When the room to be covered is rectangular in plan and more than 40 feet wide, and the side gables, if any, are comparatively narrow, the roof and ceiling can often be best supported by means of two Howe trusses, placed longitudinally of the roof and supporting smaller transverse trusses if necessary. The author has used this method of construction several times, and with economical and satisfactory results.
To illustrate this method of construction, Fig. 170 has been drawn from the working drawings of the church shown in perspective and plan by Figs. 171 and 172.
The interior of the audience room is shown by Fig. 173. The ceiling is of lath and plaster, and is divided into panels by false beams, as shown in the drawing. The general shape of the ceiling is also shown by the heavy line in Fig. 170. The width of the audience room between walls is 56 ft. 8 ins. and the length from the inner angle of the tower to the post P, either side of the pulpit opening, 51 ft. 8 ins.; the width of the gable recesses at the sides is 17 feet.
As shown by Fig. 170,the roof and ceiling are supported primarily by two Howe trusses placed as indicated by the dotted lines on the plan. The top chords of these trusses serve as one of the purlins on each side of the roof to support the rafters and the lower chords support the outer ceiling joists. To support the ceiling joists and rafters over the space between the Howe trusses, four smaller trusses, C, were placed across the space with their ends resting on the top chords of the longitudinal trusses. The tie-beams of these transverse trusses support the ceiling joists and the top chords support another set of purlins. The tie-beams of the transverse trusses drop below the ceiling joists, and are cased to correspond with the false beams used in dividing the ceiling. From an inspection of Fig. 170, it will be seen that about four-fifths of the entire roof and ceiling are supported by the two Howe trusses, L L, and these again are supported by three wooden posts (marked P on the plan), and by the inner angle of the tower, hence but very little weight comes on the walls.
The advantages of this system of roofing are: First, the system gives a greater clear height for the audience room than can be obtained with transverse trusses with the same height of walls.
This large space above the wall plate adds both to the apparent size of the audience room and to the comfort of the audience.
Second. The shape of ceiling naturally adapted to this system of trussing is appropriate and fitting to the plan.
Third. By this system no thrust is exerted on the outside walls and but little weight, while the rigidity and weight of the centre roof and ceiling really tend to stiffen the walls, and to prevent the building from racking sideways.
Fourth, economy. Trusses with horizontal chords are the simplest and easiest to construct, and where they are adapted to the shape of the roof, require the least amount of material. And not only are the trusses themselves such as any carpenter can easily construct, but the general construction is easily erected and can be put up before the side walls are completed, thus advancing the completion of the building.
As soon as the transverse trusses are in place, the centre ceiling joists may be set, and thus a convenient permanent stage is provided for the use of workmen while completing the roof. This system is as well adapted to a wooden as to a brick church, and can be used over rooms up to 100 ft. in depth and 80 ft. in width, provided that the pitch of the roof is 45 degrees or more.
74. In laying out such a roof construction the points to be studied are to space the longitudinal trusses so that they may have sufficient depth, and that they will not give too long a span to the lower rafters; also that the ceiling may be divided in pleasing proportions. As stated in Section 10, the height of a Howe truss, measured from the centres of the chords, should be one-sixth of the span when the space will permit, and never less than one-eighth for moderate spans. It is not necessary that the trusses shall be of the same span, but they should be of the same height.