CHAPTER VI.

ROOFS.

688. A Roof, in common language, is the covering over a structure, the chief object of which is to protect the building against the effects of snow and rain. It is composed of boards, shingles, slate, mastic, or other suitable materials.

Fig. 207.

The inclined pieces AC, and BC, Fig. 207, which support the roof are called rafters. When the roof is light, the roof boards DE are placed directly upon the rafters, but when the rafters are far apart say more than four feet, small pieces a, b, c, and d, called purlins,* are placed across the rafters for the purpose of receiving the roof proper. AB is a tie, and F and G represent the ends of posts. The frame ABC is called a roof truss.

689. Roof Trusses have a great variety of forms, and differ greatly in the details of their construction. All the trusses which have been discussed in the preceding pages are suitable for this purpose in many cases. Some other forms are given in the following pages.

690. General Data. roof truss is required to carry its own weight, the weight of the purlins, the weight of the

* Purlin beams are sometimes placed under the rafters.

roof above them, the force of the wind, the weight of snow when there is any, and in some cases certain local or concentrated loads, such as floors, machinery, and the like, which are suspended from the roof trusses.

691. The Weight of Snow. Freshly fallen snow weighs from five to twelve lbs. per cubic foot, although snow which is saturated with water weighs much more. Some say that snow is equivalent to from to of its depth in water, while others say that it may be equivalent to its depth of

water.

European engineers consider that six lbs. per square foot is sufficient for snow, and eight lbs. for the pressure of the wind, making fourteen lbs. for both. Trautwine thinks that not less than twenty lbs. should be allowed in the United States.

A

692. The Force of the Wind. According According to Mr. Smeaton, the pressure of the wind directly against a flat surface in a hurricane may be 32 lbs. per square foot. Tredgold recommends an allowance of 40 lbs. per square foot. gauge in Girard College broke under a strain of 42 lbs. per square foot, whilst a tornado was passing near by. During the severest gale on record at Liverpool, England, there was a pressure of 42 lbs. per square foot directly upon a flat surface. During a very violent gale in Scotland, a wind-gauge once indicated 45 lbs. per square foot. Buildings which are more or less protected will not be subjected to such high

pressures.

f

a

Fig. 208-Represents a roof truss for medium spans.

a, tie-beam of truss.

b, d, principal rafters framed into tie-beam and the king post c, and confined at their foot by an iron strap.

d, d, struts.

é, e, purlins supporting the common rafters,..

693. The truss of a roof, for ordinary bearings, consists (Fig. 208) of a horizontal beam termed the tie-beam, with which the inclined beams, termed the principal rafters, are connected by suitable joints. The principal rafters may

either abut against each other at the top or ridge, or against a king post. Inclined struts are in some cases placed between the principal rafters and king post, with which they are connected by suitable joints.

For wider bearings the short rafters (Fig. 209) abut against a straining beam at the top. Queen posts connect these pieces with the tie-beam. A king post connects the straining beam with the top of the short rafters; and struts are placed at suitable points between the rafters and king and queen posts.

Fig. 209-Represents a roof truss for wide

spans.

a, tie-beam.

b, b, principal rafters.

c, short rafters abutting against the strain

ing beam d.

e and, king and queen pcsts in pairs.

9, 9, purlins supporting common rafters h.

In each of these combinations the weight of the roof covering and the frames is supported by the points of support. The principal rafters are subjected to cross and longitudinal strains, arising from the weight of the roof covering and from their reciprocal action on each other. These strains are transmitted to the tie-beam, causing a strain of tension upon it. The struts resist the cross strain upon the rafters and prevent them from sagging; and the king and queen posts prevent the tie and straining beams from sagging and give points of support to the struts. The short rafters and straining beam form points of support which resist the cross strain on the principal rafters, and support the strain on the queen posts.

When a

694. Ties and Braces for Detached Frames. series of frames concur to one end, as, for example, the main beams of a bridge, the trusses of a roof, ribs of a centre, etc., they require to be tied together and stiffened by other beams to prevent any displacement and warping of the frames. For this purpose beams are placed in a horizontal position and notched upon each frame at suitable points to connect the whole together; while others are placed crossing each other, in a diagonal direction, between each pair of frames, with which they are united by suitable joints, to stiffen the frames and prevent them from yielding to any lateral effort. Both the ties and the diagonal braces may be either of single beams, or of beams in pairs, so arranged as to embrace between them the part of the frames with which they are connected.

695. Iron Roof Trusses. Frames of iron for roofs have been made either entirely of wrought iron, or of a combination of wrought and cast iron, or of these two last materials combined with timber. The combinations for the trusses of roofs of iron are in all respects the same as in those for timber trusses. The parts of the truss subjected to a cross strain, or to one of compression, are arranged to give the most suit

a, a, feathered struts of cast-iron.

b, b, suspension bars in pairs.

m, n, tie and straining bars.

e, e, and f, f, cross sections of beams resting in the cast-iron sockets connected with the sus pension bars.

able forms for strength, and to adapt them to the object in view. The parts subjected to a strain of extension, as the tie-beam and king and queen posts, are made either of wrought iron or timber, as may be found best adapted to the particular end proposed.

The joints are in some cases arranged by inserting the ends

of the beams, or bars, in cast-iron sockets, or shoes of a suitable form; in others the beams are united by joints arranged like those for timber frames, the joints in all cases being secured by wrought-iron bolts and keys. (Figs. 210. 211 and 212.)

Fig. 211-Represents the half of a truss of wrought iron for the new Houses of Parliament, England. The pieces of this truss are formed of bars of a rectangular section. The joints are secured by cast-iron sockets, within which the ends of the bars are secured by screw bolts.

696. Fig. 213 shows a very common form of the roofs of gas-houses.

This here shown is supposed to be made entirely of iron At the ridge is a ventilator to allow the escape of gases. The manner of joining the parts is sufficiently shown in the figure.