical exterior stain, showing some surface erosion but no peeli...
Figure 5.6 One of the newer generation of ‘breathable’ exterior paints.
Figure 5.7 Shrinkage on dark‐stained boards, showing paler joint areas.
Figure 6.1 Typical grading line in a modern softwood sawmill.
Figure 6.2 (a) Initial formation of a ‘live knot’: here, the fibres of the t...
Figure 6.3 (a) Typical cluster of ‘live’ knots in a softwood (pine). (b) Dea...
Figure 6.4 Typical resin pocket as found in spruce (seen here between knots)...
Figure 6.5 Wane: seen on many of these sawn boards, on one or both edges.
Figure 6.6 Distorted timber: in this case, it has ‘sprung’.
Figure 6.7 Compression wood: in the log (left) and in microscopic section (r...
Figure 6.8 Early copy of the Export R List.
Figure 6.9 Copy of the NHLA grading rulebook (2003 edition).
Figure 6.10 FAS grade illustration.
Figure 6.11 Interlocked grain, giving a very characteristic ‘stripe’ figure ...
Figure 6.12 ‘Pinworm’ holes in the surface of sawn timber. (NB: The ‘pinworm...
Figure 7.1 Large‐section graded softwood beams as used in a flat roof constr...
Figure 7.2 Typical strength‐grading machine.
Figure 7.3 New trussed rafters, made using ‘TR26’ softwood members.
Figure 7.4 Typical strength‐grading mark (in line with BS 4978 requirements)...
Figure 8.1 Exposed veneer edges, revealing the usual cross‐banded constructi...
Figure 8.2 Comparison between a birch hardwood plywood (left) and a conifer ...
Figure 8.3 Very poor plywood layup: note the very high number of core gaps a...
Figure 8.4 Delamination in a typical ‘uncertified’ tropical hardwood plywood...
Figure 8.5 (a) Typical plywood specimen undergoing the glue‐bond ‘shear test...
Figure 8.6 A ‘plug’ in an ‘improved‐face’ softwood veneer.
Figure 8.7 (a) CanPly stamp. (b) APA stamp.
Figure 8.8 Typical OSB surface appearance, showing the ‘strands’.
Figure 8.9 Machined and moulded MDF, which can be machined‐profiled just lik...
Figure 10.1 The ‘woody stem’ of a tree allows it to remain standing for many...
Figure 10.2 (a) Very fast‐grown softwood from a forest area with a long grow...
Figure 10.3 Selected conifers in a Swedish tree nursery, encouraged to creat...
Figure 10.4 Conifer seeds being ‘potted up’ in a Swedish tree nursery. They ...
Figure 10.5 A well‐managed forest which has now been harvested for a second ...
Figure 10.6 Typical regeneration in a previously harvested Canadian forest....
Figure 10.7 A well‐managed and ‘sustainable’ temperate hardwood forest in th...
Figure 10.8 Monterey (radiata) pine (an ‘exotic’ introduced species) growing...
Figure 10.9 The largest (and oldest) Sitka spruce in the British Isles, plan...
Figure 10.10 Elliottis pine (an introduced conifer species) is often made in...
Figure 10.11 ‘Falcata’ logs, harvested after only 8 years, being processed f...
Figure 12.1 Typical softwood door, made from a timber harvested legally from...
Figure 12.2 Typical example of the FSC logo on wood‐fibre products: in this ...
Figure 12.3 The PEFC logo currently appears primarily on commercial packages...
Figure 12.4 The FSC chain‐of‐custody ‘licence number’ stamped on a timber pr...
Figure 12.5 The SFI logo, seen here stamped on softwood in the United States...
Figure 13.1 Headlines in the United Kingdom's TTJ magazine about ‘illegal’ t...
Figure 13.2 The CE mark gets everywhere, not just on wood goods – and it has...
Figure 13.3 The three different CITES appendices.
Figure 14.1 Approximate distribution of the world's softwood forests.
Figure 15.1 Approximate distribution of the world's hardwood forests.
Figure 15.2 Leaflet published by the Woodland Trust about ash dieback (Chala...
Figure 16.1 The ‘carbon cycle’, which makes trees and wood carbon‐neutral.
Figure 16.2 Piece of furniture made from reclaimed wood.
