The "couple-close roof" is a step higher than the last, and consists of rafters, ridge, and wall-plates, with the addition of a tie fixed to the foot of each rafter, which often does duty for a ceiling joist (see fig. 346).
These roofs are suitable for spans up to 12 or 14 feet; but after they exceed the 10 feet it is necessary that the rafters should be made 4 1/2 inches x 2 1/4 inches. The tie is usually the same size as the rafter, and the joint between them called a dove-tailed halving, as fig. 347, half the thickness being cut off both rafter and tie to a dove-tail pattern, which, when in position, prevents one pulling away from the other.
It must be always borne in mind that rafters must have a support at intervals of every 8 feet, or they will sag; and if the space between ridge and plate exceeds 8 feet, additional support, in the form of a purlin or collar, must be placed between them (vide fig. 348).
This leads us to the "collar" roof, suitable for spans up to 16 or 17 feet, and designed to meet the requirements just alluded to. It has also the advantage of giving more height to the ceilings of the rooms, etc., immediately under the roof, where a tie, as in the "couple-close" roof, would be too low and in the way.
1 Scale Fig 3481.
The joints in this form of roof are precisely similar to those last named; but the roof itself, as a whole, is considerably inferior in construction, because, there being no tie at the foot of the rafters, the weight of the roof is apt to cause them to spread, pushing the walls out, and lowering parts of the roof in consequence. The rafters and collars are 4 1/2 inches by 2 1/2 inches in section, and spaced 12 inches apart, or 14 to 15 inches centre to centre, as in all other framings; but these collars are not always used to each rafter, one being framed to every sixth or seventh rafter, and a purlin - a longitudinal beam running parallel between plate.
Wood fish plates and ridge - being placed between collar and rafter (as fig. 348 a), and supporting the rafters between each of its bearings on the framed collars. The purlins and ridge are jointed in their length by bevelling, fished oints, or scarfs to resist tension and bearing, the particular strain to which they are subject.
The "bevelled" joint (fig. 348 b) is the simplest in work and most common in use, though for obvious reasons it can only be employed over bearings on walls or trusses.
Fished joints are not often used, on account of their awkwardness, and their weakness in comparison with the expense which they entail. They are, practically speaking, "butt" joints, whereof the ends are kept in position by fish-plates of wood or iron, fastened generally to each side of the beams, and connected together by through bolts (fig. 349).
The bolts should be placed "hit-and-miss" fashion, so that plates and beams are only cut once in the length of their fibres. This form of joint, when in tension, causes all the strain to be thrown on the bolts; and as these often become loosened by the shrinkage of the timbers, they then press and crush the fibres, which results in the yielding of the joints.
To obviate this as much as possible, the joints are tabled in both iron and wood alike, as fig. 350; or keyed, as fig. 351. Each of these methods, while conferring a benefit, also creates another defect, by decreasing the section and consequent strength of the timbers themselves.
These forms of joints are also suitable for lengthening posts and other upright timbers subject to compression; but, for such purposes, it is better that they should be fished on all sides, to prevent any movement laterally.
"Scarfs," as used in joints, to lengthen purlins and ridges - i.e., to resist tension - are, as a rule, made as fig. 352; or fig. 353, which is slightly more intricate; though fig. 354 is still better, as it can be tightened up by means of the wedges or keys, X.
Still, this is not really complete without the fish-plates, to allow the tightening to be done properly, as fig. 355, which may be said to be the best scarf to resist tension.
While dealing with "scarfs" - though a little out of place - it may be as well to explain, so that the difference can be noted, those that are designed to resist compression used in perpendiculars, posts, etc, and that used for bearing timbers subject to cross strain, where the top fibres are in compression and the bottom in tension, as fig. 356. The principle of scarfs for tension is a keyed bevel, while that for compression is a keyed butt joint, as square as possible, so as to distribute the weight all over its area.
Fig. 357 represents the simplest form of scarf used in posts, though this may be improved by checking, as fig. 358; and still more by wedging or keying, as fig. 359; and, further still, by fish-plates and a little tabling, whether in iron or wood, as fig. 360.
The joint to resist cross strain, applicable to breastsummers, girders, binders, joists, and other bearing timbers, is as fig. 361; the square butt joint, X, resisting the compression in the top fibres, while the oblique butt and the fish-plate and bolts resist the tension on the bottom fibres.
Before leaving the subject of joints in longitudinal and other bearing timbers we must not forget the lapped joint, though it is the least workmanlike of joints, and rarely used except in rough work. It consists in lapping one length of timber over the other, as fig. 362, and securing them together either by means of bolts, A, or straps, B.
Wall plates are scarfed, as in fig. 363, though they may be bolted together in addition.