The top covering to a house or other building; in which sense it comprises the timber work, slate, tile, lead, with whatever else is necessary to form and complete the whole. Roofs are of various forms. First, the pointed roof, in which the ridge, or the angle formed by two rafters at the point at top where they meet, is an acute angle. Secondly, the square roof, in which the angle at the ridge, formed as above, is a right angle. Thirdly, the flat roof, or rather pediment roof, which has the angle at the ridge more or less obtuse. There are various other forms, as the hip-roof, the valley-roof, the hopper-roof, the salt-box roof, the round roof; and when the covering of a building is flat, it is denominated a platform, technically, and not a roof. For a full and exact description of every kind of roof, we must refer the reader to Nicholson's Practical Builder, contenting ourselves by presenting to him a very elegant and economical arrangement for a pediment roof, recently designed by A. H. Houldsworth, Esq., for the presentation of a model of which to the Society of Arts, etc, that gentleman was awarded an honorary medal.

It is represented in the cut.

Roof 454

The advantages which this method affords, are, the saving of a considerable proportion of the timber usually employed, and the gaining for useful purposes the whole space that is contained within the roof. Mr. Houldsworth constructed a roof of this kind over the dwelling-house of a friend of his, and notwithstanding his walls were only six feet above his upper floor, he has obtained, in consequence, good lofty rooms, whilst the outside of his house appears very low; his barns, hay-lofts, etc, are built upon the same plan. A A represent the walls of the house, and B one of the timbers of the uppermost floor, resting on the sleepersff, which are let into the wall; over two other sleepers, laid in the top of the wall, are fitted two pieces of wood, D D. The principal rafters C C, forming each pair, are then secured at the bottom into the pieces D D, and are fastened to each other at the top by iron pins. Each pair of the principal rafters C C is supported by two arch pieces E E; these pieces are in their grain, and are formed on the plan recommended by Mr. Hookey, of the King's Yard, at Woolwich, to whom the country is so much indebted for this mode of converting the timber.

They are cut lengthways, by a saw, into three pieces, to within two feet of one end; are then placed in a steam-kiln, and boiled until they will bend freely, when they are fixed to a mould and left to cool; after which a few pins of wood are driven through them, to keep the pieces so cut from again flying open. The arch pieces will get a little out of shape when taken from the mould, but will be easily brought back, and when secured under the principal rafters, will fit the more firmly. The lower ends of these arch pieces are inserted in the beam B of the floor, and therein firmly pinned, while at the top they cross one another, and each butts against its opposite rafter. They are further secured by iron straps to the short pieces D D, on which the principal rafters rest, thus preventing the latter from sinking, and thrusting out the walls, and making the whole a stiff and complete framing, on which the longitudinal rafters and transverse pieces are fastened in the usual manner-

The roofs of barns or other buildings that have only a ground floor, may be constructed in the same way, care being always taken to bring the feet of the arch pieces so far down the wall as to give them a firm bearing.

Mr. Houldsworth, having already constructed several roofs of great widths on the plan described, expresses his entire confidence of being able to apply the same principle to a roof of any given span, for which timber of sufficient length could be procured. This elegant improvement, which does away with all those inconvenient timbers in roofs of the ordinary construction, called king-posts, queen-posts, braces, etc. etc, consequently leaves the whole space (as before observed), which is usually employed to no useful purpose, for the making of good lofty rooms, besides effecting a considerable saving on timber. Numerous examples of the modes of trussing girders for roofs are given under the article Beam.