The Couple Close Roof Fig. 2, has the feet of the rafters tied together by nailing the ceiling joists to them, which strengthens the roof considerably, thus enabling it to be used for wider spans than the previous form.
The Collar Beam. Roof, Fig. 3. shows the fie or collar placed higher up in the roof, to increase the space in the room below. When this type of roof is used it is necessary to have heavier common rafters than in the preceding types.
The Collar Bolt and Tie Roof (illustrated in the same figure) is a form used in wider spans, where the ceiling joists would sag in the middle if unsupported; consequently a light bolt is attached to the ridge and passes through the middle of the joists. Sometimes a plate is fixed across the top of the joists and the bolts are attached to this about four feet apart.
The King Post Truss. Fig. 4, consists of a triangular frame fastened together with mortise and tenon joints, which are secured by iron straps or bolts, as shown in the details. These frames or trusses are spaced about 8 to 10 ft. apart, and carry the ridge, purlins and pole plates which support the common rafters and the covering. They also support the ceiling joists, which are either spiked to them or framed into the lower member as shown in Fig. 4.
The drawing shows two half- trusses of different pitches, the one suitable for tile covering and the other for slates (see enlarged detail, Fig. fi). All of these drawings are in elevation, and can be drawn directly, from the examples. The walls should be drawn in first at the required distance apart, then the wall plates, and the common rafters by aid of the 30° set square. Usually 2 in. of " back " is allowed in the rafter above the angle of wall plate.
Having drawn the back line, scale off the depth of rafter and draw the under side parallel. The kmg post truss will be drawn similarly, commencing with walls and tie beam. Then find the middle and draw in a centre line, about which scale off the king post. The joints should be copied from the enlarged details. One side of the drawing shows a
A Laminat id Rib Roof
Fig. . Cross Section. Fig. 2. Detail at foot of Rib. Fig. 3. Elevation of Fig. 2
parapet finish, the other an eaves finish. Braces should be pitched at same angle as the ratters.
The Laminated Rib Roof shown in Fig. i, page 57 is an adaptation of Colonel P'my's system of laminated ribs. The details are somewhat different from those first used by the inventor, a French engineer who adopted the method of forming large arched ribs by means of thin boards bent around a drum or templet, first about the year 1820. Emy used this form of roof for very large spans up to 60 ft. and 80 ft. In these large roofs he strengthened the arch by adding extra boards at the haunches—that is, to points about two thirds its height above the springings. The design shown is suitable for a roof of moderate span, the trusses being spaced about 8 ft. apart.
A detail to enlarged scale is given in Fig. 2, which will show the construction. The downward thrust of the arch is taken by corbel stones resting upon piers, and, where the walls are not of considerable thickness, piers or buttresses must be built to counteract the spread of the ribs. The arch is formed of six 1J in. boards, 6 in. wide, steamed and bent around a drum ; they are left on the same a sufficient time, to take a considerable " set " or curve. Emy fastened the lamin.'e together with wood pins, and though noils are often used, the pins would undoubtedly be better. Screws are sometimes used, and in other cases the lamina? are bolted all together. In the present case the boards are cut back at the foot of the rib and fitted into notches in the wall post.
A front view of these notches is given in Fig. 3, which is an elevation of the lower end of truss, with the rib removed.
The rib is bolted to the truss, which is of the collar beam type, at three points. The wall post is framed into the foot of the principal rafter to resist the spreading of the latter, which is also counteracted by the heavy wall plate built into the wall, and on which sits the pole plate carrying the feet of the common rafters. These are shown sailing over the eaves, but the finish of these is immaterial; they might equally well finish behind a parapet.
Was this article helpful?