Flat Roof Truss Calculator
A flat roof truss uses parallel or near-parallel top and bottom chords with just enough slope (typically a 1/4" to 2/12 pitch) to drain water off a membrane roof, common on commercial buildings, additions, and modern low-slope homes. This calculator sizes a flat roof truss from span and minimal pitch.
Size Your Flat Roof Truss
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Estimates only. Actual costs vary by region, supplier, and site conditions.
Flat Roof Truss Design Basics
True flat roofs are never perfectly level — a flat roof truss is built with a very shallow slope (as little as 1/4 inch of fall per foot, or up to a 2/12 pitch) so water drains toward gutters or interior roof drains instead of pooling on the membrane roofing.
Because there is almost no rise to work with, flat roof trusses behave more like parallel-chord floor trusses than like a steep-pitch roof truss — the top and bottom chords are nearly the same length, and depth (not slope) is what gives the truss its strength over a wide span.
Flat and low-slope roofs need waterproof membrane roofing (not shingles), positive drainage design, and — in snow-load regions — a higher design load than a steep roof of the same span, since snow does not shed off a flat surface. Confirm your local flat-roof snow-load factor with your building department.
Frequently Asked Questions
What pitch is considered a flat roof?
Roofs from dead level up to about a 2/12 pitch (roughly 9.5°) are generally considered flat or low-slope, and require membrane roofing rather than shingles to stay watertight.
Do flat roof trusses need a steeper snow load design?
Yes — because snow does not slide off a flat or low-slope roof the way it does off a steep roof, flat roof trusses are usually designed for a higher effective snow load than a steep-pitch truss of the same span.