When the width of the room exceeds 32 feet, and an open timbered roof is desired without going to the expense of a hammer beam truss, an adaptation of the scissors truss may be adopted with a pleasing effect, and without sacrifice of strength. Fig. 137 shows a scissors truss in about its simplest form, and Fig. 138 shows what is practically the same truss more elaborately treated, the constructive members being nearly the same in each truss. Fig. 139* shows a little different treatment of the scissors truss, the main ties in this truss terminating at the foot of the king post. This is a true scissors truss, but it will require a strong fastening at the joint K, to transfer the stress in the main ties to the king post.
*Interior of Boston Highlands M. E. Church, Walter J. Paine, Architect. From the American Architect and Building News of Nov. 10, 1900.
Fig. 139. - Church Interior.
Walter J. Paine, Architect.
Either of these three trusses may be used for spans up to 60 ft. and on either wooden or masonry walls, although the trusses shown by Figs. 138 and 139 are the best for wooden buildings on account of the long braces below the truss proper. The pieces A and B should be hollow with a rod inside.
The same principles apply to the designing of these trusses as to the rough trusses, hereinafter described.
Fig. 140 shows a half elevation of an ornamental truss that is quite frequently used in churches of moderate width. The truss has the appearance of a hammer-beam truss, but is not a true truss, as it depends upon the transverse strength of the rafter and the resistance of the wall for its stability. Fig. 141 shows the conduction of the truss and the necessary size of the parts for a span of 36 feet and a spacing of 12 to 14 feet. The portion built up of 2-inch plank is made of two thicknesses, one plank being cut the full length shown, while the other is cut between the pieces that cross the long piece. All but the carved hammer beam and the king post are cased with 3/4-inch finished lumber. The author would not recommend the use of a truss of this type for a greater span than 36 feet, and even for such a span it is best to reinforce the wall by buttresses placed opposite the truss.
Fig. 143. - Interior of Church.
In the roof shown in Fig. 140 the purlins are placed quite near together, as this distributes the load more evenly on the truss, and also adds to the appearance of the ceiling. If desired, the ceiling can be divided into square panels by putting false ribs, in imitation of rafters, between the purlins.
Neither of the trusses shown in Figs. 135 and 140 are well adapted to a roof that is intersected by transepts of the same width of the nave. If transepts or side gables are desired they should be made narrow, so as to come between two trusses, as in Fig. 136, or a different type of truss should be used.
Another very simple truss for an open-timber roof is shown in Fig. 142, the appearance of the ceiling being shown by the halftone illustration, Fig. 143, which is taken from a church designed by Mr. C. C. Haight.
This makes a very effective ceiling for a small church, and the enclosed space under the ridge makes the room easier to warm and more comfortable in the summer; it also leaves the room entirely free from timbers, which is desirable in a small church.
In this design the finished ceiling is nailed to the under side of the rafters; or, if preferred, to furring strips, and mouldings are planted on to divide the ceiling into panels, and in the opinion of the author this method gives the more comfortable room and the better accoustic properties.
The truss, however, depends in a great measure upon the transverse strength of the principals and the resistance of the walls to keep it from spreading, and is only suitable for spans of 35 feet and under.
Fig. 145 - Roof. Christ Church, Oxford, England.
Fig. 146. - Roof of Eltham Palace Hall.
When trusses like those shown in Figs. 140 and 142 are used, it is desirable that the walls be stiffened either by buttresses or by offsets.
Breaks in the walls like that shown at A, Fig. 144, add very greatly to the stability of the wall, and permit of using a truss with some horizontal thrust. When they are terminated by a gable, with tracery windows, they also make a very effective feature in small churches.
Fig. 147. - Lambeth Palace.
Fig. 148.-Middle Temple Hall.
Fig. 149. -Hampton Court Palace.
Fig. 150. - Church of the Redeemer.
66. When the span of the roof exceeds 35 feet, some form of the hammer-beam truss is generally adopted, the best form being that used in Westminster Hall and shown in Fig. 48.
Elaborate open-timber roofs are quite common in England, the truss being generally of the hammer-beam type, with braced or trussed purlins between the trusses. Several examples of such roofs are shown in Figs. 145-149.
Fig. 150 shows a very handsome open-timber roof on a church at Paterson, N. J., designed by Messrs. Cady, Berg & See, the truss being of the Westminster Hall type.
Fig. 151. - St. Stephen's Church, Lynn, Mass.
Messrs. Ware & Van Brunt, Architects.
When the church has a transept of the same width as the nave, the roof over the square formed by the intersection of the two and designated the "crossing," must be roofed either by diagonal trusses, as shown in Fig. 150, or by trussed valley timbers, as shown in Fig. 151.
For open diagonal trusses the hammer-beam truss, or some truss with a centre rod, seems best adapted.
Very often, however, the valley timbers themselves may be made to support the roof, the walls at the angles being usually capable of resisting any thrust that may come upon them.
Figs. 151 and 152 show a very good and practicable method of roofing a large rectangular church by means of trusses of moderate span, and at the same time giving to the church the appearance of having a nave, transept and aisles. This plan is particularly adapted to Episcopal churches, and gives a pleasing and ecclesiastical interior at moderate cost.
By bolting the ends of the trusses together where they meet on the columns or piers, a sort of chain is formed to receive the thrust of the valley timbers, and the trusses A and B reinforced by the outside walls and buttresses offer additional resistance.
In the church shown in Fig. 151 the choir piers are connected with the main walls by stone arches. While these were built largely for the architectural effect, some rigid connection should always be made between the tops of the columns and the side and end walls.
The arrangement shown by the plan, Fig. 152, is also well adapted to plastered ceilings with concealed trusses.