67. Owing to the greater cost of the trusses and finish of open-timbered roofs, and the additional cost of heating the building, most modern churches have a suspended ceiling supported from the tie-beams of the trusses, which conceals all of the constructional features.
The ceiling, however, is generally raised in the centre, and is often domed or vaulted; it may be finished with wood, metal, or plaster, the last material being the most common. Occasionally a portion of the truss is exposed beneath the ceiling, and the latter is paneled to give the appearance of an open-timbered roof, as shown in perspective in Fig. 153 and in section in Fig. 154. Such construction is usually much cheaper than true open-timbered construction, and in small churches nearly as effective, and it has the advantage that the roof can be constructed so as to have no horizontal thrust, thus permitting of lighter walls; or, if necessary, of frame walls, besides making the room casier to heat and ventilate.
68. A very economical and quite pleasing roof construction for a small chapel is shown in Fig. 155. The truss is the regular scissors truss, with only one purlin on each side. Ceiling joists extend across the roof just above the purlins, and a false beam is built on the ceiling, as shown at A. The exposed portions of the truss are cased, and the lower portion of the centre rod is boxed in to give the appearance of a king post. The under side of the rafters and ceiling joists is plastered, but it can be finished with ceiling and paneled if desired.
The truss may be built up of four thicknesses of 1-inch boards spiked together. Two boards should extend the full length of the tie-beams laping over each other, and the other two boards cut between. At the intersection of the ties and principals the boards should lap over each other alternately, and be strongly spiked together and a bolt inserted, as shown by dotted lines, to give additional security.
The ends of the purlins are supported by rafters spiked to the side of the principals, by the casings of the principal and strut and also by spikes toe-nailed into the principals. If the roof area to be supported is much greater than that shown, hangers should be used for supporting the purlins, and the size of the truss timbers and rods may need to be increased. The ceiling joists should be supported at their centre by narrow boards nailed to the joists and to the upper end of the rafters.
With trusses of this shape the building may have transepts of the same width as the nave, the roof over the crossing being supported by diagonal trusses, with a common tie-rod in the centre.
Details for the connection of diagonal trusses are described in section 77.
69. As a rule, where a church has a suspended ceiling and the width of the building is between 32 and 45 feet the shape most commonly adopted for the ceiling is that shown by Fig. 156, as such a ceiling is about the cheapest to construct, and gives the desired height and a fairly good surface for decoration. Occasionally a curved outline is given to the ceiling as shown by the dotted line
For such roofs the scissors truss is best adapted, as its outlines correspond with those of the roof section. When the span of such a roof does not exceed 35 feet, the roof and ceiting may be economically constructed by spacing the rafters from 28 inches to 32 inches on centres and trussing each pair of rafters as shown in Figs. 157 and 158. [The dimensions given in Fig. 158 are the smallest that should be used for a shingle roof with a span of 34 ft., the trusses being spaced 2 ft., C to C] By this method no purlins are required and very little ironwork, and the amount of timber in the roof will not much exceed that of an ordinary roof without trusses.
Fig. 158. - Trussed Rafters.
It also has the advantage that the weight is evenly distributed over the walls. If the ceiling is of wood it may be nailed directly to the under side of the trusses; if of plaster the laths are nailed to furring strips or strapping fastened to the under side of the tie and collar beams. The author has adopted this method of trussing in several churches with good results. It is not, however, a desirable method of construction when there are side gables or dormers and requires a rise to the roof of at least 10 1/2 ins. in 12.
In constructing trusses of the scissors type the author has found it best, under ordinary conditions, and with spans of 36 feet and under, to build the tie-beams and rafters out of planks bolted together, giving the tie-beams a section about double that determined by calculation. By this method of construction it is easier to get good connections at the intersections and not as much ironwork is required. It is often difficult, too, to get large timbers that are well seasoned, and unless they are fairly dry the joints will shrink so as to let the truss spread a little.
In one church designed by the author the trusses, which were of the shape and dimensions shown in Fig. 159, were built up of 1-inch boards spiked together, each layer being spiked separately to the next. As the church was built in a country town, the saving over large timbers and iron rods was quite considerable. The only objection to the use of thin stuff is that in seasoning in the building the outer boards are apt to curl and separate from the inner ones, and for this reason it is desirable that a few bolts be used to hold them more securely in place.
Wooden ties, especially in the centre, are also not as desirable as rods, as there is no way of tightening them; they are also dangerous in case of fire.
In building up the tie-beams pains should be taken to see that enough long pieces are used to carry the entire tensile stress.