ROOF is the name given to the covering of a house or building of any kind, of whatever materials the roof is composed; but in carpentry, it is limited to the timber work or framing by which the external cover ing of the roof is supported.
In our article on the theory of CARPENTRY, we have already treated, in a popular manner, of the resistance of beams to cross strains, whether the straining forces act perpendicularly, or obliquely to the length of the beams, and whether the beams arc supported at one or both ends; and in our article on practical carpentry, is given a great deal of practical information on the subject of roofing in general; on circular, elliptical, and polygonal roofs; on the covering of circular roofs; on trusses or framings of carpentry; with the dimen sions and representations of some of the most celebra ted roofs that have been constructed.
In the present brief article, we purpose to treat very generally of the equilibrium of a framing of timber or an assemblage of beams; and to add a notice of some roofs and framings which have not been given in the preceding part of the work.
When the distance between two walls, or the width of a river is too great to be crossed by a single beam, the desired effect may be produced by an assemblage of two or more pieces of wood or beams.
The simplest form of a roof is when two beams are used, as in PLATE CCCCLXXXI. Fig. I, where A C is the distance to be crossed; and AB, BC, the two beams employed for this purpose, either to carry a roof or the roadway of a bridge. The first question for our consideration here is, the pitch or declivity of the roof or the angle ABC. We have already shown under CARPENTRY, that two beams, AB, AD will bear the same load, being both equal to a horizontal beam AE. If a beam AD, therefore, is just able to carry the roofing which it is to bear, the more ed beam AB, having to support a greater quantity of roofing from its greater length, will require to be stronger than All. Hence steeper roofs must alWays require stronger beams, or the same beams to be placed closer to each other in the proportion of their greater length; or the depth of the beam AB must be to that of AD as the square root of AB is to the square root of AD.
In this construction of a roof, it is obvious that the walls or abutments at A and C are supposed capable of sustaining the joists at A and C, pressing them outwards; but as the ordinary walls of houses are quite unfit to resist any such force, it becomes neces sary to resist it by another of sufficient power. This effect is obtained by introducing the beam AC, which is called a tie, from its binding together the feet A and C, of the two inclined beams AB, CB. The whole weight of the roof therefore, in place of pushing out the walls, is exerted in drawing out or stretching the tie-beam AC in the direction of its length. If we consider this tie-beam merely as a part of the roof, its place might be supplied by a chain, or wire, or rope of equal strength; but as it is often used to carry the weight of the ceiling of the room below it, and some times to carry a flooring, it is generally made strong and connected with the feet A and C of the rafter by a mortice and tenon.
When the tie-heam AC is long, it has a tendency to bend down or sway at its middle E. It therefore re quires to be sustained at that part, and this is effected by suspending it by a short beam BE, from the ridge B. This beam BE, is called the king post, and per forms the part of a string or chain. The various me thods of joining the king posts or rafters, icc. has al ready been explained under CARPENTRY, and in Plate CXXVIII. Fig. 4, kc.
When the rafters AB, BC are long, or considerably loaded, they also have a tendency to bend. In order to prevent this, braces or struts EF, EG are morticed into them at C and F, and also into joggles at the foot of the king post. By this means, the rafters have their relative strength quadrupled, in consequence of being reduced to half their original length.