The parts common to all ordinary roofs, such as tie beams, rafters, wall plates, purlins, ridges, gutters, etc., have already been considered in Part I., and it remains only to give a description of those peculiar to queen-post roofs.
These have, between them, to carry about 2/3 the weight of the tie beam, together with that of the ceiling, if any, suspended therefrom, and they frequently have to support additional loads brought upon the tie beam by the occupation of the space between the queen posts as a garret.
The heads of the queen posts are kept apart by a "straining beam," SB (Fig. 77), and the feet are tenoned into the tie beam and prevented from moving inwards by a "straining sill," SS.
The feet of the queen posts are sometimes secured by being housed on their inner sides into the tie beam, in which case the straining sill may be dispensed with.
The object of this beam has just been mentioned - its ends are supported by being housed and tenoned into the heads of the queen posts, additional security being generally afforded by cleats, C, nailed to the posts as shown.
In that figure it is shown as supporting a lead flat, in which case it may with advantage be made thicker in the centre than at the ends, so as to strengthen the beam, and to give the lead a slight slope outwards.
This is generally merely a piece of scantling lying on the tie beam, and butting against the feet of the queen posts (Fig. 77, p. 39).
The straining sill is sometimes bolted and keyed to the tie beam in the manner explained under Built up Beams in Part. I.
It may, however, be arranged, as shown in Fig. 82, so as to form a truss and give support to the centre part of the tie beam.
Binders, marked Bi, are shown in Fig. 96 framed in between the tie beams. This is sometimes a convenient arrangement for stiffening the roof. It may also be adopted when the. principals are widely spaced, in order to afford a shorter bearing for the ceiling joists.
When a long building is divided into rooms of moderate length by partition walls running across it, as shown in Fig. 84, the walls themselves play the part of the principals and carry the purlins, these latter supporting the rafters as usual.
If the purlins have to be of such a length that their scantling would be inconveniently large, they may with advantage be constructed in the form of trusses.
It will, however, be a question of economy whether it is more or less expensive to use trussed purlins than to introduce principals between the walls, so as to reduce the bearing of the purlins to such an extent that they may be formed with beams of ordinary scantling.
Fig. 84 shows a portion of a building of the description referred to above. The first space A, being of moderate length, is spanned by a purlin consisting of an ordinary beam op. The next room B, being much longer, is crossed by a purlin, q r, trussed with iron rods as described in Part I., while the roof on the third compartment, C, has a wooden purlin st trussed in . ordinary queen-post fashion.
The rafters resting upon the purlins are omitted on the near side of the roof to avoid confusing the figure. The purlins should rest upon stone templates built into the walls; these are not shown in Fig. 84.
In some cases trussed purlins are used in connection with ordinary trussed principals; these latter being placed at a considerable distance apart.
For example, in the skeleton diagram Fig. 85, the principals P P P, represented in thick lines, are 2 0 feet apart, and connected by trussed purlins (each dotted in a different manner), which are shown only on the near side of the roof; those on the farther side being omitted for the sake of clearness.
On these purlins may lie the common rafters, inclining downwards, parallel to the principal rafters, or intermediate principal rafters may be introduced, not forming part of a truss, but resting upon the purlins at x y z, and across the principal rafters horizontal common rafters may be placed.
Eoofs are sometimes constructed with horizontal rafters extending across the principals, at right angles to them, as in Fig. 96. These are in fact purlins, except that they support the roof covering directly, having no rafters upon them.
This is a strong and cheap arrangement, and specially convenient when the roof covering is in large pieces, such as sheets of corrugated iron, which can be laid on the rafters without boarding.
When boards are required, they of course extend lengthways down the slope of the roof, and their edges are thus not so liable to be soaked with wet, in case of a leak, as they are when laid parallel to the ridge.