Figure 16.3 Biomass ready for the furnace in a power plant.
Figure 16.4 ‘Biomass’ being transported for burning.
Figure 17.1 Brick, timber, and concrete. Wood has been calculated as having ...
Figure 17.2 Logs brought only tens of kilometres from forest to sawmill have...
Figure 17.3 Cement has a high level of ‘production energy’ input before it i...
Figure 17.4 Comparison of timber's embodied energy versus that of steel, whe...
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Library of Congress Cataloging‐in‐Publication DataNames: Coulson, Jim (James C.), author. | Wiley‐Blackwell (Firm), publisher.Title: A handbook for the sustainable use of timber in construction / Jim Coulson.Description: [Hoboken, NJ] : Wiley‐Blackwell, 2021. | Includes index.Identifiers: LCCN 2020025971 (print) | LCCN 2020025972 (ebook) | ISBN 9781119701095 (cloth) | ISBN 9781119701149 (adobe pdf) | ISBN 9781119701132 (epub)Subjects: LCSH: Sustainable construction. | Timber‐‐Standards. | Wood products‐‐Standards. | Sustainable buildings‐‐Design and construction.Classification: LCC TH880 .C688 2021 (print) | LCC TH880 (ebook) | DDC 691/.1‐‐dc23LC record available at https://lccn.loc.gov/2020025971LC ebook record available at https://lccn.loc.gov/2020025972
Cover Design: WileyCover Image: © Chris Rogers/Getty Images
This new edition is both an updated version of my original Wood in Construction and a combining of that volume with an update of my second book, Sustainable Use of Wood in Construction. Thus, it brings together in a single volume the technical aspects of wood and its uses with information on wood's outstandingly good environmental credentials.
The reasons for creating this new and combined edition are twofold: primarily, to make some necessary changes to the technical references – because a lot has happened in the 10 years since I first sat down to write Wood in Construction – and secondly, to bring to the foreground the highly valuable contribution that wood can make to the twin problems of climate change and global warming.
At the time I wrote the second book (which, in its updated form, makes up Part II of this revised edition), the whole aspect of wood's ‘sustainable’ credentials was beginning to be of much interest to construction professionals. However, in this new era of the 2020s and beyond, the huge contribution that both trees and their timber can make to alleviating of the world's environmental issues – especially that of global warming – has suddenly come very much to the foreground. There has never been a better – nor a more pressing – time to use wood as a construction material; and, as a consequence, there is a real global need to grow more trees (in the best correct and sustainable ways, of course!) in order to capture and ‘lock away’ as much as possible of the world's excesses of atmospheric carbon.
I hope that you will gain a great deal from this new expanded and revised edition of a book that it has been my pleasure once again to write. I have spent a lifetime trying to help the specifiers, users, and traders of this ‘wonder material’ to better understand how it works. Now you too can gain a much better understanding of how to legally and sustainably harness wood's amazing properties, and so play a vital role in helping us to care for this planet of ours.
Trees were well established on the earth long before mankind ever appeared – and they will still be here long after we have ceased be around, trying to wreck it!
This new book also includes an extra chapter on the principles of Timber Engineering, written by Iain Thew: who is a qualified structural engineer with a wealth of understanding about timber construction. It was my pleasure to introduce Iain to the delights of Wood Science when he worked for me, over a decade ago.
Jim CoulsonBedale, North Yorkshire, March 2020
The first thing that we need to get absolutely clear in our minds, right from the start of this book, is that there really is no such thing as ‘wood’… (‘What!?’ I hear you say, ‘Have you gone mad?’) Well, of course, there definitely is the stuff that grows on trees (or, more correctly, the stuff that grows inside trees): but what I am trying to get at here is that there is not one individual, unique, and singular substance with infinite properties and uses that can simply be referred to just as ‘wood’. There is no one highly specialised species of tree which can give us a material that will do every single job without any problems and with no prior thinking or preparation, no matter how simple that job might be.
‘Wood’, as most laymen are apt to use that word, is merely a catch‐all term that covers a quite staggering range of possibilities in terms of appearance, abilities, and potential uses: from the hard‐wearing to the hardly worth bothering with; from the very strong and durable to the very weak and rottable. My aim in this book is to show you that any particular named species of wood can be very different in its properties – and therefore in its usefulness for a specific job – to some other vaguely similar species.
