Height. The height of the truss, always measured from "centre to centre" of the chords, should never be "less than one-ninth of the span," for spans up to 36 feet, or than one-tenth of the span for spans from 40 to 80 feet. As a general rule, a height of from one-seventh to one-sixth of the span will be most economical.
[Note. - Howe trusses can be built with a height of one-twelfth of the span, when the latter exceeds 36 ft., or with the braces at an inclination of only 300, but these proportions will greatly increase the stresses and necessitate larger timbers and rods.]
If the position of one or more of the purlins is fixed by some special requirement, then the panels should be so arranged that the upper end of a brace will come under the purlin, and that the inclination of none of the braces will be less than 36o.
Although it is generally better to have the truss symmetrical about the centre, it is not absolutely necessary, nor is it necessary that the panels be of uniform width.
When the truss is "not symmetrically loaded" however, it may be necessary to reverse the brace in one of the centre panels. This point is more fully considered in Chapter VIII (Architectural Terra Cotta).
Counter Braces. If there is any chance of the truss being more heavily loaded on one side of the centre than on the other, counter-braces - that is, braces in the opposite direction from the regular braces - should be inserted in the centre panels, as shown by the dotted lines in Figs. 22 and 23. When there is no likelihood of one side of the truss being more heavily loaded than the other, counter-braces will not be needed. Where and to what extent counter-braces should be used is explained in Chapter VIII (Architectural Terra Cotta).
Bearing on Wall or 'Post. The point where the centre lines of the end brace and of the tie-beam intersect should always come over the support, if possible, and generally at least 6 ins. beyond the inner face of the wall. This applies to all trusses.