When an octagonal roof has a raised ceiling, and is surmounted by a lantern, which must be entirely open and unobstructed, as is often the case with church roofs, the problem of supporting the roof, especially if wooden construction must be used, becomes more difficult.
The simplest method of supporting such a roof is that shown by Tig. 191, in which the roof is supported by eight hip rafters, abutting at the top against a heavy curb or plate, which also supports the lantern. The outward thrust of the hip rafters is resisted by the wall plate, which must be made a complete octagon, and connected at the angles so that the octagonal ring cannot be broken.
As long as the plate and connections hold intact the rafters cannot spread.
The hips may be made of a single timber or they may be trussed, in the manner indicated at A, A. To prevent distortion of the frame from unequal distribution of snow or from wind, diagonal ties should be introduced in each panel, as shown at B, B, although if the roof were boarded diagonally these ties might be omitted. The curb plate should also be made as rigid as possible by proper connection at the angles. The best construction for the plate would undoubtedly be a heavy steel angle bar with strong riveted joints.
It would be better on such a roof either to run the common rafters horizontally from hip to hip, or else to have a number of purlins, so that the common rafters cannot push outward on the plate between the angles.
The principal objection to the adoption of this mode of construction, especially on churches, is that it is not always practicable to make the plate continuous around the roof on account of gables, chimneys, etc. With gables, however, the difficulty can be overcome by using a flat steel plate, which can be built in the brickwork, and, in fact, a steel plate is much to be preferred to a wooden plate, even on top of the wall, for the reason that the angle connections can be made more perfectly with a steel plate than with a wooden one, and it should be borne in mind that with this construction, if a single joint should fail, the stability of the whole roof would be endangered. With this construction the ceiling would naturally be hung from the roof framing, and its shape may be varied almost in-definitely.
Another method of supporting an octagonal roof with a raised ceiling and an open lantern is shown by Fig. 193. The building for which this construction is drawn is shown in elevation by Fig. 192 and in plan by Fig. 194. A section through the roof and ceiling, taken on a centre line, is shown at the right, Fig. 195. With a roof of this pitch it is possible to place two scissors trusses across the building as shown by the dotted lines on the plan and by the drawing, Fig. 193, which can be kept within the space bounded by the roof and ceiling, and from these trusses the roof and ceiling may be supported,leaving the space under the lantern entirely clear. The lantern is supported by two secondary trusses, B,and two still smaller trusses, C. A portion of the rafters are supported by purlins, while others are braced or trussed, as shown in the section, Fig.
The position of the hip rafters is shown by the dotted lines, Fig. 193.
The section at the left, Fig. 195, is taken on the line of truss A, so as to show the exact size of the space in which the truss may be built. The purlins would have to be raised in the centre to receive the rafters of the front panel.
With this construction there would be no thrust on the walls from the trusses, and very little, if any, from the common rafters, so that it would not be necessary to carry the plate through the gables, although it would be a very wise safeguard to connect the ends of the wooden plates each side of the gables by an iron bar, say £x4 inches laid in the gable wall. When using scissors trusses, particularly when the angle between the Messrs. Link & Rosenheim, St. Louis, Architects;
Fig. 196. - An Iron Synagogue Roof and Dome, St. Louis, Mo.
Mr. Julius Baier, St. Louis, Mo.. Engineer rafters and tie is very small, it is always well to use whatever assistance may be obtained from the walls or other sources to prevent spreading.
Whether the construction shown in Fig. 191 or that shown in Fig. 193 will be the best to use on a given building will depend upon the particular conditions, and also upon whether the construction is to be of wood or steel. If the church shown was to have a steel roof, the writer would be inclined to use the construction indicated by Fig. 191, or else that described in Sections 83 and 84, while for wooden construction he would use that shown by Fig. 193.