Wood in Traditional Building 1: Oak
Wood is, along with stone and earth, one of the principal materials used in the construction of buildings, and particularly older buildings. It is important to have some understanding of the nature of wood, its uses in the older house and some sympathy for its virtues as well as its limitations.
Timber is used in a wide variety of applications, and the most important of these are the support structure for floors; the roof timbers and associated work; and the interior finishing timber. Timber is also used in the construction of interior walls and in many areas in the construction of supporting walls.
There are three timbers commonly found in older buildings in France, namely oak, poplar and pine. Other timbers are often found as parts of outhouses and sheds.
Oak (Quercus spp) is without question the most important constructional timber in most of France, apart from the mountainous areas and the south-west coastal region. It is a very dense and hard wood, and grows harder with age. It is very resistant, though there are more resistant woods. French oak is grown specifically to produce the long, straight beams of massive section which are such a prominent feature of the older French house. As I write this I am sitting under one such beam, which is over thirty feet (9m) long, and is 18 inches square!
For all of its benefits of strength, stiffness and durability, oak does have some considerable disadvantages. The first is that it tends to “shake” or to split along its length as it dries. This is less of a problem than may at first be thought, as a lengthwise shake may not seriously affect the load-bearing capacity of the beam; however, if the timber shakes at the end of the beam, it can splay enough to crack the masonry it is embedded in. However such cracks are a “one-off” and once the timber has stabilised will get no worse providing associated problems, such as rainwater ingress, are prevented.
Warping and stability
Oak tends to warp as it seasons, so you need to take great care in stacking it to dry. Partly for this reason and partly out of impatience, old-time builders often used their oak green, in the hope that fixing it into the masonry would restrict the warp. Unfortunately oak is stronger than that, and it is not uncommon to see an oak timber which has distorted or cracked a stone wall.
The heartwood of oak, that is to say the strong inner core of the timber, is very different from the sapwood, which forms the outer two inches or so of the tree, under the bark. The sapwood is the living part of the tree. It is quite soft and contains many nutrients, whereas the heartwood is the architecture of the tree, the strong, rigid core around which the tree lives. As such it has to be strong and tough, and it is effectively dead even while the tree is standing.
Oak sapwood is often included in the beams found in older houses. Because the sapwood is so rich in nutrients it is a favourite target for insect attack, and it is common to find beams where the sapwood parts have been thoroughly riddled. However it is always the case that the heartwood remains untouched, as oak heartwood is simply too hard for the insects to eat. Even Death Watch Beetle, which drills a hole you can put your little finger through into softer timber, balks at oak heartwood and leaves it alone. Provided that any infestation is not ongoing, and that the structural strength of the timber is not compromised, this sort of attack could safely be left alone.
However, in order to protect furniture or new timber, it is a good idea to treat the affected area as follows; thoroughly brush down the area, then treat it with a proprietary wood preservative as per the manufacturer’s recommendations. Once this has dried, you can treat the timber to two coats of dilute linseed oil. In fact you can dilute the first of these coats with one of the coats of preservative. However please note that linseed oil will tend to darken the timber, which you may not wish to do.
Finally, whether linseed oil has been used or not, treat the timber with a good quality beeswax polish, rubbing it in liberally so that all the wormholes are filled. There is no need to chop out the affected wood, unless it is literally crumbling. In many cases this would simply disfigure an attractive piece of timber, for no good reason at all.
Fresh water is the bane of most timbers, and oak is no different. Though the heartwood is again more resistant than the sapwood, if kept continually in damp conditions, the wood, this time including the heartwood, will rot. Clearly this may have serious implications when structural timbers are affected, and as the damage will continue to get worse until something is done to stop it, the cause of the water penetration must be investigated and corrected at once, and any damaged timber treated or replaced.
Oak is not a difficult timber to work, though working it is slow. It requires the use of very sharp tools and attention to technique. Nailing into oak heartwood is impractical unless a pilot hole for the nail is drilled first, as common nails will just bend. The ability of seasoned oak to resist four-inch nails being welted at by a large man wielding a very big hammer is genuinely awesome. Drilling into oak takes care, as twist bits will jam and burn unless they are frequently cleared.
Oak is very heavy, and there is a real risk of serious damage or injury being caused if a large timber falls. It is also quite sharp, and splinters of oak are very painful. When working with this timber, therefore, it makes sense to wear proper protective clothing including steel-toecapped boots, and leather gloves. You will need a strong support to do anything more than the most basic work on oak; not only is the timber heavy, but it is hard to work, and even with sharp tools a good bit of weight is needed to cut it. A flimsy support is likely to collapse, with the possibility of the worker being injured by the sharp tools in use! So, no short cuts.
Oak can be worked using ordinary hand tools, but because it is so hard, power tools are a big help. A power plane with a rebating attachment will prove its worth over and over, as will a good router and selection of bits. Chisels and other hand tools should be of the best quality, and should be kept very sharp. Oak has pronounced grain and you should work in sympathy with this. Well seasoned oak has a tendency to split if attacked with blunt tools, so again, attention to technique is important.
Oak does not like steel fastenings of any kind. This is because the sap of oak is quite acid, which is one of the factors which contribute to its longevity; it tends to pickle itself as it ages. However that same quality will quickly attack the metal fixings. “Green” or unseasoned oak, which is often used in large work, is particularly corrosive. The reaction between the oak and the metal affects both, by the way, so nails and screws will waste away and at the same time the wood around them will become soft or ‘nail-sick’.
Because of this oak ideally should be fixed using wooden fixings, such as mortise and tenon joints with dowels or trenails. Although sometimes the use of metal fixings is the only practical course of action to take, a little bit of thought in the design of timber work will often yield a better solution. Having said that, oak is lovely timber to work with. It has a unique scent and a tightly packed grain which produces a fine surface.
Oak is best used for big structural timbers where its strength is vital or for finishing work where you can show off your handiwork and the prettiness of the wood. There are other timbers which are more suitable for rough work.
Originally published at Rod Fleming’s World.