An obvious comparison is with what we mean when we use the word ‘metal’. If you should go along to a broker or a stockist of metals, then the first thing you're likely to be asked is exactly what type of job you intend to do with the ‘metal’ you're looking to buy. The answer to that question will govern the properties you will want that ‘metal’ to possess. Do you require it to have a high tensile strength, or a good degree of ductility, or a shiny surface, or a light weight, or what? If you can't specify precisely what you need your ‘metal’ for, then you may be offered a whole slew of options: ranging from steel, to brass, to copper – or tin, or lead, or mercury (which, of course, is liquid at room temperature), or even calcium (yes, although it's a major part of your bones, it's actually a metal!). All of these ‘metals’ are – as you probably know – very different from one another, with huge variations in their physical and chemical properties; but all of them fit the vague and general description of being a sort of ‘metal’. Why, then, should we presume that the situation is any different when it comes to the ‘wonder material’ that we call wood?
Probably a good question to ask would be: ‘Why do so many people assume that “wood” is all that they need to ask for and specify?’ Even those professionals who try to take more care about what they do or write often think that they've done enough by asking just for a ‘hardwood’ or a ‘softwood’ – as though that somehow defines more accurately the properties that they require of their material. Even such apparently extra clarity is simply not good enough, as I want now to show you in this revised and updated volume of mine.
Every single individual species of wood has certain very specific properties; and therefore, it must follow, certain potentially good uses and certain other not‐quite‐so‐good uses. Many wood species may have other things about them that we might do best to avoid, or at least restrict. Thus, the individual properties of this immensely variable material will be subtly – or maybe even greatly – different as we move from one species to another. In essence, no two ‘woods’ are quite the same as one another; just as no two ‘metals’ are exactly the same.
Sometimes, of course, the differences in properties are quite minor, and they will not significantly affect the outcome if one species is used instead of another. But quite often, the differences between wood species options can be absolutely vast – the equivalent of using chalk instead of cheese. (I know nobody builds with cheese – but sometimes, they might just as well, for all the good their chosen material does!)
At least 60 000 (and still counting) different species of wood are estimated to have been discovered and described by botanists and wood scientists to date. You may thus begin to see that you really do need to know a whole lot more than perhaps you thought you did in order to begin to understand exactly what sort of ‘wood’ you should be asking for – and, of course, what species.
As already hinted at, however, it's not only a question of species – vitally important though that is. The quality and the grade of the timber are also very significant factors in getting the best performance at the best price, as are a number of different processes and treatments that can (and quite often should) be applied to it, once its species and final quality have been decided upon.
Some of these other processes include moisture content (mc; this has to do with how wood dries), treatment (i.e. preservatives), coatings and finishes (paints and stains), and care during delivery and storage. All of these things are, in my humble opinion, equally essential factors in getting a good job done properly when using timber. Not to mention all the additional complexities that are involved in specifying and using wood‐based board products, such as plywood, chipboard, and medium density fibreboard (MDF). I will explain the most important of these different factors and processes in somewhat greater detail in later chapters; for now, I want to begin the journey toward your timber enlightenment by looking at the way that wood is actually made in trees – and what complex things it is made of.
Figure 1.1 shows a diagram of a typical tree (just for now, it doesn't matter about the individual tree species or wood type). It demonstrates that a tree has a number of different elements or components, some of which provide pathways for liquids and nutrients to be moved around within the tree's trunk as it grows. First of all, we should see that the tree's root system takes up moisture from the soil and transfers it vertically up the trunk, all the way to the leaves; then, the leaves undertake the really amazing process of extracting carbon dioxide from the atmosphere to manufacture – quite literally, out of thin air – the material that we know as wood. I explain in the next section more details of the chemical process (known as photosynthesis) whereby this miracle happens – but first you need to understand a little of how it happens within the tree.
