This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
In discussing roofs, attention will here be confined to the timber part of the structure, leaving the covering to be dealt with under the section on Roofing; and the descriptions will stop short at those kinds of roof where architectural and engineering skill and appliances are called into requisition. Roofs of an cvery-day character may be divided into 2 classes - " lean-to " roofs including those which have only one slope, a gradual fall from one side to the other; while "span" roofs have 2 slopes descending from an apex at or near the centre.
The simplest kind of lean-to roof, adapted only for covering a shed of short span, and with a very light roofing material, is shown in Fig. G57. Here the back wall a has the upper ends of the rafters b simply built into it, at distances of 14-18 in. apart centre to centre, while the lower ends rest upon the front wall e and overhang it sufficiently to cast the rain-water off free of the wall. In Fig. 658, the top and bottom ends of the rafters a rest upon wall-plates b let into the walls, and running their whole length, while the extreme lower end of the rafters carry a guttering c for conveying away the rain-water. Other forms of guttering for the ends of rafters are shown in Figs. 659, 660. In Fig. 659, the rafter a resting on the wall b, has a triangular block of wood c nailed to it outside the line of the wall, affording support to a zinc or iron gutter e, having one edge lying under the roofing material. At any point in the length of the gutter a hole is made for the insertion of a vertical pipe d for conveying the water away down the outside of the wall. In Fig. 660, the rafter a is recessed at b for the reception of the gutter, a pipe c from which passes down the front of the wall d.
Fig. 661 illustrates a wooden gutter a attached by nails to the ends of the rafters b, and provided with a pipe c, bent underneath so that it may run down close to the wall d.
When a wider span is needed in a lean-to roof, a tie-beam has to be introduced, to counteract the outward thrust of the roof which would tend to force the walls asunder. Fig. 662 shows the arrangement adopted. The rafter a rests at its upper end on the wall-plate b and at its lower end on the tie-beam c, which in its turn is supported in a horizontal position on the wall-plates d, e in the back and front walls f, g. As the front wall g is carried up above the bottom edge of the roof, forming a parapet surmounted by a coping, instead of lying underneath it as before, another form of gutter is demanded. This as seen at h, consists of sheet metal running up underneath the roofing material far enough to form a trough. Another contrivance for guttering along a parapet wall is shown in Fig. 663, and is termed a "bridged" gutter. The rafters a, butting against the wall-plate b carried by the wall c, support a "bridging-piece" d of small scantling, on which lies a board flooring e bearing the sheet metal (zinc or lead) gutter f.
When the roof is required to possess greater strength than can be obtained with the use of a simple tie-beam, the construction assumes a more complicated character, as seen in Fig. 664. Here the tie-beam a rests as before on the wall-plates b, c, but at the back end it supports a king-post d, from which the strut e passes to sustain the " principal " rafter /, whose upper end butts against a fillet on the king-post d while its lower end is borne by the tie-beam a. Running parallel with the walls, and carried by the "principal" rafters f, is the "purlin" g, whose duty is to hold up the "common" rafters h on which the roofing material is laid. The common rafters lie at intervals of 14 in. centre to centre, while the principal rafters are generally about 10 ft. apart. The upper end of the strut e (Fig. 664) is joined to the under side of the principal rafter / by a tenon, which may be either simple (a) or angular (b), Fig. 665. In Fig. 666 is seen a way of joining the purlin to the rafters : the purlin a is led into grooves in the faces of the common and principal rafters b, c respectively and butts against the block d wedged into the upper face of the principal rafter c.
The feet of the struts may either butt against the sloping shoulders of the king-post as at a (Fig. 667) or be tenoned in as at b.
Ordinary span roofs with various modifications are illustrated in Fig. 668. In A which is the simplest form, the rafters a rest at foot on the wall-plates h, to which they are only nailed, while at their upper ends they either butt against each other as at c, or are crossed and nailed as at d. Obviously this is a very slender structure, and quite unfitted to bear any considerable weight of roofing material. B, C represent progressive steps in strengthening this form of roof, by the introduction of one or more " collar beams," which prevent the collapse of the sloping rafters, and give their name to this modification of the span roof. In B, the rafters a, measuring usually about 6 1/2 in. by 1 1/4 in. and carrying a covering of 1-in. boarding b, butt against the ridge-pole c at top, and are cut out for the reception of the wall-plates d at bottom. At rather more than 1/3 of the height from the wall-plate to the ridgepole, the rafters are tied by the collar-beams e, having the same dimensions as the rafters, and which may be simply nailed to them at the ends, or halved in, as here shown. C differs from B only in having a second collar-beam f, and the extra support of a purlin g let into the rafters and the lower collar-beam e.
