Purlin Spacing Calculator
Calculate how many purlins you need for a metal roof or pole barn from the building length and your purlin spacing.
How this calculator works
Purlins run across the roof framing at a set spacing, measured center to center, to support the roofing panels above. To find the total count, convert the building length to inches, divide by the spacing in inches, and add one purlin for the starting end — since a purlin sits at both ends of the run, not just in the gaps between them.
Formula: Purlin count = floor(building length in inches ÷ spacing in inches) + 1 (one purlin at each end plus the ones in between).
Worked example
A 40 ft building, purlins at 24 in on center:
- 40 ft × 12 = 480 in
- 480 ÷ 24 = 20
- 20 + 1 = 21 purlins
Notes
Purlin spacing is set by the roofing panel’s maximum allowable span, which varies by panel gauge and profile. Check your roofing manufacturer’s span table for the maximum spacing your panel allows, then use that spacing here to find the count.
Steel purlins (C or Z shape) commonly run 24 in on center for standard 26-gauge panels, and tighter for lighter-gauge material.
How to use
Enter your building’s length and the purlin spacing you’re planning to use, based on your roofing panel’s allowable span. The calculator returns the number of purlins needed along that run, which you can use as a starting point for material ordering.
Counts purlins from spacing you provide. Maximum allowable purlin spacing depends on your specific roofing panel — confirm with the panel manufacturer’s span table.
Frequently asked questions
Why does the count add 1 to the division result?
A purlin runs at both ends of the building's length, not just in the gaps between them. Dividing length by spacing alone counts the gaps, so adding 1 accounts for the starting purlin at the first end, matching how purlins are actually laid out across a roof.
How do I know what spacing to enter?
Purlin spacing is set by your roofing panel's maximum allowable span, which varies by panel gauge and profile. Check your roofing manufacturer's span table for the maximum spacing your panel allows, then enter that spacing here to get the purlin count.
What's a typical purlin spacing for a metal roof?
Steel purlins (C-shape or Z-shape) commonly run 24 in on center for standard 26-gauge panels. Lighter-gauge panels usually need tighter spacing, so always confirm against your specific panel's span table rather than assuming 24 in applies.
Does this count include purlins around openings or eaves?
No. This calculator counts evenly spaced purlins along a straight building length only. Extra purlins around skylights, roof penetrations, or eave and ridge detailing are additional and typically specified separately by your building's framing plan.
Estimates only. Verify quantities with your supplier before purchasing.