In constructing the force diagram for any truss, it is requisite to determine the points of the truss which are to serve as points of support (see Figs. 70, 72, etc.), and to ascertain the amount of strain, or loading, which will occur at every such point.
The points of support along the rafters will be required to sustain the roofing timbers, the planking, the slating, the snow, and the force of the wind. The points along the tie-beam will have to sustain the weight of the ceiling and the flooring of a loft within the roof, if there be one, together with the loading upon this floor. The weight of the truss itself must be added to the weight of roof and ceiling.
215. - Load on Roof per Superficial Foot - In any important work, each of the items in Art. 214 should be carefully estimated, in making up the load to be carried. For ordinary roofs, the weights may be taken per foot superficial, as follows:
Roof-beams or jack-rafters,
This is for the superficial foot of the inclined roof. For the foot horizontal, the augmentation of load due to the angle of the roof will be in proportion to its steepness. In ordinary cases, the twelve pounds of the inclined surface will not be far from fifteen pounds upon the horizontal foot. For the roof-load we may take as follows:
Total on roof,
per square foot horizontal.
This estimate is for a roof of moderate inclination, say one in which the height does not exceed 1/4 of the span. Upon a steeper roof the snow would not gather so heavily, but the wind, on the contrary, would exert a greater force. Again, the wind acting on one side of a roof may drift the snow from that side, and perhaps add it to that already lodged upon the opposite side. These two, the wind and the snow, are compensating forces. The action of the snow is vertical: that of the wind is horizontal, or nearly so. The power of the wind in this latitude is not more than thirty pounds upon a superficial foot of a vertical surface; except, perhaps, on elevated places, as mountain-tops for example, where it should be taken as high as fifty pounds per foot of vertical surface.