The depth of these trusses at the centre is usually determined by the roofing material that is to be used. Thus, slate should not be used on a roof in which the rise is not equal to one-third of the span; the rise for wood shingles should not be less than one-fourth of the span, and for corrugated iron not less than one-fifth of the span. The minimum rise for other roofings is given in Chapter III (Layout Of Trussed Roofs - Bracing Of The Roof And Trusses).
Fig. 72. - Ten-Panel Fink Truss.
Fig. 73. - Twelve-Panel Fink Truss.
Considering the construction of the roof and the weight of the trusses, the most economical pitch for a roof is about 1/4 the span, or what is commonly called a quarter pitch, the rise of the rafters being 6" in 12" or 26 degrees and 34 minutes. When the rise is less than 1/6 of the span some other type of truss will generally be required. When the inclination of the roof is determined almost entirely by the question of economy the rise is generally made from 6 to 7 inches in 12 inches.