Wood in Construction

How to Avoid Costly Mistakes

Jim Coulson

• Publisher: Wiley-Blackwell
• Year: 2011
• Print ISBN:
  9780470657775
• Edition: 1
• eBook ISBN:
  9781119968252
• Topic: Technology & Engineering
• Subtopic: Construction & Architectural Engineering
• Index: Technology & Engineering

Wood in Construction – How to Avoid Costly Mistakes focuses on the basic principles and appropriate use of wood in construction and illustrates how to avoid or minimise problems, to ensure that wood performs as expected when used in a construction application. Based on the author's extensive experience of manufacturing processes and practical applications in the timber, construction, joinery, shop-fitting and furniture industries, Wood in Construction provides a guide to using wood in building in the real world. It describes the main causes of difficulty when using wood, and shows how to avoid or minimise problems, reducing the difficulties for the architect, engineer or specifier, builder and building owner.

Technical enough to explain why things should be done in specific ways, but also practical enough to demonstrate how to use wood correctly and avoid doing the wrong things, this is an invaluable resource for construction specifiers (architects, engineers), carpenters, structural engineers, building surveyors, small/medium sized builders.

 

1

Wood as a Material

The very first thing to get absolutely clear at the start, is that there is no such thing as ‘wood’! Of course, there is the stuff that grows on trees (or rather, the stuff that grows inside trees): but what I mean to say here, is that there is not one individual, unique and single substance that can simply be referred to just as ‘wood’. There is no one, unique material that will do every single job without any problems and with no prior thought, no matter what you might require it to do for you.

The stuff that we know as ‘wood’ – and as most laymen are apt to use that term – is merely a catch-all word that covers a whole range of possibilities in terms of appearance and abilities. From the hard-wearing to the hardly worth bothering with: or from the very strong and durable to the very weak and rottable. So, in this book, I aim to show that any given species of wood is very different in its properties – and therefore in its usefulness – to some other vaguely similar sort of wood, but which happens to be of a different species.

An obvious comparison could be with the idea of what we mean by the word ‘metal’. If you should go along to a stockist of metals, then the first thing you’re likely to be asked is exactly what job you intend to do with that ‘metal’. And the answer to that, in turn, will govern the likely properties that 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 something else? And if you don’t specify more precisely what you need this particular ‘metal’ for, then you may be offered a whole range of possibilities: ranging from steel, to brass, to copper – or tin, or lead, or mercury (which is liquid at room temperature) or even calcium (yes, although it’s in your bones, it’s a metal!). All of these genuine ‘metals’ are very different from one another, with huge variations in their physical and chemical properties; but all of them fit that initial, vague and general description of being a sort of ‘metal’. So why should the situation be any different when it comes to wood?

A good question to ask would be: ‘Why do so many people assume that ‘wood’ is all that they need to specify’? Even those who 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 in their material. But even that apparent improvement in the material’s description is simply not enough, as I hope this book will show.

Every single, individual species of wood has certain very specific properties and therefore, it must follow, certain potential uses. But it also has certain other things about it that we might do best to avoid, or at least restrict: and those individual properties of this immensely variable material will then be subtly – or perhaps greatly – different from one species of wood to another. In essence, no two ‘woods’ are the quite same as one another; just as no two ‘metals’ are quite the same. And quite often, the differences in performance between different wood species can be very large indeed.

Sometimes, of course, these differences in properties are quite minor; and they will not significantly affect the outcome, where one species has been used instead of another. But sometimes, the differences between alternative wood species can be absolutely vast – such that it would be 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 it does!)

There are at least 60 000 (and still counting) different species of wood in the world, which have so far been discovered and described by botanists or by Wood Scientists: so you should now begin to see that you really do need to know a whole lot more than perhaps you thought you needed to, in order to begin to understand exactly what sort of ‘wood’ you should be asking for. And, of course, what you should really be using.

But it’s not only a question of the wood species – vitally important though that is. The Quality and the Grade of the timber that are to be used are also very significant factors in getting the best performance from timber, at the best price: as are a number of different processes and treatments that can (and quite often should) be done to the timber, once its wood species and final quality have been decided upon.

Some of these other processes are: moisture content (drying), treatment (preservation), finishes (paints and stains) and taking care of the timber during delivery and storage. All of these things are, in my humble opinion, quite 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 or chipboard or MDF. I will explain the most important of these different factors and different processes in greater detail, in some of the later chapters. But for now, I want to begin the process of your timber education by looking at what wood is actually made of.

1.1 Cellulose

All wood cells are made predominantly from cellulose. It’s true that both the chemistry and 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 get you to appreciate the wonderful properties of this unique material. For now, 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 called cellulose.

Figure 1.1 How trees make wood and oxygen

Cellulose is made by (and within) the tree itself, using as building blocks the sugars and starches that have recently been manufactured in the tree’s leaves: and these chemicals in turn were obtained by harnessing the energy of sunlight, under the influence of chlorophyll (that green stuff). In fact, every tree (and almost every living plant, for that matter) is a fantastic, natural chemical factory.

