This section is from the book "The American House Carpenter", by R. G. Hatfield. Also available from Amazon: The American House Carpenter.

The various items comprising the total load upon a roof are the covering, the truss, the wind, snow, the plastering or other kind of ceiling, and the load which may be deposited upon a floor formed in the roof; or, the total load -

M= C+T+ W+S + P+L.

The value per foot horizontal for these has been found as follows: C=2lp/s; T= 0.0154lp; W = F h2/12. For 5 the value must be taken according to circumstances, as in Art. 220. So, also, the value of P and L are to be assigned as required for each particular case, as in Art. 216. The total load, therefore, with these substitutions, will be -

M = 2 lp/s+ 0.0154lp + F h2/l2+S + P+L;

which reduces to -

M = lp (2/s+ 0.0154) + F h2/l2 +S+P+L; (96.)

in which / is the length of the rafter; p is the weight of the covering per foot superficial, including the roof boards or slats, the jack-rafters, etc.; s is the span of the roof; h is the vertical height above a horizontal line passing through the feet of the rafters; F is the force of the wind per square foot against a vertical surface; S is the weight of snow per square foot horizontal; P is the weight per superficial foot of the ceiling at the tie-beam; and Z, the load per superficial foot in the roof, including weight of flooring and floor-timbers. The dimensions, s, l, and h, are each in feet; the weight of p, F, S, P, and L are each in pounds. The value of p is for a square foot of the inclined surface.

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