The annexed is the simplest form of a roof, and will help to explain the subject of carpentry in other respects. The beam A, called the tie-beam, is of such a length as to rest on the side walls of the house at each of its ends, and is supposed to be of such dimensions in depth and thickness as would render it inadequate to support much more than its own weight. The two sloping rafters B B, are called principals; they are mortised into the tie-beam at their ends by a joint, shown in the lower figure, by which they are provided with a firm abutment, to prevent the ends from slipping outwards; and in order to prevent the principal from starting upwards out of the mortise, it is strapped down to the tie-beam by an iron strap, bolted or screwed to both timbers.
P is termed a king-post, and is cut out with a head and foot, the former to receive the upper ends of the principals, which, being cut square, abut firmly against the sloping face of the head. The sloping principals hold up the king-post, and the tie-beam is supported from the latter by a stirrup-shaped strap, that goes under the beam, and is bolted, or screwed, to the post on each side. To prevent the principals from bending by the strain, or by the weight of the roof covering, the struts C C, are placed, abutting against the bevelled part of the foot of the king-post, and are strapped to the principals, or mortised into them.
The number of tie-beams, with their trusses, &c., of course depends on the length of the roof, or the material with which it is to be covered. A longitudinal scantling, or thin beam, called a purline, E, is laid lengthwise, resting on the principals over the ends of the struts, and is secured to the former by a spike, or else by being notched down on to the principal. These purlines support the common rafters R, which abut at their feet against a longitudinal scantling S, lying on, and halved down on, the tie-beams; at their upper ends, the rafters R rest against a ridge-piece, or thin plank, let edgeways into the head of the king-post. The rafters are placed about a foot apart, and on to them are nailed the laths or battens to carry the tiles or slates.
The Mortise and other Joints.
In constructing roofs, floors, and other structures of timber, the various beams are framed, or fastened together, by certain processes calculated to insure strength and permanence in the framing, which ought to be understood, and their names remembered.
The Mortise and Tenon joint is used when one beam is to be attached to, and supported by, another, without resting on it, but so that the beams may be in the same plane. The mortise is a hole cut into, or through, the side of the one beam, into which hole the end of the other, cut down to fit the form of the hole, is inserted and fastened. It is obviously necessary to consider two things in determining the size and form of the mortise and tenon. First, that by the former the one beam may not be too much weakened, and yet that it should be large enough to give the tenon that fits into it, sufficient strength to enable the beam to carry the weight intended.
If the one beam is horizontal, and the other to stand perpendicularly upon it, the tenon need only be large enough to retain the upright beam in its place. The foregoing figures are the most usual forms of mortises and tenons, and will explain their use and principle.
It is obvious that two mortises never should come opposite each other on the two sides of the same beam.