A trussed roof employs the following timbers, - tie-beams, principal rafters, collar-beams, struts, purlines, and common rafters.

Tie-beams are the large and long timbers which lie in a horizontal position, and extend across the building at the base of the roof. They are usually subjected to two kinds of strain. One is that which is exerted by the principal rafters: the other is the cross-strain, and may be produced by the weight of the ceiling below, or a load upon the beams themselves. In mortising tie-beams, as little wood should be removed as the nature of the case will allow. Tenons may be small, their use being simply to retain each piece in its proper place. If the figuring laid down in this work is followed, the tie-beams of each design will be of sufficient size to resist the strains exerted by the inclined parts, and the rods will resist the cross-strain.

The weakest part of a tie-beam, and hence the one demanding most attention, is at its ends about the foot of the rafters. To strengthen this part, it is the usual practice to bolt pieces of strong white oak or Carolina pine to the under side of the beam. The pieces should be as thick as half the depth of the beam, and of sufficient length to extend from the end thereof to three feet beyond the heel of the rafters. (See S, Fig. 2, Plate VI.) Objections to the use of strengthening-pieces have been made, because they present a joint or seam where dampness may gather, and produce decay in the wood; also because they are in effect a camber to the beam, exerting a thrust on the walls of the building. To a certain extent, these objections are valid; but neither is of sufficient moment to outweigh the benefit produced. The objection first named may be entirely obviated by thoroughly painting the pieces when the work is put together. The practice of cambering a tie-beam, by tightening the rods till the beam is curved upwards, cannot be considered advisable; for, if sufficient camber is produced to give the beam additional strength by its partaking of the nature of an arch, this is more than counteracted by injury to the walls of the building. A large ceiling, if entirely level, presents an optical delusion, leading the beholder to believe that the surface has a sag, or downward curve. In furring such ceilings, a rise of an inch in twenty feet will obviate the difficulty. While the carpenter is cautious in cambering beams for either of the purposes named, or any of like nature, he should remember that there will be a settlement from the shrinkage of the timbers, till each part has found a solid bearing. Hence the rods should be kept tightened; and, when the work is completed, the centre of the beam should be slightly curved upwards, that the tendency named may be counteracted.

Principal Rafters are the large inclined timbers which support the purlins: they should be of the same thickness as the tie-beam, and about four-fifths as deep. To a beam seven by ten inches, the rafter would be seven by eight inches. In some examples of framing, as that shown on Plate XI., Fig. 1, one rafter is placed above another; in which case, both should be of the same size, having pieces of oak-board, an inch and five-eighths thick and four inches wide, let into each rafter five-eighths of an inch, leaving the rafters three-eighths of an inch apart for the passage of air between them: the pieces should be perfectly dry, and tightly driven into the grooves. The timbers should be bolted together with bolts five-eighths of an inch in diameter.

Collar-beams are the horizontal timbers which lie between the heads of principal rafters. They are also known as straining-beams. As their use is to prevent the rafters approaching each other, their dimensions may be the same as the timbers named. In designs where these beams are liable to sag, they should be supported with struts, as seen at A, Plate VIII. The case not unfrequently occurs where collar-beams are serviceable as tie-beams, and thereby strengthen the principal tie-beam: an example of this kind may be seen at B and C, Plate XI. In cases of this kind, separate rods will be required. The top-truss, being needed as a truss, will require rods of its own to make it complete in itself; the main-beam being suspended by other rods.

Struts are the inclined pieces which support the principal rafters. The ends of struts should always be framed with a shoulder an inch and a half wide, and sloping from this to the end of the piece. It may be remarked here, that the ends of all braces (whatever their position) should be formed with a shoulder of like nature, proportional to the size of the piece. Struts, being always in a state of compression, need not be pinned to the beam or piece they support, a short tenon being all that is required to keep the parts in their proper place. The width of struts should be the thickness of the principal rafter; and they should be about half as thick as the rafter is deep. The carpenter should make it an invariable rule to place the curved or cambered side of a timber upwards, whenever such cambered side exists.

Purlins are the horizontal timbers extending from truss to truss to support the common rafters. They should always be framed or bridged over the principal rafters, by notching into the back of them and breast of the purlins, each half an inch, making an inch when the work has been put together. Their size is determined by their length of bearing and distance apart. "When the trusses are within ten feet from centres, and the purlins less than eight feet apart on the principal rafters, they may be to them in thickness and depth, respectively, as five to eight. They should not be cut into lengths which will reach only over single spaces, but continued whole; and, when they are put on, they should be made to break joints, by the use of short lengths at the end of every other one. It is to be remembered, that the joints should always be made over the principal rafter. In cases where the roof is large, and exposed to the direct action of heavy storms, the purlins should be braced, like the posts and girts of a side-wall.

Common Rafters are the outside timbers of a roof, and are used simply to support the boarding. Being uniformly loaded, only light pieces are required; but they should always be jointed over the purlins, and never placed more than eighteen or twenty inches apart from centres. If the bearing is not more than eight feet, they may be two by six inches; but, where it is more, their depth should be proportionally increased. They should be notched into a half-inch, to keep them from sliding off the purlin; but the purlin itself should remain entire.