Water from the soil is moved upwards through the cells which make up the first few growth layers of wood immediately below the bark (we will describe and discuss this part of the tree in greater detail a bit later; for now, let's just get to grips with the overall process). Once in the leaves, it is combined with the carbon dioxide extracted from the air, assisted by chlorophyll (that's the ‘green stuff’ in leaves, which is a catalyst: something that assists a chemical reaction but takes no active part in it) and powered by the energy from the sun. We can thus see that wood as a material actually traps the sun's energy and stores it away: this is why one of the earliest uses of wood by humanity was – and still is – as a source of fuel. But the main thing which wood captures and stores away (in a process which we call ‘sequestration’) is carbon, in the form of CO2. So trees do us and our planet an enormous amount of benefit, simply as a sideline to their making the wood substance that they themselves require just in order to stand there and be trees!
The chemicals which are made in the leaves, using the power of sunlight, then need to be got back down into the trunk, so that the tree can complete the clever process of making wood. That downward transfer of nutrients is performed by the inner bark, which (for some reason that is lost in the mists of time) has been granted two ‘official’ names: the ‘bast’ and the ‘phloem’. If you want to imagine what phloem (let's just stick to the one name for now!) looks and feels like, then just think of a cork in a wine bottle. Not one of the newer, plastic ‘corks’ (and, of course, definitely not a screw top), but the traditional one that you can still find in the necks of many Spanish or Portuguese wines and which you have to wrestle out with a corkscrew. Perhaps surprisingly, that cork is pure phloem: it is literally the inner bark of a very special tree – the cork oak – which has been harvested for that very purpose for countless centuries, because its phloem happens to be particularly thick and abundant. But all phloem – that is, all the inner bark of all trees – is essentially like that: spongy and full of pathways, which conduct the nutrients that the leaves have made back down the trunk to be fed inside the tree and used to make its essential ‘woody’ ingredients. And chief amongst these ingredients – the chemical which contributes most to the overall properties of wood as a material – is cellulose.
Figure 1.1Schematic of how photosynthesis works within a living tree.
All wood cells are made predominantly from cellulose (that is, around 70–80% of overall wood ‘bulk’, depending upon species and growth habit). It is true that both the chemistry and the physics of wood are somewhat more complex than this simple statement would imply, but I don't need to go too deeply into the chemistry and physics here in order to allow you to appreciate the wonderful properties of this unique material. Suffice it to say that the main ingredient of wood – and, therefore, what gives this natural material most of its significant properties – is the organic substance which we term ‘cellulose’ (so named because it is the primary ingredient of many plant cells). However, trees also make a ‘waste product’ which most of the living things on this planet require to be able to live: oxygen. Thus, not only do trees make a material which we find incredibly useful, not only do they trap and store away (sequester) planet‐harming carbon dioxide, but as a bonus they also keep us alive with oxygen supplies! How many other building materials can claim to do so much for the planet and for humanity?
Cellulose is made by (and within) the tree itself, using as its basic building blocks the sugars and starches manufactured in the leaves by photosynthesis (as already discussed). So, in fact, every tree (and almost every green living plant, for that matter) is a fantastic, natural chemical factory. Utilising nothing more complex than water, drawn up from the ground via its root system, and adding to it the carbon dioxide that it literally sucks out of the air through its leaves, this wonderful ‘chemical plant’ combines the most basic of ingredients by shuffling their atoms and molecules around to produce completely new substances.
Figure 1.2 shows how a tree ends up with cellulose just by recombining the basic atoms of water and carbon dioxide. But this does not happen immediately. In order to begin the process, the tree first uses six molecules of H2O (water) plus six molecules of CO2 (carbon dioxide) to manufacture – as a first step – a single molecule of sugar, whose ‘makeup’ is C6H12O6. An extremely useful byproduct of this chemistry – certainly so far as we humans are concerned – is 12 ‘spare’ atoms of oxygen, which are helpfully released into the atmosphere in the form of six molecules of O2.
Figure 1.2How trees make wood and give us oxygen.
After making itself a supply of basic ‘carbohydrates’ (that is, sugars and starches; these are quite similar in their chemical construction, all using only the atoms C, H, and O), the growing tree uses them to manufacture cellulose (C6H10O5) and other essential chemical ingredients, as and when they are needed. As it does so, it releases one ‘spare’ molecule of water – which is reabsorbed into the tree so that nothing gets wasted. Simple!
Having seen that a tree can conveniently make its own cellulose (plus a few other important ingredients, which we will discuss in a bit more detail later), we should perhaps try to learn something about this particular substance. The most fantastic thing about cellulose is that it is strong: very strong, indeed. It is, in effect, a natural type of carbon fibre, invented by Mother Nature long before humans ever started to get clever with chemistry and physics.
