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Two checks set purlin spacing. First, the roofing panel can only span so far between purlins at the design load. Second, each purlin picks up a strip of roof equal to the spacing, so wider spacing means more load per purlin and a bigger section. Take the smaller allowable spacing of the two.
- 30 psf load, purlin span 25 ft, try 5 ft spacing.
- w = 5 × 30 = 150 lb/ft; M = 150 × 25²/8 = 11,720 lb·ft.
- Governs = smaller of panel span and purlin capacity.
Reference tables
Typical metal-roof purlin spacing
| Panel / load | Common spacing |
|---|---|
| Standing seam, light load | 5–6 ft |
| Through-fastened, medium | 4–5 ft |
| Heavy snow region | 3–4 ft |
A number the tables do not give: at 30 psf, moving purlins from 5 ft to 4 ft spacing cuts the load per purlin (and its moment) by about 20%.
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FAQ
What limits purlin spacing?
The lesser of the roof panel’s allowable span and the purlin’s bending/deflection capacity at the design load.
Does snow region change it?
Yes — higher ground snow raises the psf, which either tightens spacing or calls for a heavier purlin.
C-purlin or Z-purlin?
Both work; Z-purlins nest and lap for continuity over supports, often allowing longer spans. Use the manufacturer’s load tables.
Does this replace design?
No — metal building purlins are engineered with manufacturer tables and load combinations. This shows the load path only.
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