A roof is a framing which covers die top of the whole building, as a protection against the elements. In different countries, having different climates, and being subject to different kinds of weather, it is of course necessary that the roofs of buildings should be designed and constructed so as to meet those variations of climate. For instance, the northern countries of the world have steep-pitched roofs, to throw off the snow and prevent the water from sucking-up between the laps of the different materials used as a covering; while in hot countries, where there is little, and that very heavy rain, the roofs are constructed on the flat. Therefore the framing of wood-roofs must be suited to the weight requirements of the covering which it has to carry, according to the climate. Lead, zinc, copper, stone, or asphalte coverings are generally laid on flat roofs; slates on pitches of about 30 degrees from the horizon; and tiles or wood-shingles on a 45-degrees pitch.

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Fig. 333.

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Fig. 334.

Before proceeding to deal with each roof separately on its own merits it may not be out of place to give the student an idea of the origin of the different kinds.

The first and simplest form of a roof is as fig. 333, from which, supposing the rafters are, as it were, hinged at A, B, and C, it may be imagined that as soon as the weight on the rafters exceeds a certain maximum, the walls will be pushed out by the extra weight or thrust brought down on them by the rafters (see fig. 334).

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Fig. 335.

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To obviate this, the collar first, and afterwards the "tie," were added as a means of tying in the rafters: first, by the collar fixed part of the way between the wall-plate and ridge, A and B, as fig. 335; and then, as the length of the span and rafters increased, it was found necessary to tie in the feet of them, as fig. 335, by means of the tie-beam.

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The next difficulty encountered, as the spans were increased, and consequently the length of the tie-beams, was that the tie-beam was apt to sag in the middle from its own weight over such long bearings (see fig. 336);to counteract which the king-post in shorter spans, and two queens in longer spans, were put in to sling up, as it were, the tie-beam to the ridge or apex of the rafters in the one case, and to the straining beam - which did duty for two apexes - in the other, as figs. 337 and 338.

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The last defect to be remedied was the sagging of the rafters between the wall-plate and apex; and this was done by means of struts, S S, so named because they "strut" up the rafter from the king-post or queen, as the case may be (vide fig. 339).

The above description will explain the origin of the different members of the ordinary truss; and as spans and trusses increase, and their consequent defects arise, so indefinite additional ties and struts are added to the frame to counteract them.

Some kinds of roof-trusses are a combination of wood and iron, the rafters and struts generally being of wood, and the suspenders and ties either of iron or wood, according to circumstances.

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Couple Roof

The couple roof - the simplest of the whole series - consists of the following parts - viz., wall-plates, bedded on the walls, and running longitudinally; and two rafters, raised from each wall-plate to the pitch required from the wall-plate line as horizon. At the apex or meeting-place of these two rafters the ridge is fixed, running longitudinally, and generally parallel to the wall-plates or walls {vide figs. 340 and 341).

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Fig. 342.

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Fig. 343.

This form of roof is only suitable for spans under 10 or 11 feet, the ratters being spaced 13 inches apart, and usually 3 1/2 inches x 2 1/2 inches in section. The ridge is generally 7 inches x 1 1/2 inches, and the wall-plates 3 inches deep and either 4 1/2 inches or 9 inches wide, on bed, according to circumstances.

The joints required in its construction are very simple, and can all be made by the use of the saw; that at the ridge being as fig. 342, and that at the junction of the foot of the rafter and wall-plate being as fig. 343 or fig. 344; the former being used where there is an outside eaves-gutter, and the latter where there is a lead gutter between two roofs meeting at the wall-plate from opposite directions. Fig. 345 shows the mode of connecting the rafters and wall-plates for overhanging eaves.

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Fig. 344.