62. As a rule the shape of the trusses used for church roofs is determined more by the external or internal appearance desired for the building than by economical or mechanical considerations, and hence such roofs present peculiarities of construction which place them in a distinct class.
Open timber roofs seem particularly appropriate to churches, and especially to those which are of the Gothic style of architecture. They are also frequently used over large halls and occasionally over a two-story hall in a private dwelling.
In this country wooden ceilings having the appearance of being the actual roof are frequently placed beneath the roof, but in such cases the actual supports of the roof are concealed, and such roofs will therefore be treated under another head, "open timber roofs" being intended to include only those in which the larger part, if not all, of the trusses is exposed to the audience.
Open timber construction is not well adapted to an irregular plan or to broken roofs, as the beauty of this work depends in a great measure upon symmetry and repetition of the forms. It is also desirable that exposed roof construction be as simple as possible in its contructive features, as in irregular or complicated construction it is difficult to make the connections so that they will have ample strength and at the same time a neat appearance. As a rule open timber work is only employed where the roof is a simple pitch roof, with perhaps another roof of the same kind intersecting it.
Open trusses are usually made either entirely of wood, or the few rods that are used are cased in imitation of timbers. Occasionally light tie-bars are used to connect the ends of the hammer beams, but in such cases they are made as inconspicuous as possible.
The construction of an open timber roof is essentially the same as that explained in Chapter III (Layout Of Trussed Roofs - Bracing Of The Roof And Trusses) for supporting pitch roofs, the only difference being that the purlins have to be finished, and also the rafters, if they are exposed, and the trusses are made more ornamental.
In fact, the principal interest attaching to an open roof, from a constructive point of view, is in the character of the trusses and the manner in which they are built.
In churches of the cathedral type, that is, with a nave lighted through clerestory windows, the roof of the nave is generally made of rather a low pitch, and is frequently supported by trusses of the king post type, but in which the braces are supplanted by open panels, as shown in Fig. 134. The span of such trusses is usually not more than 30 feet, and by making the cross sections of the tie-beam and principals about one-fourth larger than would be used if the truss was to be built in the ordinary way, they will have sufficient strength without the brace. For spans of more than 30 feet the braces should be put in, cutting through the panels, or a queen post truss with collar beam should be used.
In trusses built as shown in Fig. 134 the author would advise placing concealed rods in the centre of the truss and under the purlin, as these can be tightened after the panel work is in until the parts are made to fit very tightly together.
The brackets against the wall are not necessary for the support of the truss, but they materially stiffen the building when the wind is blowing against either side.
In nearly all other types of churches, especially when finished with an open roof, the side walls are usually comparatively low, so that a horizontal tie-beam would be objectionable, and hence the trusses in such buildings are usually built in some form of the hammer beam or scissors types.*
Fig. 136. - Interior View Showing Truss, Fig. 135.
63. Probably the simplest method of constructing an open timber roof, when the span is from 30 to 35 feet, is that shown in Figs. 135 and 136, which show the roof construction of a small Episcopal church in Denver. In this instance all of the rafters are exposed, and also the under side of the roof boards, which are 1 1/8 inches thick and dressed and beaded on the under side.
The principals are kept 6 inches below the roof boards and a rafter placed against each side of them, as is shown in Section A, Fig. 135. This was done to permit of keeping the purlin beneath the rafters. The trusses and purlins are made of solid hard pine, dressed and varnished. The best manner of supporting the ends of the purlins where all of the work is exposed is by means of a Duplex Hanger, as this gives ample strength and makes a neat joint. When the purlins are cased the hangers can be completely covered up
In trusses of this shape the most difficult joint to make is that at B, and the method here shown is the best that the writer has yet seen where the work is exposed.
There is one serious objection to this form of construction from a practical point of view, in that such roofs are very cold in winter and hot in summer, owing to there being but a thin covering between the room and the outer air. When the roof boards are exposed they should be matched, unless laid in two thicknesses, and building paper or Cabot's "quilt" should be laid under the slate or shingles.
*For a description of these types of trusses, see Sections 14, 15 and 21, Chapter I (Foundations On Firm Soils. Staking Out The Building).
The writer strongly favors ceiling on the under side of the rafters, as shown in Fig. 137, and planting false ribs on the ceiling if a paneled effect is desired.