The best form of roof truss or principal to be used for a given span is determined by the following considerations: -

1. The parts of the truss between the points of support should not be so long as to have any tendency to bend under the thrust; therefore the length of the parts under compression should not exceed twenty times their smallest dimension. This will be explained in Part IV.

2. The distance apart of the purlins should not be so great as to necessitate the use of either purlins or rafters too large for convenience or economy.

3. The tie beam should be supported at such small intervals that it need not be too large for economy or convenience.

It has been found by experience that these objects can be attained by limiting the distance between the points of support on the principal rafter to 8 feet.

In determining the form of truss for any given span, it is therefore necessary first to decide the pitch, then roughly to draw the principal rafters in position, ascertain their length, divide them into portions 8 feet long, and place a strut under each point of division.

By this it will be seen that a king-post truss is adapted for roofs with principal rafters 16 feet long, i.e. those having a span of 30 feet. A queen-post truss would be adapted to a roof with principal rafters 24 feet long, that is of about 45 feet span.

For greater spans with longer principal rafters, roofs such as that in Figs. 8o to 83 must be used.