Primarily, it is the very strong chemical bond between the carbon atoms in the cellulose molecules that gives wood its incredible strength. (Chemists call these molecules ‘long‐chain’ molecules, because of their highly organised, elongated, and linked‐together structure.) A good demonstration of the fact that cellulose (and therefore wood) has such very high strength, which it gets from the linked atoms of carbon in its molecular chains, is provided by an experiment conducted at a major UK university (Leeds, if I recall correctly) in the 1960s. This experiment consisted of pulling apart two equal‐weight strands – one made of European pine and one of a high‐tensile steel wire – using a special testing machine, called a ‘tensometer’ (which pulls things apart in tension). The researchers measured the force that it took to snap each strand – and found that, weight for weight, wood was actually stronger, in tension, than steel!
However, the picture is not quite as straightforward as perhaps I've implied when it comes to establishing exactly why and how wood is so strong. As well as knowing its chemistry – that is, that wood is made up of very strongly‐linked molecules of cellulose – we also need to consider its physical structure when it comes to looking at how it performs when it is used for any particular job. I now need to tell you about the way wood is – quite literally – put together, in order that you can properly understand how best to use it.
Figure 1.3 shows us an ‘exploded’ view of a typical softwood cell, which is called a ‘tracheid’ (we will refer to this cell type in more detail a bit later, so don't worry too much about it for now). To see just how intricate wood's structure is, I would like you to look carefully at the different component parts of the cell wall, as shown in the diagram.
Figure 1.3‘Exploded’ view of a typical softwood tracheid, showing the different cell‐wall layers.
The first two main elements – that is, the parts that divide (or separate) the individual wood cells from one another – are not of any great significance to this part of our story, so we can more or less ignore them for now; for completeness, they are called the middle lamella and the primary cell wall. The part of the wood cell wall that is really, really important to our understanding of fundamental wood properties and behaviours is the secondary wall; this is divided, as you can see, into three distinct ‘layers’, which are called, for convenience, ‘S1’, ‘S2’, and ‘S3’.
Apart from cellulose, there are two other chemical ingredients of wood which you need to be aware of. One is a substance (or rather, a collection of similar substances) called ‘hemicelluloses’; these may perhaps be thought of as incompletely‐formed cellulose molecules, which act as a kind of ‘filler’ or ‘bulking agent’ within the cell walls and play some part in altering the physical properties of wood. The other is ‘lignin’, a sort of natural adhesive which infiltrates the cell walls and effectively sticks all of their ingredients together. At this stage, a simple analogy may perhaps help us to understand the complex makeup of wood cells a little more clearly.
We may adapt the idea of a fibreglass resin‐bonded material (such as is used for lightweight car bodies and kayaks, etc.) to describe how wood is put together on the submicroscopic scale. (Stop Press: Some very new research – which has not yet been fully digested, nor fully accepted everywhere – would seem to suggest that this ‘fibreglass’ analogy may be a little too simplistic. But for now, it is a good enough approximation to do the present job, if I am to help you to better understand the highly complex structure of wood.) In fibreglass, very thin strands of glass (which at that scale are flexible rather than brittle) are set into a ‘mat’ of resin, which keeps them together; thus, between the two ingredients (glass and resin), we end up with a material which has infinitely superior properties to either substance on its own, with respect to its strength and moulding abilities.
In wood cell walls, the equivalent to the resin‐bonding ‘mat’ is the combination of the lignin and the hemicelluloses: in fact, lignin is chemically quite similar to some plastic resins that are used as adhesives – the eventual hope is that we may be able to extract lignin from wood and use it as an alternative to oil‐based plastics (but that's another story). But what is wood's equivalent to the glass fibre strands that give fibreglass its strength? The answer is cellulose ‘microfibrils’.
