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Exposed-fastener metal roof panels are screwed to rows of wood purlins running the length of the roof. The 1st Coast Metal Roofing Supply installation guide for its Armour Rib and PBR panels accepts 1 in or 2 in nominal purlins and recommends a maximum spacing of 24 in on centre; its 5-V crimp panels need solid decking. The Purlin Spacing Calculator takes the building width, length, pitch and eave overhang, finds the slope length with the pitch factor √(144 + rise²) ÷ 12, divides it into equal spaces no wider than your maximum, adds the row at the eave and counts the boards. A 24 × 40 ft gable at 4/12 with 12 in overhangs has 13.70 ft of slope, so 7 spaces of 23.5 in and 8 rows per side: 16 rows, 640 lineal feet or 48 boards of 16 ft.
Exposed-fastener metal roof panels are screwed to rows of wood purlins. The 1st Coast Metal Roofing Supply installation guide for its Armour Rib and PBR panels accepts 1 in or 2 in nominal purlins at a maximum of 24 in on centre; 5-V crimp panels need solid decking. Rows per slope = slope length ÷ spacing, rounded up, plus one for the eave row; slope length = (run + overhang) × √(144 + rise²) ÷ 12.
| Width | Pitch | Slope length | Rows per side | Actual spacing |
|---|---|---|---|---|
| 20 ft | 3/12 | 11.34 ft | 7 | 22.7 in |
| 24 ft | 4/12 | 13.70 ft | 8 | 23.5 in |
| 24 ft | 6/12 | 14.53 ft | 9 | 21.8 in |
| 30 ft | 4/12 | 16.87 ft | 10 | 22.5 in |
| 30 ft | 6/12 | 17.89 ft | 10 | 23.9 in |
| 36 ft | 4/12 | 20.03 ft | 12 | 21.8 in |
| 40 ft | 4/12 | 22.14 ft | 13 | 22.1 in |
A purlin that shortens a rafter span must be no smaller than the rafters and continuous, braced with 2x4s at 45° or steeper to a bearing wall, with braces no more than 4 ft apart and no more than 8 ft unbraced.
24 in on centre is the usual maximum for residential exposed-fastener panels. The 1st Coast Metal Roofing Supply installation guide for its Armour Rib and PBR panels recommends a maximum of 24 in on centre and accepts 1 in or 2 in nominal purlins; 5-V crimp panels need solid decking instead. Always check the load table for your panel, gauge and wind zone.
On each slope, divide the slope length (eave to ridge, including the overhang) in inches by the spacing and round up, then add one for the row at the eave. A 24 ft wide gable at 4/12 with 12 in overhangs has 13.7 ft of slope: 164 ÷ 24 = 6.9, so 7 spaces and 8 rows per side — 16 rows, or 640 lineal feet on a 40 ft building.
Both are used. 1st Coast lists 1 in and 2 in nominal lumber as acceptable purlins and has product approvals for 29 gauge panels over 1x4 southern pine purlins on plywood and over 2x4 purlins. 2x4s give the screws more wood to bite and are the usual choice on open framing; 1x4s are common over an existing deck or shingles.
Yes. Where the old decking is doubtful or one layer of shingles stays in place, 1st Coast recommends new 1x4 or 2x4 purlins fastened through the decking into the rafters to give the panels a solid, flat base.
In rafter framing a purlin is a continuous beam under the rafters that shortens their span. IRC R802.4.5 requires it to be no smaller than the rafters, braced with 2x4s at 45° or steeper to a bearing wall, with braces no more than 4 ft apart and no more than 8 ft long unbraced.
Next, the fasteners: the metal roof screw calculator puts a row of screws on every purlin.
Need the whole roof at once? The free roofing calculator works out roof area, squares, shingle bundles, underlayment, drip edge, ridge cap and installed cost from one set of measurements.