This section is from the book "Modern Buildings, Their Planning, Construction And Equipment Vol1", by G. A. T. Middleton. Also available from Amazon: Modern Buildings.
Pieces of timber are then cut to fit tightly between the rafters, to which they are securely spiked. These act as purlins, and are placed where shown in Fig. 282. The object of the purlins is to stiffen the structure and to provide a means of trimming the rafters, which should not be greater than 15 inches apart, and diminish in number towards the apex of the roof. The short rafters need only be spiked to the purlins.
3. Triple - Rafter or Trussed-Rafter Roofs.- - In this kind of roof the covering is supported upon a triple system of rafters, as shown in Fig. 283.
Each rafter in the lowest system forms part of a triangulated framework called a Truss. These trusses are usually spaced at distances of about 10 feet centre to centre. The rafters forming part of the trusses are called Principal Rafters, and support horizontal timbers called Purlins at intervals. Upon the purlins a third system of rafters, called Common Rafters, are supported. Trusses consist of Principal Rafters, Struts, Posts, and Ties.
Principal Rafters should be supported at regular intervals by means of struts. Their lower ends are connected to the ends of the tie beam by either of the joints shown in Fig. 264, while the upper ends are connected to the heads of the post by means of a housed and tenoned joint.
Struts are employed to support the principal rafters, to which they are connected by means of a tenoned abutment joint. The lower ends of the struts are tenoned to shoulders cut at the bottom of the posts, so that the weight carried by the strut is transmitted to the post. The trusses should be so designed that the struts may be inclined as much as possible to the horizontal.
Posts are formed with sloping shoulders at both ends, against which the principal rafters and struts abut. The lower end of posts are stub tenoned to the tie beams, to prevent them from moving out of place laterally.
Ties are timbers used for connecting the feet of the principal rafters. Their middles are prevented from sagging by connecting them to the posts by means of straps. The ends of tie beams should not be built into the walls, but should be inserted in pockets formed for them, and if possible should be ventilated from the outer atmosphere to render them less likely to decay.
It can be readily seen, in the simpler forms of trusses, that the principal rafters and struts are in a state of compression, while the posts and tie beams are in a state of tension. This is invariably the case with properly designed trusses when not subjected to excessive wind pressures.
Trusses are strongest when designed so that each member is subject to a direct compression or tension. This condition is most nearly fulfilled when the axes of jointed timbers meet at one point.
The horizontal members which carry the common rafters between the trusses and transmit the load thereto are called purlins, and should be arranged to come directly over the struts. They should be notched or cogged over or housed into the principal rafter. They should never be spaced at greater intervals than 10 feet, otherwise the common rafters will have to be made very large to prevent sagging; but in good work the trusses are so designed that the spacing may not be greater than 8 feet from centre to centre of purlin. When the trusses are farther apart than 10 feet the purlins should be strutted.
To prevent the purlins from tilting, cleats are fixed against the lower face of the purlin.
This type of truss is shown in Fig. 283, and may be used up to 30 feet span, if the pitch be not so steep as to make the unsupported lengths of the common rafters too great.
The junction of the timbers at the head of the king post is usually secured by means of three-way straps on either side of the truss, bolted to the timbers. The tie beam is prevented from sagging by means of a strap, which is tightened up with a gib and cotter joint, as shown at A to a large scale. The purlins are placed in such a position that they may divide the common rafters into equal spans, and the slope of the struts is determined by arranging them to come immediately beneath the purlins.
As it is difficult to form a joint in wood suitable for resisting tension a King rod is often substituted for a King post in this type of truss, as shown in Fig. 284. Where a King rod is used a slight variation in construction is needed. Thus a straining piece must be fixed to the top of the tie beam to give a suitable abutment for the struts, while at the junction of the rafters an iron plate is fixed to prevent them from rising; in which case large cleats have to be fixed to the rafters to support the ridge piece.
When a ceiling is required the tie must be of wood, but where appearance is no object a tie rod of iron is less expensive. In this case a cast-iron socket is used for the upper ends of the principal rafters, having a groove in its head to support the ridge piece, and a hole for the insertion of the ring bolt.