Different names are given to roofs according to their form.
A "Lean-to"1 roof has only one side or slope, which lies between two walls or other supports one higher than the other. See Fig. 86.
A M Roof consists of two ordinary triangular roofs side by side (Fig. 87).
A V Roof (shown in section, Fig. 88) has two slopes inclining inwards from side walls towards a gutter, which rests upon beams running along the centre of the building, and supported by the party walls PW.
A Flat-topped Roof is one in which the apex of the triangle is cut off flat, as in Fig. 96
A Curb or Mansard Roof is one in which the apex of a high triangle is cut off and replaced by a flatter summit, as in Fig. 89, and Fig. 98, p. 50.
A Conical Roof is shaped like a cone.
An Ogee Roof has sides of which the lower portions are convex outwards and the upper portion concave, thus forming curves resembling that after which they are named. These two last descriptions are seldom required, and will not be further noticed.
1 Sc. To-fall.
The Ridge is the line formed by the meeting of the slopes of a roof at the summit.
The Eaves are the lower edges of the slopes, which rest upon the walls or project over them.
A Gable is formed when the end wall of a building is carried up so as to terminate the roof, as in Fig. 87.
A Hipped Roofl is sloped back at the ends as in Fig. 90. These terminating slopes are called the "hipped ends."
Hips2 are the salient angles formed by the intersection of the sides and ends.
Valleys are the intersections similarly formed in re-entering angles (see VR VR, Fig. 91).
A Pavilion Roof is hipped uniformly at both ends, as shown in Fig 90.
If a roof terminates in gables, only ordinary principals are required in its construction, but if it is cut into by another roof or is hipped back at the ends special arrangements have to be made for the valleys and hips.
When a simple couple roof is "hipped," deep and narrow " hip rafters"3 HE, Fig. 91, are carried from the end of the ridge to the angles of the end of the building, and short rafters, called "jack rafters" jr, are cut to fit in between the hip rafters and the wall plates.
The same course is followed in the valley caused by the intersection of two roofs,"valley rafters " or valley pieces being introduced, as at VR VR in Fig. 91.
In framed roofs the jack rafters fit in between the hip rafters and the wall plates, or between the valley pieces and the ridges.
In the former figures, HE HE are the hip rafters, VR VR the valley rafters, DD the dragon pieces in the angles (see p. 48), TT the trimmers carrying the rafters round openings left in the roof for chimneys, skylights, etc.
1 Sc. Piend roof. 2 Sc. Piends. 3 Sc. Piend rafters.
Fig. 92 shows in section the collar-tie roof trimmed to clear the chimney, and in elevation the gable end G, and the end hipped back at F.
Fig. 91. Scale, 1/10 inch=l foot. Plan.
Fig. 92. Sectional Elevation on A B.
In a larger roof, such for instance as requires king-post trusses with purlins, as in Fig. 93, the length of the purlin, PP, on the hipped end would be too great to be left without support; in such a case a half king-post truss may be introduced at KT.
In larger roofs flat-topped trusses must be introduced at intervals in the hipped ends to carry the rafters.
In a construction such as that described above, the hip rafter being very long and heavy requires to be well supported at its lower end, or it would thrust out the corner of the building; moreover the angle requires to be tied together.
These objects are fulfilled by the arrangements shown in Figs. 94, 95. The foot of the hip rafter is tenoned into a mortise in the dragon beam1 D, one end of which is notched into the wall plate wp, while the other is furnished with a strong tusk tenon which passes through a hole in the angle brace A.
After the hip rafter is fixed it is tightened up by driving a pin into the hole h.
Trimming.2 - Wherever rafters come across any obstacle, such as a chimney, they must be trimmed in the same way as described in Part I. for floor joists. Thus in Fig. 91 the rafters tr tr would be made thicker than the others, and a trimmer, T, framed in between them. The rafters are similarly trimmed in order to leave openings for skylights, etc., as shown at T Fig. 186, p. 97.
1 Or dragging-tie. s Sc. Bridling.
The roof in Fig. 61, p. 28, is trimmed to clear the chimney; the trimmer is shown in section at T.
The trimmers are often placed vertically, and sometimes supported in the centre by corbels protruding from the chimney.
Fig. 96 shows a method of forming a very nearly flat top to a queen-post roof.
Fig. 96. Scale, ¼ inch to 1 foot.
The straining beam, SB, is made slightly thicker in the centre, so as to raise the joists, jjj, supporting the lead fiat, sufficiently to throw off the wet. The rolls for the lead are not shown (see Part I.)
Sometimes rafters at a very flat slope are introduced above an ordinary straining beam to carry the joists.
As a considerable weight comes upon the straining beam, it receives additional support from two struts branching inwards from the feet of the queen posts, and kept asunder by a small straining piece. Fig. 96 shows also binders framed in between the tie beams, as described at p. 42.
Mansard or Curb Roofs were originally introduced in the days of very steep roofs, with a view to diminish their excessive height by cutting of the apex and substituting for it a summit of flatter slope.
This form of roof is condemned by Tredgold as being ungraceful in form, causing loss of room as compared with the original roof of high pitch, and further on account of the difficulty of freeing the gutters from snow. It is, moreover, a dangerous structure on account of its inflammability.
It is, however, much in use at the present time, as it affords an economical attic story, and is considered by many to be more picturesque than the flatter roofs, while it is certainly much cheaper and less exposed than those of high pitch.
There are several ways of describing the outline of a Mansard roof.1 Fig. 97 shows Belidor's method,2 which is the one most usually adopted. It consists in dividing the semicircle, described on the span 0 5, into five equal parts at the points 1, 2, 3, 4, 5 ; the highest point r is then marked, lines joining 0 1 and 4 5 form the sides of the true roof, while 1 r and r 4 give those of the false roof or summit. Fig. 98 shows an ordinary form of Mansard roof.
Those trusses that come immediately over a partition may be much strengthened if connected with it by prolonging the king post downwards so as to form the centre post of the partition (see Part I.)
Those for king-post roofs adapted for spans of from 20 to 30 feet are given in Part I.
1 Theoretically the beams should be in equilibrium, and support each other without fastenings : to do this they should be arranged in the form they would assume if loosely connected at the ends, and then inverted and allowed to hang in a catenary curve.
2 From Newlands' Carpenter's and Joiner's Assistant.