This section is from the book "Modern Buildings, Their Planning, Construction And Equipment Vol1", by G. A. T. Middleton. Also available from Amazon: Modern Buildings.
The surfaces of a roof may be inclined at practically any angle, varying from the almost horizontal to the almost vertical. The material with which the roof is covered generally decides this angle, which is known as its pitch.
The following table shows the materials in general use for roof coverings, and the minimum inclinations as given by Hurst: -
Kind of Covering.
Inclination to the Horizon.
Ratio of Rise to Span.
Weight of Material per Foot super.
Asphalted felt .......
Slates, Large ......
,, Ordinary ....
Thin slabs of stone ............
Plain tiles ...........
Thatch of straw .......
A steeper pitch is required for some materials than others, because they are more liable to strip off under a high wind, and rain is more readily driven in between the joints. In exposed situations the pitch of roofs covered with slate or tile should be steeper than that indicated in the above table. When a roof is steeply pitched, rooms are usually constructed within it, and are lit by means of dormer windows.
Like floors, roofs may be divided into three kinds, according to the arrangement of their timbering, as follows: -
1. Single-Rafter Roofs.
2. Double-Rafter Roofs
3. Triple-Rafter Roofs.
1. Single-Rafter Roofs are such that the roof covering is supported upon a single system of rafters not greater than 15 inches apart. It is only used when the span is small - not greater than 20 feet. Fig. 280 shows several examples of single-rafter roofs.
Lean-to Roofs are formed in a single slope, as shown at A, the upper end of the rafters being spiked to a wall-plate supported on a corbel, and the lower end bird's-mouthed to a wall-plate on a lower wall. This roof should not be used for a span greater than 10 feet, unless the rafters are strutted at their centres from the wall. When a wall occurs conveniently at about the middle of a space to be roofed over, two lean-to roofs may be used, sloping down toward the centre, where a lead gutter is formed. A double lean-to roof is sometimes called a V-roof, on account of the shape of its section.
Couple or Span Roofs are formed as shown at B, the upper ends of the rafters being abutted against and spiked to a Ridge board, while the lower ends are bird's-mouthed over and spiked to a wall-plate. This form of roof should not be used for spans greater than 10 feet, unless the walls be very firm, as an outward thrust is exerted by the feet of the rafters.
Couple-Close Roofs are similar to the above, but have the feet of the rafters tied together by means of Tie beams spiked to the rafters, as shown at C., The soundest roof is procured by tying the feet of every pair of rafters, and indeed this is necessary when a ceiling is to be attached to the ties; but when the roof is open a tie is rarely used more frequently than one for every third or fourth pair of rafters. This roof may be employed for spans up to 20 feet. At C, Fig. 280, is shown a roof over a span of 8 feet, but if larger roofs are to be constructed in this way the ridge boards and rafters should be made 1/2 inch deeper and the ties 1/4 inch deeper for every additional foot of span, the width remaining the same in all cases.
When the span exceeds 12 feet it is more economical to suspend the ties by means of bolts spaced at intervals of about 6 feet. The ties between the bolts are housed into and spiked to a horizontal timber which is suspended by the bolts, as shown at D. When suspension bolts are used the depth of the ties may be half that found by the above rule.
Collar-Beam Roofs are formed like couple roofs with a beam spiked to the middle of the rafters, as shown at E, where the span is assumed to be 8 feet; but when larger roofs are required the scantlings may be determined by the same rule as was given for couple-close roofs. This type of roof is employed when a greater amount of head room is required than is given by a couple-close roof, but it is not a sound roof, as it always exerts a thrust upon the walls. The collar, being used to prevent the rafters from sagging, is in a state of compression, and does not tie the rafters together as it is generally supposed to do.
2. Double Rafts or Purlin Roofs are composed of two series of timbers, as shown in Fig. 281, in which it will be seen that the roofs are composed of common rafters supported by means of purlins, for which reason this kind of roof is often called a purlin roof.
These roofs can be used for any span whatever when the gable walls are close together, or when the rafters can be strutted from a floor or central wall.
In Fig. 281 the outline of the roof is that known as a "Mansard," the upper portion being practically a couple-close roof, the rafters resting upon stout outer purlins which are tied together by the ceiling joists, and which are securely spiked between them and the intermediate purlins; or if greater security is required, the purlins may be tied together by means of one or two 3/4-inch bolts.
The lower rafters are practically independent of the upper portion of the roof, being merely bearers for the roof boarding of the steeper part, and are secured by spiking them, at the upper end to the purlin, and at the lower end to the wall-plate. The feet of these lower rafters do not need tying, as their inclination to the vertical is so small.
Fig. 282 shows the method of constructing an octagonal trussed roof, and the method of building it is as follows: A wall-plate is laid upon the wall, to which it is anchored by means of iron straps, to prevent it from slipping off when the inclined thrust of the rafters comes upon it. The plates are halved and bolted at the angles, so that they mutually tie one another in, and the bolts for securing the feet of the rafters are inserted before the pieces of wall-plate are laid. The lower ends of two angular rafters are then slipped over the projecting ends of these bolts, and securely nailed to the sixteen-sided block at the apex, which is temporally held in position. The two angular rafters, at right angles to the first two, are then slipped over the bolts at their lower ends and securely nailed to the apex block, and the bolts at the feet of the four rafters are tightened up. The structure thus far completed will be self-supporting. The four remaining angular rafters and the eight rafters resting upon the middles of the sides of the octagon are next fixed.