The head of the king post sometimes becomes so compressed by the rafters that the fibres are crushed, and the post sinks, allowing the tie beam to sag. On this account oak king posts have been used, as they are less compressible, and it is a strong reason in favour of king bolts.
It is therefore necessary that the head of the king post and of the principal rafters should be fastened together with straps as shown. Occasionally provision is made for tightening the joint by means of gibs and cotters similar to those for the foot of the king post described at p. 163.
The joints between the ends of the principal rafters and the head of the king post should be left a little open at b b, Fig. 323, so as to allow the ends of the rafters to bear inwards when the roof settles, without crushing the angles at b b.
The foot of the king post is kept in position by a stub tenon, and secured to the tie beam either by a strap with adjusting wedges, as above described; or by a common stirrup; or by a bolt passing through the tie beam and secured into a nut fixed within the king post; or by a very bad arrangement consisting of a dovetailed tenon penetrating the whole depth of the tie beam, and secured by a wedge as described at p. 70.
When the truss is first put together, the foot of the king post should be kept well clear of the tie beam, so that in case of a general depression of the roof it may not bear upon it. The tie beam is raised by the adjustment of the cottered joint after the roof has taken its bearing.
Suspending Pieces, as described at p. 7 5, may sometimes be used instead of king posts.
The Struts 1 prevent the principal rafters from sagging in the middle. In order to guard against any cross strain whatever coming upon the rafters, the heads of the struts should be almost immediately under the purlins1 (see Fig. 325), but this cannot always be arranged without inclining the struts at too flat an angle. The more upright they are, the better they are placed for bearing the strain upon them.
1 Also called Braces.
The heads of the struts are tenoned into the principal rafters, as shown in Fig. 325, and their feet into the foot of the king-post.
The struts, being under compression,2 should be made of full length and of very dry stuff, for unless well seasoned they will shrink (even in length) and allow the rafters to bend.
The Principal Rafters are tenoned at the upper end into the head of the king post (Fig. 323), and at the lower end into the tie beam, as shown in Fig. 325 and at p. 71, the joints being secured by straps or bolts, as described at pages 74 and 79.
The heads are sometimes secured in cast-iron sockets, and the feet in shoes (see p. 82).
The principal rafters carry the purlins, which are notched to fit them, the back of the rafter being sometimes cogged to receive the notch.
Each principal rafter is supported near its centre, close below the purlin, by a strut tenoned into it, as shown in Fig. 325.
This halves the bearing, and therefore greatly increases the strength and stiffness of the rafter.
A shrinkage in the depth of the rafters will cause them to sag, or to separate from the struts. If they shrink in length, the king post will subside between them, and will tend to bear upon the tie beam instead of holding it up.
The Purlins 3 are beams running longitudinally from Principal to Principal as supports for the common rafters.
They are sometimes framed in between the principal rafters, but this weakens the latter and is a bad construction.
The purlins are generally notched where they rest upon the principal rafters, so as to keep the latter rigidly apart; but if a cog is formed upon the back of the rafter it should be very wide, so as to leave the latter as nearly intact as possible.
As an additional precaution, the purlins are supported by blocks of wood (cl, Fig. 325) called "cleats," which may be housed into the backs of the rafters, as shown in Fig. 3 2 5, or merely spiked to them, as in Fig. 329.
1 The exact position of the head of the strut should be a little lower than the centre of the purlin, so as to meet the resultant of the forces caused by the wind-pressure and the weight of the roof.
2 See Part IV. 3 In some parts of England called side timbers or side wavers.
The best position for purlins is immediately over the head of the strut, as before mentioned (see p. 165 and Fig. 325), in order that they may cause no cross strain on the principal rafters; but they are generally placed (see Fig. 3 21) so as to support the common rafter at equal intervals, without reference to their exact position as regards the struts, the heads of which are placed as nearly under them as possible.
