How to build a house

by Eugène-Emmanuel Viollet-le-Duc

“Now that you know what is the simplest roof-truss, let us go up into the roofs.”

These roofs were old, and had been repaired and strengthened many times, and formed a complication of timbers difficult enough to understand.

“Formerly,” said Eugène, “that is, more than a century ago, they used to make roofs such as you see here: every rafter was framed, that is, each of the rafters composed a truss, except the tie-beam, which was introduced only at intervals. Then wood was in plenty, and they scarcely thought of economising it. At present it is less abundant, and there is a difficulty in procuring a considerable number of pieces of large dimensions. The noble forests that covered the soil of France have been foolishly wasted, and long timbers of heart of oak are rare. It has therefore been necessary to economise them. The expedient has been adopted of placing strong trusses at a distance of about 12 feet from each other. On these trusses have been placed purlins, which are the horizontal pieces you see on this side; and on these purlins longer or shorter rafters have been placed to receive the lathing for the tiles, or the battens for the slates. But all timber roofing should be fixed upon sleepers, which are those horizontal pieces resting on the top of the walls, which bind and isolate the tie-beams from the masonry; for it is to be observed that timber is preserved for an indefinite time in the free dry air, but soon decays in contact with a moist body, such as stone is. Look here at this piece of wood, almost buried in the masonry; it is nearly reduced to touchwood, while the blade above, which is in the free dry air, is as free from rot as if it were new.

“Formerly upper floors were made by putting joists resting on beams and the walls. These joists and beams remained visible, as you may see still in the kitchen and the large hall on the ground floor, which serves as a store-room. The air therefore could circulate round these timbers, and they might last for centuries. But it was considered that thus exposed they were not pleasant to look at—that they were not clean, and allowed spiders to spin their webs in the interspaces. Laths were therefore nailed under these joists, and this lathwork plastered so as to form what we call a ceiling. Timbers thus inclosed and deprived of air, ‘heated’ (as carpenters call it), that is, they fermented and soon began to decay. In fact, floorings with exposed joists which had resisted the action of time for centuries decayed and broke down in a short time after being inclosed. I may add that formerly, before using timber in building, they took the precaution of leaving it exposed for some years to the action of the sun and rain. They even kept it for some time in water, to free it from the sap (for sap is the ferment which makes wood rot). When the timber, after having been barked and roughly squared, had remained in the open air for five or six years, it was used. But now-a-days we are in a hurry, and make use of timber that has not been cut more than a year. It is not dry, it retains its sap, and if it is then enclosed it ferments rapidly, so that in a few years the largest beams are completely rotten. Prudent architects therefore hesitate to use wood for floors. Yet its use—even if only partially dried—would not entail serious inconvenience if it was not covered with plaster. The worst that could happen would be the occurrence of cracks and shrinkings. It would dry when in use, as it would have dried in the open air.

“There is no great disadvantage, then, in employing wood newly cut for roof-timbers, which are generally left exposed. They dry where they are. They warp, but do not perish of dry-rot.

“As we shall not be able to find wood absolutely dry for your sister’s house, we shall leave the floor-joists visible, and endeavour by simple and economical means to render them not unsightly.

“But you ought to be well acquainted with the qualities of timber. I will not tell you that nature has caused these large vegetable growths which we employ to grow for our pleasure or use. Nature is, I think, very little concerned as to whether the oak or the fir would serve any of our purposes; and if human intelligence has been able to take advantage of these materials that spring up before us, it is after having recognized and verified their properties by experience. Unfortunately, it would seem as if the results of this experience did not tend to increase; and judging from the way in which building-timber is most frequently employed, we might be led to suppose that we are less informed than were our predecessors, or that we have lost that habit of observation with which they were familiar.

“Wood, being composed of fibres more or less lax or compact, possesses a considerable power of resistance to a pressure exerted along these fibres, but is easily bent or crushed under a pressure exerted across these same fibres. Thus a log of wood 4 inches in diameter and a yard or so long, placed on end, will support, without being crushed or contorted, a pressure of 40,000 lbs.; whereas this weight will break or crush it if placed horizontally, as you would crush a reed under your foot. Take a thoroughly sound bit of straw, 4 inches long, and place your finger on one end of it, holding the straw vertically on a table; you will have to press pretty strongly on it to bend it, while the least pressure on the same straw placed horizontally will flatten it. The straw is a tube. A tree consists of a series of tubes, some enveloping others. The more numerous, close and fine these tubes are, the more does the trunk resist pressure, either in the direction of its length or its thickness. But this shows us that to enable the wood to retain its power of resistance we must employ it as nature gives it to us; and in fact this was done formerly. Each piece of timber was cut from a tree of larger or smaller size, as the case required, but they did not split the tree lengthwise to get several pieces of timber; for the heart being harder and more compact than the sap-wood (which is the spongy envelope beneath the bark), and the concentric layers of wood being the closer and tougher in proportion to their nearness to the bark, if you split a tree in two lengthwise one of its faces is much more resisting than the other, the equilibrium is disturbed, and flexure is easily produced under a weight. The outer layers, being the more recent, are more spongy and lax in texture than the older layers that are near the heart; consequently the process of drying makes these outer layers shrink more than the inner: hence curvature. Let A (Fig. [14]) be a split piece of wood; the layers B are harder and more compact than those marked C, which contain more moisture and whose fibres are softer. In drying, therefore, this piece of wood will present a hollow bend on the outer side, as I show at D. If the wood is left entire, as at E, the effects of drying will neutralize each other, and the piece will remain straight.

Fig. 14.