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The total distance measured along the roof slope, from eave to ridge, that purlins need to span across.
Along the SLOPE, not on plan. Purlins are laid out at intervals down the sloping surface, so the distance that has to be divided is the sloping one — and the steeper the roof, the further the two diverge. A plan measurement on a steep roof produces one row too few, which shows up as the last bay at the ridge being wider than the roofing sheet can span.
The maximum allowable spacing between purlin rows, from the roofing material or structural design.
The SMALLER of the two limits, because both apply: the sheet's own span capacity, which usually governs on a light roof, and the structural design, which usually governs on a heavy one. It is a maximum, so the built layout is this or closer and never further. Put the remainder bay at the eave rather than letting it fall at the ridge, where the sheet laps and the flashing both have to cope with it.
Purlin rows needed
11 rows
- Bays between purlin rows
- 10
- Purlin centres up the slope
- 1.95 ft
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Purlin Row Spacing Calculator: 11 rows — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- Number of purlin rows = ceil(roof run length ÷ maximum purlin spacing) + 1, providing intermediate support between widely-spaced rafters or trusses
Inputs used
- Roof Run Length (Along Slope)
- 19.5 ft
- Maximum Purlin Spacing
- 2 ft
Intermediate steps
- Bays between purlin rows
- 10
- Purlin centres up the slope
- 1.95 ft
What this calculation does not cover
- Rows are counted here, purlins are not. A row longer than the stock lengths available has to be made up from more than one piece joined or lapped over a rafter, and the cleats, straps or blocks fixing every purlin-to-rafter crossing sit outside this division, so the answer still has to be turned into lengths and fixings before it becomes an order.
- The run entered is a single slope, eave to ridge, so a duopitch roof takes the figure twice; where one member at the apex serves both faces, the top row produced by the added end row is that same purlin counted once on each side.
- Only one of a purlin's two directions is on this page. The spacing asked for is measured up the slope, across the rows, whereas what each purlin actually spans is the gap between the rafters or trusses beneath it, and that gap is never entered, so bringing the rows closer together does nothing for a section that is too light for the rafter centres it crosses.
- Any maximum spacing from 10 mm (0.39 in) to 1.5 m (5 ft) is accepted and divided into the run without being weighed against the covering it carries, the load resting on it or the wind pulling up on it; that figure has to arrive already settled, from the sheeting or tile manufacturer's span rating or from the structural design.
- A clear, unbroken slope is assumed from eave to ridge. Rooflights, chimneys and dormers interrupt rows and want trimming members around the opening, and any row deliberately shifted to land on a ridge board, a valley or a change of covering is a departure from the even division this count is built on.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-05 · in the site-wide review of 2026-09-06 · v1.0.1
Regulatory standards & verification citations1
- Number of purlin rows = ceil(roof run length ÷ maximum purlin spacing) + 1, providing intermediate support between widely-spaced rafters or trusses
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