This section is from the book "The Building Trades Pocketbook", by International Correspondence Schools. Also available from Amazon: Building Trades Pocketbook: a Handy Manual of reference on Building Construction.
When the slope of a roof is over 12 in. rise per foot of horizontal run, a snow and accidental load of 8 lb. per sq. ft. is ample. When the slope is under 12 in. rise per foot of run, a snow and accidental load of 12 lb. per sq. ft. should be used. The show load acts vertically, and therefore should be added to the dead load in designing roof trusses. The snow load may be neglected when a high wind pressure has been considered, as a great wind storm would very likely remove all the snow from the roof.
The wind is considered as blowing in a horizontal direction, but the resulting pressure upon the roof is always taken normal (at right angles) to the slope. The wind pressure against a vertical plane depends on the velocity of the wind, and, as ascertained by the U. S. Signal Service at Mt. Washington, N. H.. is as follows:
Velocity. Pressure. (ML per Hr.) (Lb. per Sq. Ft.)
The wind pressure upon a cylindrical surface is one-half that upon a flat surface of the same height and width.
Since the wind is considered as traveling in a horizontal direction, it is evident that the more nearly vertical the slope of the roof, the greater will be the pressure, and the more nearly horizontal the slope, the less will be the pressure. Table VII gives the pressure exerted upon roofs of different slopes, by a wind pressure of 40 lb. per sq. ft. on a vertical plane, which is equivalent in intensity to a violent hurricane.
Foot of Run.
Proportion of Rise to Span.
Normal to Slope.
In addition to wind and snow loads upon roofs, the weight of the principals or roof trusses, including the other features of the construction, should be figured in the estimate. For light roofs having a span of not over 50 ft., and not required to support any ceiling, the weight of the steel construction may be taken at 5 lb. per sq. ft.; for greater spans, 1 lb. per sq. ft. should be added for each 10 ft. increase in the span.