For symmetrical trusses having panels of uniform width, and uniformly loaded, the stresses in the different parts will be proportional to the span, number of panels, height of truss, spacing of trusses and the load per square foot.

It is therefore practicable to tabulate the strains in trusses of a given number of panels for different loads, spans, spacings and height. For the convenience of architects and builders the author has prepared the following table, which gives the required dimensions for wooden trusses having "six panels" or braces, and for heights of one-sixth and one-eighth of the span, and supporting a flat roof of either tin, sheet-iron or composition, with a plastered ceiling supported by the tie-beam.

These dimensions apply only "to trusses which have purlins placed at the upper joints" and where the height is at least equal to' that given in the table; a greater height may be used with the same dimensions, but a less height materially increases the strains-and will require an increase in the dimensions. It is also important that the panels be all of the same width (within a few inches) and always six in number.

If there is no ceiling to be supported the lower chord may be of the same size as the upper chord. Where the rafters rest directly on the top chord the dimensions of the latter must be greatly increased, and special calculations should be made therefor.

Six-panel trusses were chosen for the reason that this number of panels will generally be found the most economical of material up to spans of 60 feet. For greater spans seven or eight-panel trusses will probably be more economical where a ceiling is to be supported.

Wherever the conditions of load, span, height and spacing are not exactly as given in the table, or for localities where a greater allowance must be made for snow, "special calculations" should be made of the strains and corresponding dimensions, but even in such cases the table will serve somewhat as a check upon the calculations, and will be convenient for making preliminary drawings and estimates.

Table I. - Dimensions For Six-Panel Howe Trusses, To Support A Flat (Composition) Roof And Plastered Ceiling Purlins to be Placed at Upper Joints

Computed for heights of one-sixth and one-eighth of the span and for a snow load of 16 pounds per square foot. Chords and braces, Norway pine; verticals, wrought iron rods.