forces may be composed, and the single force, acting in a certain direction, be calculated.

Leaving the subject of the composition and resolution of forces, after a statement of the

principle, we may proceed to explain the construction and arrangement of those parts of a building which be

long to the carpenter. And, first of all, we may speak of roofs.

The Construction of Roofs.

The simplest method of constructing a roof is to place horizontal timbers from wall to wall, but this method is only suited to very short bearings, and does not readily throw off the water which may fall upon its covering. The Egyptians constructed flat roofs. To prevent this inconvenience, a roof may be made as an inclined plane; and such a construction has advantages, though its want of uniformity and beauty, and also its want of strength, proportioned to the amount of timber employed, are objections to its use; but still it is stronger than the flat roof, and readily carries off the water that may fall upon it. The best form for a roof is that in which there are two sides, equally inclined to the horizon, and resting in a line called the ridge of the roof. The angle which the inclined side forms with the horizon is called the pitch. In countries where there is a cold climate, and snow is apt to fall in large quantities, the roof is high; in warm countries the roof is low. In Gothic architecture the roof is generally high pitched, and it is so consonant with the style that it often forms a prominent feature in these buildings. There are not so many advantages in high pitched roofs as most persons suppose, and there are many disadvantages. The additional force of the wind upon a high roof is a serious objection, and when parapets are employed it is so far from preventing the effects of a heavy fall of rain or snow that the gutters are so filled that the pipes cannot carry off the water fast enough, or, being stopped by the dirt carried down by the velocity of the water, an overflow is occasioned. The height of roofs is now generally between one third and one sixth of the span.

It is the carpenter's business to frame the timbers of roofs, and sometimes he is required to design them, and he should therefore know how to obtain the strength and other qualities required, with the smallest possible amount of timber.

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.

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The timbers which compose a roof are known by different names, according to the uses for which they are employed, and the situations 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

ID

A

B

Fig. 3.

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 rafters 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 heads of the

that which connects the posts, is called a straining-beam; and

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 proportioned. 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.

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

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