Work out purlin spacing instantly with clear inputs, formula shown and shareable results.
Purlin spacing is set by how far the roof sheeting can span. Dividing the rafter length by the maximum permitted sheeting span and rounding up gives a whole number of equal spans, and the number of purlin lines is one more than the number of spans because both eaves and ridge need support.
Spacing
Spans = ceil(rafter length / maximum sheeting span); spacing = rafter length / spans
Purlin quantity
Total length = (spans + 1) x building length x 2 slopes
Preliminary estimate only. Purlin and sheeting spans must be confirmed against the manufacturer's load-span tables and checked by a qualified engineer for wind uplift.
Spans = ceil(rafter length / maximum sheeting span); spacing = rafter length / spans. Purlin spacing is set by how far the roof sheeting can span.
Deflection and, for walkable roofs, point load from foot traffic. Manufacturers publish span tables by profile, gauge and whether the span is single, double or continuous.
This calculator takes 3 inputs: Rafter length up the slope, Maximum sheeting span, Building length. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.