The "king-post" is the next roof which circumstances, as it were, bring before the carpenter, and to his help into the bargain, as has been explained earlier in the course of these notes.

This form of roof is practically the beginning of all "trusses," which are complete framings in themselves, spanning from wall to wall, and doing duty for the cross walls, in that they support, in their turn, the ridge and purlins, which require a bearing every 8 or 10 feet Trusses should be no more than 8 or 9 feet apart, and have a 9-inch bearing on each wall.

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Fig. 364.

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Fig. 365.

The tie-beams of trusses are generally cogged on to wall-plates, or wood-templates about 30 inches long, built in the walls, as fig. 364; or, another method is to dowel them on to stone pads, as fig. 365.

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Fig. 366. 1" Scale.

Fig. 366 represents a king-post roof-truss. P R is the principal rafter, 5 inches deep and 4 inches thick; T the tie-beam, 9X4 inches; S the struts, 4x4 inches; and the king-post, K, is 5 1/2 x 4 inches at X; but, as it has to be cut out to give a bearing for the struts and principal rafter, it really comes out of 9 x 4 inch stuff, being 9 inches wide both at top and bottom, reduced between to 5 1/2 inches, as shown.

A very simple way to familiarity with and recollection of the safe sizes of the various members of a king-post roof is as follows: Divide the span by 5, and call the quotient inches, which is the required thickness; the struts should be always square, and middle of the kingpost a little wider than its thickness; and the setting out of the bevels will then give the actual size of the stuff from which it is to be cut Assuming 9 inches and 5 inches as the standard depths for tie-beams and principal rafters for 20 feet span respectively, add 1 inch to each for every 5 feet additional span. Should the dividing process give the answer with a half-inch, it is usual to take it off for slated roofs, which incline at an angle of from 26 to 30 degrees, or about "one quarter pitch" - that is to say, the ridge is a quarter of the span in height above the tie-beam; but for tiled roofs add 1 inch for the odd parts, as tiled roofs have more weight to carry, and they are usually inclined at an angle of 45 degrees, being what are called " half-pitched" roofs.

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Fig. 367.

Flat-pitched roofs are not so strong as those that are pitched higher. The nearer to the perpendicular that wood is fixed the stronger it is. This is shown by the fact that the horizontal thrust of a pair of rafters is proportionate to the length of the oblique line drawn, at right angles from the foot of the rafters, to the perpendicular dropped from the apex. See fig. 367, where it will be seen that the flatter the roof is the longer the oblique line.

The joints of a king-post truss, in feet, all consist of mortises and tenons entering but a short distance into the timbers; and they have all bevelled shoulders, which ought, wherever possible, to be at right angles to the incline of the roof.

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Fig 366. 1" Scale.

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1" Scale. Fig 369.

Fig. 368 is the joint between king-post and principal rafters at the apex supporting the ridge, a pair of 2 1/2 x 3/8 inch wrought-iron straps, bolted from side to side, completing the joint; or in practice a through-bolt from A to B will answer the same purpose, though not so good.

Fig. 369 explains the joint between the strut and principal rafter carrying the purlin. This requires no ironwork.

Fig. 370 is an elevation, and fig. 371 a section, of the joint between king-post, tie-beam, and struts, which should explain itself.

The truss is either tightened up to what is called a "camber" by means of the stirrup-strap, with the gibs and keys or cotters, as shown, or by a bolt which goes up through the centre of the tie-beam into the king-post, where a nut is let in through a hole to tighten it up.

The camber is the rise (about 1/2 an inch in every 10 feet) given to the centre of the tie-beam; so that when the roof settles, the tie-beam may stay at its level, instead of dropping below it (vide respectively figs. 373 and 373).

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Fig. 372.

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Fig. 373.

The joint between the tie-beam and the foot of the principal rafter is called a butting joint, being as fig. 374; which is completed either by a 2 1/2 X 3/8 inch wrought-iron heel-strap encircling the joint, as shown, or by a bolt put obliquely through the centre of the tie-beam and rafter. The mortise and tenon are similar to fig. 375. Instead of this "butting" joint, a bridle joint is sometimes used (see fig. 376), which will show that bridle joints are really the converse of the butting joint or a mortise and tenon, when applied to other framing.

They are not very often used, though they are another form of joint.

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Fig 374. 1" scale.

The bridle should never be more than one-fifth of the thickness of the timber, on account of the weakening of the cheeks, which would be very liable to be wrenched off if they were not considerably more than the bridle in thickness.

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Fig. 375.

Sometimes king-post roof-trusses are constructed with the tie-beam in two thicknesses, one notched to each side of the foot of the principal rafters and king-post, as in fig. 377.

The other joints are the same as in the ordinary truss, with the exception that the ridge is boxed lower down into the king, so that the top of the common rafters is level with the top of the principal rafters (fig. 378).

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Fig. 376.

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Elevation.

Fig. 377. 1" scale.

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Fig. 376. 1" scale.

This, of course, necessitates the purlins being framed into the principal backs, which is done by means of a tusk tenon {vide ante, fig. 281).

King-post trusses are suitable for spans up to 30 feet.