A Handbook for the Sustainable Use of Timber in Construction

Jim Coulson, Iain Thew

• Publisher: Wiley-Blackwell
• Year: 2020
• Print ISBN:
  9781119701095
• Edition: 1
• eBook ISBN:
  9781119701132
• Topic: Technology & Engineering
• Subtopic: Construction & Architectural Engineering
• Index: Technology & Engineering

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.

PART ONE
How Timber Works: Wood as a Material — Its Main Processes and Uses

1
Wood as a Material

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.

1.1 Tree growth and wood formation

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 Portu...

Table of contents

• Cover
• Title Page
• Copyright
• Preface
• Part One How Timber Works: Wood as a Material — Its Main Processes and Uses
• Part Two Using Timber and Wood-Based Products in Legal and Sustainable Way
• 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
• Index
• End User License Agreement