Simply by utilising nothing more complex than water, drawn up from the ground via the tree’s root system, and then adding to it some Carbon Dioxide that is literally sucked out of the air, this wonderful ‘chemical plant’ then combines those most basic of ingredients, by simply shuffling the atoms and molecules around to make completely new ingredients out of them.

To make cellulose, the tree uses six molecules of H₂O (water) plus six molecules of CO₂ (carbon dioxide) to fabricate – as a first step – a single molecule of sugar (C₆H₁₂O₆). An extremely useful by-product of this chemistry – certainly so far as we humans are concerned – are 12 ‘spare’ atoms of Oxygen (see Figure 1.1), which are helpfully released into our atmosphere in the form of six molecules of O₂.

After making itself a supply of carbohydrates (that is, sugar plus starch – which is really quite similar in its chemical construction: using as it does, only the atoms of H, O & C), the growing tree then uses this newly-produced food supply to manufacture cellulose (C₆H₁₀O₅) for itself: and as it does so, it then releases one ‘spare’ molecule of water. To complete the picture, this excess molecule of water is simply absorbed into the tree, so that nothing is wasted.

Having seen that the tree can conveniently make its own cellulose, we should then perhaps try to learn something about that particular substance. And 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 Mankind ever got clever with chemistry.

It is the hugely strong chemical bond between the atoms of Carbon in the molecules of cellulose that gives wood its high strength. (These molecules are called, by chemists, ‘long chain’ molecules, because of their highly-organised, elongated and linked-together structure.)

Cellulose (and therefore wood) has, as I’ve just said, very high strength, which comes from the linked atoms of Carbon in its molecular chains. This amazing strength was shown way back in the 1960s: where an experiment was carried out at a major university, to prove just how incredibly strong wood can be. The experiment consisted of pulling apart two equal-weight strands: one made of European pine and one made of a high-tensile steel wire, using a special machine, called a ‘tensometer’ (which pulls things apart in tension). Then they measured the force that it took to snap each strand: and from this experiment, it was demonstrated that (weight for weight) wood is 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 the physical structure of wood when we are looking at how it performs when we actually try to use it to do any job with. So 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.

1.2 Grain

Trees (and therefore of course, wood) have an inherent ‘grain’ structure. Grain is one of those common yet very over-used words, that laymen love to bandy about all the time when referring to wood in all sorts of ways: not least when describing its appearance (which is wrong). The word ‘grain’ has a very specific meaning: so it is important that I should help you to use this term correctly from now on.

First of all, what grain is not is that nice, wavy (or 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. I wouldn’t mind betting that most of you have used the word ‘grain’ in that context: and I suspect that perhaps many of you still do.

But that’s not right. The correct name for this nice, decorative surface pattern on a piece of timber is the word ‘figure’ (see Figure 1.2). Figure can often (although not always) show us what the real grain is up to; but it is decidedly not the same thing as the ‘grain’ of the wood. Sometimes, mis-reading the figure and thinking it is the grain can lead to physical damage: and sometimes it can lead to unnecessary rejection of the timber, for example when undertaking strength grading (a topic that I will discuss in a later chapter).

Figure 1.2 Example of figure(pattern) on the surface of timber

So, if it is not the pattern that you can see on the surface of the wood, then what exactly is grain?

Well, in my book (literally, as well as metaphorically!) the term ‘grain’ specifically relates to the direction of the wood fibres: that is, the way they grow up and along the trunk of the tree; or the way they are aligned along the length of a board or a plank of wood (see Figure 1.3). The principal vertical (or longitudinal) cells in the tree trunk – which for now, we’ll refer to simply as ‘fibres’ – are relatively long (a few millimetres in softwoods) but they are very narrow, and they generally grow quite straight: along the main axis of the tree’s trunk or stem.

These basic wood cells grow in the form of hollow tubes: which have a relatively thin cell wall, and 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. 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 want to come back to the detail of the correct drying of timber later.)

Figure 1.3 Wood surface showing grain(wood fibres)

These tube-like ‘wood fibres’ all point more or less in the same direction (along the tree trunk). So please remember from now on, that you should (and I definitely will!) only use the word ‘grain’ to mean one thing: ‘the direction of the wood fibres’.

You should now see that, if we cut up a tree in a good and efficient way, such as in a sawmill, we will (hopefully) find that the wood fibres that were in the tree will line up...

Table of contents

• Cover
• Title page
• Copyright page
• Preface
• 1 Wood as a Material
• 2 More on Timber: Some Stuff About the UK Timber Trade
• 3 Water in Wood: Moisture Content and the Drying of Timber
• 4 Specifying Timber – for Indoor or Outdoor Uses
• 5 The Quality of Timber: Grading for Appearance
• 6 Strength Grading and Strength Classes of Timber
• 7 Wood Preservatives and Wood Finishes
• 8 Principal Softwoods Used in the UK
• 9 A Selection of Hardwoods Used in the UK
• 10 Wood-based Sheet Materials
• Appendix 1: A Glossary of Wood and Timber Terms Used in the Timber and Construction Industries
• Appendix 2: A Select Bibliography of Some Useful Technical References About Wood
• Appendix 3: Some Helpful Technical, Advisory and Trade Bodies Concerned with Timber
• Index