Purlin Spacing Calculator
This purlin spacing calculator lays out the purlins on a gable or mono pitch roof. Enter the building width, roof pitch, eave overhang, and the maximum purlin spacing your roof sheeting allows, and the purlin spacing calculator returns the number of purlins per slope, the actual even spacing, the rafter length along the slope, and the total length of purlins to order.
Results
The Purlin Spacing Calculator is part of our Structural collection. Browse more structural to size, convert, and check your work alongside this tool.
What the purlin spacing calculator does
Purlins are the horizontal members that run across the rafters or trusses and carry the roof sheeting. The sheeting manufacturer states how far the sheets may span between purlins for a given load, and the purlins must be close enough to meet that limit. The purlin spacing calculator finds the slope length of the rafter, divides it into equal gaps no wider than your maximum, and counts the purlins with one at the eave and one at the ridge.
Purlin spacing formula
The calculator first finds the length of the rafter along the slope:
W is the building width and n the number of slopes, 2 for a gable and 1 for a mono pitch. The slope is then split into equal gaps:
The total purlin length is the number of purlin lines times the building length.
Roof purlin spacing example
A 10 m wide gable building with a 15 degree pitch, 0.5 m overhang, and a sheet that may span 1.2 m between purlins:
Both slopes need 12 purlin lines. For a 20 m long building that is 240 m of purlin, before laps and waste.
How to use the purlin calculator
Choose meters or feet and the roof type. Enter the building width between the eave walls, the roof pitch in degrees, and the horizontal eave overhang. Enter the maximum purlin spacing from the sheet maker for your load and sheet thickness. Add the building length if you want the total purlin length. Press Calculate.
Reading the purlin layout result
The actual spacing is always equal to or less than your maximum, since the slope is divided into equal gaps. Many builders place the first purlin a little in from the eave and the last close to the ridge so the sheets overhang the gutter and the ridge cap has a fixing; adjust the end positions and keep the inner gaps within the limit. Check the purlin size itself for the span between rafters and the roof loads, which this tool does not do.
When to use a purlin spacing calculation
- Laying out purlins for a metal roof on a shed or barn
- Counting how many purlins to order for a steel building
- Checking purlin spacing for metal roof sheets of a given thickness
- Planning purlins for a carport or patio roof
- Comparing sheeting options by their maximum purlin spacing
Common purlin spacing mistakes
- Measuring spacing on plan. Purlin spacing is measured along the slope.
- Using the light load span. Snow and wind can halve the allowed span.
- Forgetting the overhang. The eave purlin often sits on the overhang.
- No ridge purlin. The ridge cap needs a purlin to fix to.
Tips for roof purlin spacing
- Take the maximum spacing from the sheet maker for your exact profile and thickness.
- Use closer purlins at the edges and corners where wind uplift is highest.
- Order extra purlin length for laps and offcuts.
Maximum purlin spacing and roof loads
Sheet manufacturers publish span tables that link sheet thickness, profile depth, and load to the largest purlin spacing. A 0.5 mm trapezoidal sheet that spans 1.8 m under a light roof load may only span 1.2 m under heavy snow, and the end spans next to the eave often need closer purlins than the inner spans. Wind uplift near the edges and corners also calls for tighter spacing or extra fasteners. When you know the rise and run rather than the angle, the roof pitch calculator converts them to degrees for this tool.
Purlin layout and rafter length
The slope length here runs from the overhang edge to the ridge line. Your rafter or truss top chord may be a little different once you add the ridge detail and fascia, and the rafter length calculator works out the exact cutting length. With the purlins set, the roof area for ordering sheets, fixings, and underlay follows from the roofing calculator.
Typical maximum purlin spacing chart
| Roof sheeting | Typical maximum spacing |
|---|---|
| Corrugated steel sheet, 0.4 to 0.5 mm | 0.9 to 1.2 m (3 to 4 ft) |
| Trapezoidal steel sheet, 0.5 mm | 1.5 to 1.8 m (5 to 6 ft) |
| Trapezoidal steel sheet, 0.7 mm | 2.0 to 2.5 m (6.5 to 8 ft) |
| Insulated sandwich panel, 40 to 60 mm | 2.0 to 3.0 m (6.5 to 10 ft) |
| Corrugated fiber cement | about 1.4 m (4.5 ft) |
| Corrugated polycarbonate | 0.6 to 0.9 m (2 to 3 ft) |
Purlins per slope for common roofs (0.5 m overhang)
| Width | Pitch | Max spacing | Slope length | Purlins per slope | Spacing |
|---|---|---|---|---|---|
| 8 m | 10° | 1.2 m | 4.569 m | 5 | 1.142 m |
| 10 m | 15° | 1.2 m | 5.694 m | 6 | 1.139 m |
| 12 m | 20° | 1.5 m | 6.917 m | 6 | 1.383 m |
| 15 m | 10° | 1.8 m | 8.123 m | 6 | 1.625 m |
| 20 m | 5° | 2.0 m | 10.54 m | 7 | 1.757 m |
Purlin Spacing Calculator FAQ
How far apart should purlins be?
As far as the roof sheeting allows for your load: often 0.9 to 1.2 m for thin corrugated sheet and 1.5 to 2.5 m for trapezoidal steel sheet. Check the manufacturer span table.
How do you calculate the number of purlins?
Divide the rafter slope length by the maximum spacing, round up to get the gaps, and add one. A 5.694 m slope at 1.2 m maximum needs 6 purlins.
Does roof pitch change purlin spacing?
The spacing is measured along the slope, so a steeper roof has a longer rafter and may need more purlins for the same building width.
Do snow and wind loads matter?
Yes. Higher loads shorten the allowed sheet span, so use the maximum spacing for your design load, not the light load value.