Microfibrils are made from pure chains of cellulose; they can only be detected by means of electron microscopy. We saw earlier that cellulose is incredibly strong in tension, and it is the particular arrangement of these cellulose microfibrils which makes it so. Note in Figure 1.3 that the microfibrils (which are shown as fine lines within the wall layers themselves) are arranged in a criss‐cross pattern in both the S1 and S3 layers, which are relatively thin. The reason for this is that the microfibrils provide a sort of ‘corset’ effect, which braces the cells against bursting when wood is loaded vertically in compression. But you can also see that the S2 layer has its microfibrils arranged only vertically (well, more or less vertically: the angle alters subtly with the type of wood tissue laid down – but I'm now in danger of making the whole thing over‐complex, so let's just leave it as we've drawn it in the diagram) and not criss‐crossed over one another. Furthermore, the S2 layer itself is by far the thickest of all of the cell wall layers, by some considerable margin. It is the differently‐detailed structure – and therefore the physical performance – of the S2 layer that most governs the behaviour of wood cells on the submicroscopic level, and thus the behaviour of wood itself on the larger (or gross) level – which we can actually see and measure.
If you look again at the S2 layer, you should now see that its main structure provides a more‐or‐less vertical alignment of the majority of the cellulose microfibrils in the wood cell wall. In doing so, it gives each wood cell – and therefore, on the gross scale, wood itself, as a material – a very great resistance to being pulled apart in tension (if you remember from earlier, we said that the carbon bonds in cellulose are incredibly strong in tension). And remember that this very, very tiny molecular structure is being held in place by the lignin and hemicelluloses, allowing the microfibrils to do their ‘strength’ job properly.
The upshot of this incredibly intricate arrangement of microfibrils – which are concentrated longitudinally mostly in the S2 layer, but are also helpfully arranged in a criss‐cross pattern in the S1 and S3 layers (and, as you can see, in the primary wall as well) – means that wood in its gross properties can resist both compression and tension (especially tension) whenever it is subjected to all the stresses and strains of everyday life: both as a tree and as the material that we use. Clever, or what?
The arrangement of the S2 layer's microfibrils also has a profound influence on the behaviour of wood in its reaction to moisture – but I will leave that explanation until a little later. I am still, at present, dealing with wood's fundamental structure and how that influences many of its working properties. So, having for the moment dealt with how cellulose makes wood fundamentally what it is, I will carry on with my explanation of exactly how wood's structure works.
Trees (and therefore, of course, wood) have an inherent ‘grain’ structure. Grain is one of those very common and yet very overused words that laymen love to bandy about when referring to wood in all sorts of ways – not least when describing its appearance (which is quite wrong, as I will explain). The word ‘grain’ has in fact a very precise meaning, so I feel it's important that I should get you to use it correctly.
First of all, what grain is not is that very interesting, sometimes wavy, sometimes stripy or curly – and thus often highly decorative – pattern which we so often see on the surface of a piece of planed or sawn timber. (And I wouldn't mind betting that most of you have used the word ‘grain’ in that context: in fact, I suspect that perhaps many of you, or your colleagues, still do.) The correct name for the decorative surface pattern on a piece of timber is its ‘figure’ (see Figure 1.4). The figure in wood can often (although not always) show us what the grain may be doing; but it is decidedly not the same thing as the actual ‘grain’ of the wood. Sometimes, misreading the figure and thinking it is the grain can lead to physical damage of the wood you are trying to use, and sometimes it can lead to unnecessary rejection of the timber – for example, when undertaking strength grading (a topic that I will discuss in detail in Chapter 7).
So, if it is not the nice pattern that you can see on the surface of the wood, then what exactly is grain? Well, in my book (quite literally, as well as metaphorically!) the term ‘grain’ specifically relates to the direction of the wood fibres: that is, the way they grow within, along, and up the trunk of the tree; or the way in which they are aligned along the length of a cut board or plank of wood (see Figure 1.5). The principal vertical (or longitudinal) individual cells in the tree trunk – which, for now, we'll refer to simply as ‘fibres’ – are relatively long (say, about a millimetre in softwoods, but much shorter in hardwoods), but they are extremely narrow, and they generally grow quite straight: as I have said, along and up the main axis of the trunk.
Figure 1.4Example of figure (i.e. the pattern) on the surface of timber.
Figure 1.5Wood surface showing the wood's grain (its ‘fibres’).
These basic wood cells (of whatever wood type) grow mostly in the form of hollow tubes, which have a relatively thin cell wall, with a hole (known as the cell ‘cavity’ or ‘lumen’) that runs all the way down their middle. In the living tree, this lumen or cell cavity is full of sap (‘tree juice’). But when a tree is cut down, the sap dries out (sooner or later), leaving the ‘dry’ wood essentially as a network of relatively long but narrow, hollow tubes, full of air. (I will return to the subject of the correct drying of timber and its consequences in Chapter 3.)
