Church Roofs Longitudinal Trusses 300178

Fig. 174.

The number of panels in the longitudinal trusses should be arranged so that the secondary trusses will come over the upper end of a strut. The centre panels of the Howe trusses should also be counter braced, to provide for any inequality in the loads. The posts which support the main trusses should extend to the basement floor, resting upon iron plates set on piers of masonry. If the basement has a concrete floor the iron plate: should be an inch or more above the concrete, so that the ends of the posts will not rot. The posts should also be braced in at least two directions. Occasionally the walls of the church and tower may be utilized for supporting the trusses, but in a long building it is generally more economical to use posts, as was done in the building described, as in that way the span of the trusses can be materially shortened. From the posts to the end walls the roof and ceiling may be supported by beams, braced from the post and wall, or by short trusses.

In the construction illustrated, the longitudinal trusses were studded up on the inside to receive the lathing for the plaster frieze and the ends of the cove furring. Only one piece of this studding is shown. The lower lengths of rafters were also braced from the bottom chords of the longitudinal trusses by 1x6 boards, which are not shown in the drawing.

Fig. 174 shows the application of this system of trussing to a smaller church, where the transverse trusses are not required. A building 42 feet wide and 64 feet long can be roofed in this way with only two trusses.

When the ceiling beams are raised above the lower chord, as in Fig. 174, they should be supported on "ledger boards" or planks, bolted to the struts and to occasional uprights, so that they will have no tendency to push out the bottom of the trusses.

In one church designed by the author, but which, being built at a distance, he did not superintend, the contractor supported the centre ceiling beams by simply spiking them to the tops of the curved ribs, and without extending them to the trusses, as indicated by the drawing. The result was that the weight of the ceiling coming on the curved ribs caused them to spread the trusses and thus to push out the side walls. The nuts on the vertical rods in the Howe trusses should be at the top, where they can be got at, or better still, turn buckles may be placed near the centre of the rods, so that if the trusses "sag" from shrinkage, they may be raised by shortening the rods.

The application of longitudinal Howe trusses to the support of roofs with vaulted ceilings is described in Chapter V (Building Stones).