50. The most economical method of supporting a pitch roof of wide span depends upon many circumstances, such as the character of the building, the pitch of the roof and nature of the roofing, the -width of the building, the shape of the ceiling below and whether the roof is to be of wood or iron.

The pitch of a roof, by which is meant the inclination of the rafters with a horizontal line, has a very close relation to the cost, and in buildings of a purely utilitarian character should be carefully considered.

In measuring or designating the pitch of a roof three different methods are in vogue. Architects often speak of the pitch in terms of the angle, measured in degrees, but builders more frequently designate the pitch in terms of the proportion of height to span. Thus, a roof having a span of 36 feet and a height at the centre of 9 feet, would be called a one-fourth pitch; if the centre height were 12 feet it would be a one-third pitch, and so on. The latter method, however, is apt to be confusing at times, and the former is not always easy to measure, so that the writer prefers to designate the pitch in terms of the rise in one horizontal foot, as shown in Fig. 118. This designation is both accurate and easy to lay out.

The most economical pitch for roofs supported by wooden trusses, with a horizontal tie-beam, is a rise of 6 inches to the foot, or approximately 26 degrees. This gives about the minimum amount of roofing material, with a sufficient depth to the truss and efficient resistance to the weather. For roofs supported by trusses of the scissor or hammer-beam types the pitch should be 45 degrees or over.

On dwellings and public buildings, where the roof is an important factor in the external appearance, the pitch of the roof is most frequently decided by architectural considerations, but in every case it should be such that the water from rain or snow will run off without being driven under the roofing.

The following table gives what is generally considered as the least desirable inclination for different kinds of roofing, and also the equivalent designations for the pitch:

Fig. 11s.

Table III. Least Desirable Pitch For Roofing Materials

*While tiles may be used with this pitch, they do not show to advantage; a rise of 10 to 14 ins. is much more desirable.

So-called "ready roofings" consisting of an asphaltic composition on canvas or burlap may be laid on any pitch exceeding one inch in twelve. For factories, warehouses, etc., these roofing materials form a light, economical and fairly durable covering.