Shiver me timbers

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Ours is a stone culture so whereas the Canadians, Germans and Scandinavians know their wood we are only beginning to learn about the various properties of different types of timber. "Cultures that know about wood handle it with care, whereas we are inclined to treat wood as something to discard. We don't have enough knowledge to make informed decisions about how to use its," says Ciaran O'Connor, assistant principal architect at the OPW and author of Woodspec - a Guide to Designing, Detailing and Specifying Timber in Ireland, available from Coford.

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Choosing wood to use in furniture and construction isn't just a matter of picking something you like the look of, it should also involve finding out about the wood's properties: some types of wood expand and contract more than others. Beech floors move, for instance, so this needs to be factored into the design and building work. Some timbers indent more easily and some will rot faster than others if subjected to too much moisture. While there are acres of pine floors in Ireland, they are not actually the best choice for flooring as they mark easily. They may be cheap to lay, but they are not as durable so if you're thinking long term you should opt for a tougher wood such as maple or white oak. It pays to know your woods as, for example, not all oaks are the same - white oak is tougher than red oak.

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Wood is divided into softwoods and hardwoods. This is a botanical term rather than describing the properties of the wood - softwood comes from coniferous trees and hardwoods from deciduous trees. Both groups contain woods which vary in density, strength and resistance to decay and colour - but, by coincidence, softwoods are generally soft (exceptions include pitch pine, larch and yew) while hardwoods vary. Hardwoods generally have greater natural durability than softwoods but it's not always the case: certain softwoods, such as larch are reasonably durable, whereas using beech outside "would be an absolute disaster", according to Ciaran O'Connor.

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The durability ratings of wood are obtained through field trials - basically by sticking wood into the ground and seeing how long it lasts, although this can be done in less time in a laboratory by subjecting the wood to fungi and insects. "In most buildings applications, timber will never be exposed to such harsh conditions," says Professor Tom Woolley, co-author of The Green Building Handbook. So if your wood is to be structural - i.e. holding your building up as beams, joists, lintels and purlins - it needs to be very durable. If it's to be used on the exterior, as a window frame for instance, it needs to be able to cope with high moisture. Wooden window frames have got a bad name because they rot - yet the reason this happens is that the wrong wood has been used, or that it has been badly fitted and poorly maintained. Because the Irish climate isn't too easy to cope with, detailing of timber work is crucial to its survival, according to Ciaran.

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Wood has also got a bad name for cracking and twisting, but this happens when the wood has a high moisture content and hasn't been dried properly. When wood is first felled it is wet and needs to be dried before being used. To be scientific about it, wood used on an exterior should have between 18 and 20 per cent moisture and interior wood should have between 8 and 12 per cent. If it has over 22 per cent moisture it will be susceptible to rot because fungi and other organisms thrive in wet conditions. While various professionals use moisture meters we have to trust the word of suppliers - a quick test is to pick it up. "It shouldn't feel really heavy, like a lump of stone," says Ciaran, who designed the Phoenix Park Visitor Centre, a timber structure.

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In Ireland, durable hardwoods should be used on exteriors - if you are using a softwood it needs to be pressure treated (a special chemical is applied and the wood is put in a pressure chamber in which the chemical displaces the moisture - this adds about 3 per cent to the cost of the wood). "A lot of people in DIY shops will say wood is preservative treated," says Ciaran, "but it may only be treated to a minimum standard. Wood is graded from Hazard Class 1 to 5, with 1 being used inside a roof, 3 being used as external cladding on a window and 5 used on sea walls. Something may have only been treated to class 1 which would be like saying the exterior of a house has been painted when it's only had one coat - it won't last for long. We need to demand certain standards and some of the better mills are now educating their staff and producing properly graded timber. Certain systems, such as Junckers flooring (widely available) are made to high standards. Accuracy in sawing is also improving - it used to be terrible. Something that's 3mm out may not sound a lot but it does matter."

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Knowing the properties of timber prevents problems such as damage through using the wrong nails. Oak, for instance, contains tannic acid which reacts with ferrous materials, so if you use the wrong type of nail you'll end up with black spots all over the wood. "Knowing what nail to use is such a simple thing but so many people in Ireland just don't know about this so you can end up with cladding and flooring looking as if they have the measles," says Ciaran.

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Because wood is used as fuel, it's often assumed that buildings with lots of wood in them will burn more quickly but this isn't true. While the surface will char it can take a long time for the wood to burn through and any uncharred part of the wood will stay structurally strong, whereas other materials may buckle or fracture. Wood burns at a predetermined rate no matter what the ferocity of the fire (so you can even calculate how long a building will stand up for in a fire), whereas steel, for instance, will stay intact until a certain temperature and then it will suddenly buckle. In some cases, such as on certain wood panelling, the timber needs to be treated with fire retardant to meet stringent requirements.

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Certification that wood has come from a managed source is slightly akin to organic certification in that there are different schemes. One is the FSC (Forest Stewardship Council) certification, which verifies that your timber has been obtained from sustainable sources. Other environmental issues are the energy used in transport - Irish timber uses less energy to get to your site than American wood. The semi-state body Coillte has just received FSC status. It supplies saw mills around the country and also has its own mill, in Dundrum, Co Tipperary, which supplies Irish hardwood that is dried and treated to correct standards. Some American and Scandinavian suppliers don't subscribe to the FSC scheme as they see it as too stringent, but they do manage their forests in a sustainable manner. While many West African countries fell large tracts of forest, Ghana has a sustainable forest management system. Unlike oil and coal, wood is a renewable resource, so if you keep growing trees you'll always have timber.