474. Building timber into new walls is often a cause of decay, as the lime and damp brickwork are active agents in producing putrefaction, particularly where the scrapings of roads are used instead of sand for mortar. Hence it is that bond-timbers, wall-plates, and the ends of girders, joists, and lintels, are so frequently found in a state of decay. The old builders used to bed the ends of their girders and joists in loam, instead of mortar, as directed in the act of parliament for rebuilding the city of London.§ In this place it may not be amiss to point out the dangerous consequences of building walls so that their principal support depends on timber. The usual method of putting bond-timber in walls is to lay it next the inside; this bond often decays, and of course leaves the wall resting only upon the external course or courses of bricks; and fractures, bulges, or absolute failures are the natural consequences. This evil is in some degree avoided by placing the bond in the middle of the wall so that there is brickwork on each side, and avoiding the use of continued bond for nailing battens or other fixtures to. ||

But if the powerful lateral pressure of flat arches were avoided, so many ties or bond-timbers would not be necessary. The improper use of arches produces more fractures in buildings than any other cause. Nothing can be more rare; it was found necessary to underpin, at immense cost, three of the large houses in Grosvenor Place, London, which had settled from this cause. In one of the houses the floors were not less than three indies out of level. The wood platform, which was of yellow fir, seven inches thick, was found completely rotten. A similar accident happened at Norfolk House, St. James's Square, where oak planking had been used.

* Bowden'a ' Treatise on Dry Rot,' p. 70.

† Chapman on ' Preservation of Timber,' p. 14.

‡Ibid., p. 73. § 19 Car. II., cap. 3.

|| In bad foundations it is sometimes the practice to build on a platform of timber; but unless this platform be continually wet, the timber is certain to decay, which will allow the walls to settle, and probably rend the building to pieces. Instances of this kind are by no means absurd than the construction of the fronts of London houses; they exhibit a continued series of stretchers and ties. Each range of arches is a line of stretchers, and the bond and wall-plates are the ties; and as the arches are close-jointed and well fitted on the outside, but open-jointed and indifferently built in the inside, when the building settles it has a strong tendency to bulge outwards. Thus timber ties are necessary to secure an ill-constructed wall, which consequently cannot be more durable than the timber.

As an example of the danger of trusting to timber in supporting heavy stone or brick work, the failure of the curb of the brick dome of the Church of St. Mark, at Venice, may be cited. This dome was built upon a curb of larch timber, put together in thicknesses, with the joints crossed, and was intended to resist the tendency which a dome has to spread outwards at the base. In 1729 a large crack and several smaller ones were observed in the dome. On examination the wooden curb was found to be in a completely rotten state, and it was necessary to raise a scaffold from the bottom to secure the dome from ruin. After it was secured from falling, the wooden curb was removed, and a course of stone, with a strong band of iron, was put in its place.*

The bad effects resulting from damp walls is still further increased by hasty finishing. To enclose with plastering and joiners' work the walls and timbers while they are in a damp state, is the most certain means of causing the building to fall into a premature state of decay.

* Rondelet, *L'Art de Batir,' tome iv., p. 256; or ' Encyclopedie Methodlque,' Dict. Architecture, art. Coupole.

475. There is another cause that affects all wood most materially, which is the application of paint, tar, or pitch, before the wood has been thoroughly dried. The nature of these bodies prevents all evaporation, and confines the internal moisture, which is the cause of sudden decay. Mr. Bramley remarks, that both oak and fir posts were brought into a premature state of decay by their having been painted prior to a due evaporation of their moisture,* and painting affords no protection to timber against dry rot.

On the other hand, the doors, pews, and carved work of many old churches have never been painted, and yet they are often found to be perfectly sound, after having existed above a century.† In Chester, Exeter, and other old cities, where much timber was formerly used, even for the external parts of buildings, it appears to be sound and perfect, though black with age, and has never been painted.

Semple mentions an instance of some field-gates made of home fir, part of which, being near the mansion, were painted, while the rest, being in distant parts of the grounds, were not painted. Those which were painted soon became quite rotten, but others that were not painted continued firm. ‡

Painted floor-cloths are very injurious to wooden floors, as they soon cause decay by preventing the access of atmospheric air and by retaining whatever dampness the boards may absorb. Carpets are not quite so injurious, but still they assist in retarding free evaporation.