D is a modification of B, necessitated by the introduction of a ventilator in the roof: a is the collar-beam supporting the purlin b and rafters c as well as the uprights d of the ventilator. In E a new feature occurs in the shape of a " strut" or "brace" supported by a "tie-beam." Here the tie-beam a resting on the wall-plates b carries at its ends " pole-plates " c let in, and which in their turn bear the lower ends of the rafters d, butting at the apex against the ridge-pole e. To reduce the strain in the middle of the rafters, the struts / are employed, receiving their support from the ends of the straining sill g against which they abut.
When a strong roof of say 20 ft. span is required, the truss principle is fully carried out, as in the " king-post" roof, Fig. 669. Here tie-beams a measuring 9 in. by 4 in. are placed at intervals of about 10 ft. resting on the walls and wall-plates b. From the centre of each tie-beam rises the king-post c, measuring 5 in. by 3 in.; abutting against its lower shoulders on each side are struts d, 3 1/2 in. by 2 1/2 in., reaching to the middle of the principal rafters e, 6 in. by 3 in., whose feet rest on the tie-beam a, while their heads fit under the upper shoulders of the king-post c. Just outside the line of the wall-plate and that of the feet of the principal rafters, the tie-beams a have pole-plates f, 4 in. sq., let into their upper faces; and running midway along the principal rafters, just over the point where they are sustained by the struts d, are purlins g, 8 in. by 3 in. The pole-plates f, purlins g, and ridge-pole h (8 in. by 1 1/2 in.), between them carry the common rafters i, 3 1/2 in. by 2 in.
The king-post roof, from the manner of arranging the timbers, precludes any use being made of the roof space. When this space is a desideratum, the queen-post roof is better suited, two examples of which are seen in Fig. 670, the form A being adapted to a high-pitched roof, while B is accommodated to a low pitch. In A, the tie-buam a, 9 in. by 4 in., resting on the wall-plates b, 5 in. by 3 in., carries a straining sill e, 4 in. sq., separating the feet of the queen-posts d, 4 1/2 in. by 4 in., which stand on the tie-beam a, and support at top the straining beam e, 7 in. by 4 in., and the upper ends of the principal rafters f, 5 1/2 in. by 4 in. Struts g, 4 in. by 3 in., run from the lower shoulders of the queen-posts to the middle of the principal rafters, whose lower ends rest on the tie-beam a. The common rafters h, 4 in. by 2 in., abut against the ridge-pole i, at top, and are borne by the tops of the queen-posts d, the purlin h, 7 1/2 in. by 4 in., and the pole-plate I, 4 in. sq. These measurements are suited to a roof of 30 ft. span. The space m is available for a room. In the form B, the timbers are differently arranged to suit the low roof.
The tie-beam a is 11 in. by 6 in., the queen-posts &, c are 6 in. sq. and 6 in. by 4 in. respectively, the straining beam d is 8 in. by 6 in., the principal rafter e is 6 in. sq., the struts f, g are 6 in. by 4 in., and the common rafters h are 4 1/2 in. by 2 in.
Fig. 671 illustrates two ways of constructing a "curb" or "mansard" roof, which enables a capacious and well-lit apartment to be formed in the roof. In A, the tie-beam a measures 12 in. by 4 in., the struts b are 6 in. by 4 in., the upper tie-beam c is 8 in. by 4 in., the king-post d is 4 in. sq., the principal rafters e are 6 in. by 4 in., the purlin f is 5 in. by 4 in., and the common rafters g are 4 in. by 2 in. The apartment may bo lit by the window h or by a "dormer" window. The arrangement B is suited to a roof of wider span, and is strengthened by the stout king-post a and by 2 struts b from the upper king-post c.
The manner of joining the upper ends of the struts to the upper shoulders of the king-post is shown in Fig. 672: the ridge-pole drops into the recess a in the top of the king-post b, and the struts c are let into the shoulders of b either by a simple tenon d or an angular tenon e.
The form of gutter for the bottom between 2 span roofs is shown in Fig. 673. The tie-beam a carries a strip of quartering b, against which abut the lower ends of the rafters c; a bridging-piece d supports the floor of the gutter e.