These tube‐like ‘wood fibres’ all point more or less in the same direction (along the axis of the tree trunk, or the length of a board or plank, as I've said). So please remember from now on that you should (and I definitely will!) only use the word ‘grain’ to mean one thing: ‘grain in timber is the direction of the wood fibres’ – and that is the most important word: ‘direction’.
You should now see that, if we cut up a log in a good and efficient way, such as usually happens in a sawmill, we will (hopefully) find that the wood fibres that were in the tree trunk will still line up so that they are more or less parallel with the long axis of the board or plank of wood that we have produced. And if the cutting has been done well, then the grain will run pretty straight along its length. I hope you will now understand that the strong molecular ‘chains’ of cellulose (which, as I said earlier, make up the bulk of the wood‐fibre walls in the S2 layer) can contribute their very high ‘chemical‐bond’ strength to the physical ‘along‐the‐grain’ strength performance of the wood.
It is thus immediately possible to state one definitive fact about a basic property of wood: ‘the straighter the grain, the stronger the piece of timber’.
Unfortunately, there is always a plus and a minus where wood is concerned (as you will see several times in the course of this book). The ‘plus’ is that the cellulose makes wood very strong when loaded along the grain. But the ‘minus’ is that all of the long, thin wood cells that are in the tree or along the plank of timber are, as we saw earlier, stuck together by a natural sort of glue, known as ‘lignin’.
Lignin is a very complex chemical whose structure is still not fully understood. But one thing that we do know about it is that it is does not hold the wood cells together very strongly in tension, side‐by‐side. Therefore, the wood fibres may be pulled apart relatively easily in this direction – and it is this sideways orientation that we usually refer to as being ‘across the grain’.
So, because of both its chemical makeup (mainly cellulose and lignin) and its very particular microscopic physical structure (a whole bunch of long, very strong, tube‐like ‘fibres’ which point along the line of the grain but are nonetheless stuck together relatively weakly), wood ends up being a very unusual material in terms of its strength performance. Wood scientists like to say that wood is anisotropic: a posh word that basically means, ‘it behaves quite differently under different directions of loading’ – as I will now explain.
Consider a brick, or a block of concrete. Load it (by, for example, squashing it beneath the weight of a wall in a building) and it will resist the load – which is a compressive force – to the best of its ability. As you might reasonably expect, a brick or a block of concrete is capable of resisting load pretty equally, in all of the usual three directions: width, breadth, and depth. By the same token, a steel joist will be more or less equally strong when it is loaded in bending or in tension, in each of the same three directions. In fact, just about all of our building materials, by and large, behave more or less equally in terms of their strength in all of the different directions of loading. All of them, that is, except for wood – and that is because of wood's highly ‘anisotropic’ nature, which it possesses on account of its very special ‘grain’ structure.
Compared with our other building materials, wood really is very unusual. It is – as I've just explained – incredibly strong along the grain (and remember, that's along the direction of its fibres). But it is very weak across the grain (that is, sideways, across both the width and the breadth of a timber member) (see Figure 1.6). This strength difference – taken as an average amongst most common wood species – is about 40 times greater in tension along the grain than it is across the grain. Just think about that for a second: 40 times! That's an incredible difference in the behavioural properties of one single material – and one that is dependent only upon its direction of loading.
Figure 1.6Directions: along and across the grain.
So that's why everyone who uses timber should always try to use it in such a way that they can capitalise on its long‐grain strength whilst at the same time minimising its cross‐grain weakness. And they should do so by making sure that any imposed loads are carried as much as possible along the grain, not across it.
But strength is not the only property of wood that may be influenced by the direction of the grain.
Wood reacts with moisture. Or to be more accurate, it reacts to changes in its mc; and those changes are themselves influenced very strongly by the relative humidity (RH) of the atmosphere in which it is used or stored. I will discuss drying methods and the significance of water in wood in much greater detail in Chapter 3; and there, too, I will explain the vital need to get the wood's mc right. But for the moment, I want to touch on just one essential concept that is related directly to grain orientation: ‘movement’ (see
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