A piece of timber, in whatever way it may be placed, except when vertical, will bend or sag, that is to say, its upper side will form itself into a concave surface. The more horizontal the timber is placed the more it will always sag, and as the distance between the points on which it rests is increased, so it has greater liabilities of bending. To prevent this effect as much as possible, arrangements must be made for the support of the beam in some intermediate points. Now, it may be supported from either above or below. If there should be any walls between those on which the ends of the timber rest, these will be sufficient for all the purposes required; if not, the same result must be produced by a system of framing.

The timbers which compose a roof are known by different names, according to the uses for which they are employed, and the situa tions in which they are placed. The principal timbers of a roof are the following, but they are not all used in every roof: the tiebeams, wall-plates, collar-beams, king-posts, queen-posts, struts, principal rafters, common rafters, ridge-piece, collar-beams, purlins, and pole-plates.

The TIE-BEAM (A), fig. 3, is a horizontal piece of timber, which

extends from wall to wall, and rests upon the WALL-PLATES (B) at each end. It is employed for the purpose of connecting the feet of the principal rafters (C), which would otherwise have a tendency to push out the walls by their own weight, and the weight of the materials placed upon them. In roofs of large span, it is necessary that the tie-beam should be well supported in some point or points, between the ends on which it is supported, for if this be not done it will sag and draw either one or both of the principal raf ters towards its centre, and thus destroy the stability of the framing. The KING-POST (D) is sometimes used for this purpose. It

is a piece of timber placed in a vertical position, connecting the point where the two principal rafters meet, and the centre of the tie-beam.

When the king-post is not thought to be sufficient to support the pressure which may be on the framing, QUEEN-POSTS (B), fig. 4, may be used, which are pieces of timber placed in an upright position,

supporting severally the two rafters, and equidistant from the centre of the truss. The horizontal piece of timber (C) which connects the heads of the queen-posts, is called a straining-beam; and that which connects their base, so as to prevent the struts from pushing them nearer to each other, is called a straining cill. Those pieces which are placed in pairs, to assist in supporting the principal rafters, are called struts; they are frequently used to unite the rafters and the base of the king-post. Any horizontal timber above the tie-beam is called a collar-beam. The ridge-piece (H) is that piece of timber which forms the apex of the roof, and is supported by the heads of the principal rafters or the king-posts, and in its turn supports one end of the common rafters. A poleplate is a beam over the walls, supported by the principal rafters or the tie-beam, and is intended to carry the lower ends of the common rafters. Purlins (E) are horizontal timbers, between the pole-plates and ridge-piece. The small spars (cc), which are parallel to the principal rafters, and are supported by the ridgeplate, purlins, and pole-plates, are called common rafters.

The Dimensions of Timbers used in a Roof.

However accurately a roof may be designed, it is unfit for its purpose if the dimensions of the parts be not accurately propor tioned. To accomplish this, some experience is required, and a

knowledge of the strength of timbers, under particular circum

stances.

There are two things to be secured-a sufficient strength to support the weights to be carried without sagging, and to do that without burdening the walls or other parts of the building over which the roof is thrown. This is not always an easy task, for roofs are sometimes to be made in such forms as prevent the adoption of those means which would otherwise immediately accomplish the object. Sometimes a very large roof must be made flat, at other times a lantern-light must be provided in its centre; and, in a third case, it may be necessary to erect a dome. In designing for these and other roofs, attention should be paid to the character and success of similar works already executed, and the artist should study the points of similarity and difference between these and his own work, so as to provide against dangers, which may peculiarly affect his building.

Examples of Roofs.

Fig. 5 is a roof, the rafters of which are only sup ported by a collar-beam (C), which acts in part as a tie; but this arrangement is so feeble, that it should never be used over a space where the span is more than fifteen feet.

Fig. 5.

In fig. 6 there is the addition of a tie-beam (A), and the strain is here thrown from the collar to the tie-beam; the former being compressed, the latter in a state of tension. As there is no arrange

A

Fig. 6.

ment in this truss to support the tie-beam, and to prevent it from sagging, it is unfit for a span of more than twenty-five feet.

To prevent the inconveniences resulting from the sagging of the tie-beam, a king-post (P) and struts (SS) may be introduced, as

S

Fig. 7.

shown in fig. 7. This form of roof is very well adapted for a span of twenty-five feet.

For a span of thirty to five-and-forty feet, the truss represented

Fig. 8.

in fig. 8 is very well suited, and is now very commonly adopted by architects and builders.

Floors.

The timbers which support the flooring boards, and the ceiling of a room beneath, are called, in carpentry, the naked flooring. There are three kinds of naked flooring-single, double, and framed.

Single flooring is that in which there is but one series of joists, as shown in fig. 9, where A A A are joists, and B the flooring-boards. To make a single floor as strong as possible, the joists should be thin but deep, sufficient thickness being always allowed for the nailing of the flooring boards. Two inches by six is the smallest

A

B

B

Fig. 9.

dimension for joists; for a length of twenty feet they should be about three inches thick, and twelve inches deep.

Sometimes the joists cannot have in a particular place a bearing upon the walls, and then a piece of timber is framed between the nearest joists. This is done where flues, fire-places, and stairs interfere. The timber thus used is called a trimmer, and the two joists on which it is supported are called trimming-joists, and should be made a little stronger than the common joists. Thus, in fig. 10,

A

B

B

Fig. 10.

AA are common joists, BB trimming joists, and C a trimmer. When the bearing is more than seven or eight feet, the joists should be strutted; that is to say, short pieces of board should be fitted between the joists, so as to form a continued line from wall to wall. These struts greatly strengthen the floor, and prevent the joists from sinking; but it is not desirable to mortice them into the joists, as that process has the effect of weakening the joists themselves.