Two purlins are frequently used on each slope of the common king-post roof (see Fig. 329), and are necessary when the common rafters are so long as to require support at more than one intermediate point; in such a case, however, a queen-post truss should be used.
Purlins should be used in as great lengths as possible,1 but when the roof is a long one they necessarily require to be "scarfed" or "fished." It is better to connect the lengths of the purlin by butt joints fished on each side than by scarfing them. The latter, however, is the most usual practice. Each purlin should, however, in any case extend over at least two "bays," 2 and the scarf should in every case be immediately over a principal truss or partition wall.
In some cases the Principals are placed at considerable distances apart, and the purlins trussed to span the intervals.
The Ridge Piece is a board from 1 to 2 1/2 inches thick (E, Fig. 323) let into the head of the king post and running throughout the length of the roof; against it the common rafters abut.
If the ridge is to be covered with lead, it is surmounted by a long cylindrical piece of wood called the Ridge roll3 (see Figs. 323, 479); but with slate or earthenware ridging the roll is not necessary, and the ridge piece is simply blocked off to fit the covering intended (see Fig 455).
The Common Rafters4 are bevelled at the upper end to abut against the ridge piece, and are nailed to it. They may be notched out to fit the head of the king post, as in Fig. 322, or they may pass above it, as in Fig. 323. In the centre they are notched to fit the purlin, and at the lower end nailed and generally notched upon the pole plate.
1 The reasons for this will be explained in Part IV.
2 A "bay" is the interval between two Principals.
3 Sc. sometimes called Ridge pole. 4 Sc. Spars.
They should, of course, always be in one piece, and are generally made about 2 inches broad, and placed about 12 inches apart.
When the common rafters are broken through by a chimney or other obstacle upon which they cannot rest, they should be trimmed round it in the same way as floor joists are trimmed round a fireplace (see p. 122) or other opening. The section, Fig. 469, shows an example of this.
The trimmers (tt, Fig. 469) are sometimes placed vertically, but the best and strongest method is to fix them at right angles to the rafters, as shown.
The rafters are generally notched on to the trimmers instead of being tenoned into them, and the trimmer is often supported by a corbel protruding from the chimney.
The common rafters are sometimes placed as purlins horizontally, parallel to the ridge, in which case heavy timbers are avoided, and the Principals are more rigidly connected.
The Roof Boarding2 is nailed upon the common rafters to receive the slates or other covering.
It is generally placed horizontally, running parallel to the ridge; but, in some cases, is made to lie diagonally across the rafters.
When the rafters are placed horizontally, the boarding, of course, runs down the slope of the roof; thus its ends instead of its sides are presented to the descending wet, and it is more likely to be preserved from decay.
Battens are frequently used in common roofs to carry the slates or tiles. If so, they are nailed at right angles to the common rafters, parallel to one another, and at the "gauge" or distance apart required for the covering to be used (see p. 208, and Fig 454).
The Pole Plates3 generally rest on the ends of the tie beams, being as a rule notched and spiked to them, and run parallel to the length of the roof. They receive the ends of the common rafters, which are generally birdsmouthed, to fit upon the plate.
1 The eaves of a roof are the lower edges of the side slopes. 2 Sc. Sarking. 3 Sc. Poll plate.
The pole plate is usually placed immediately over the wall plate, so that the weight upon it is transmitted directly through the latter to the wall (see Figs. 321, 322), but in some cases the position has to be altered. For instance, in Fig. 325, where the joint between the rafter and tie beam rests upon the wall, the pole plate, in order to be immediately over the wall plate, would necessarily be placed so as to cut into and weaken the principal rafter as shown in dotted lines.
This position for the pole plate, though objectionable for the reason stated, is sometimes advisable - for example, in the case where room is required to form a gutter between it and the blocking course (see Fig. 476), or in the valley between two roofs where a wide gutter is required (Fig. 473).
Generally speaking, the position and size of the pole plates depend upon the form of the roof, method of fixing the gutters, etc., and are very varied.