Purlin Size Calculator
Enter the span between rafters or trusses, the purlin spacing, the material, the roof covering and the slope length. The calculator gives the purlin depth, a stock size and how many purlins each side needs.
- Purlin depth
- Stock size
- Purlins needed
This is a planning guide. Snow and wind loads may need bigger purlins.
Timber purlins are about span ÷ 20 deep, steel about span ÷ 40. A 4 m span with purlins 1.2 m apart under a metal sheet roof needs about a 200 mm deep timber purlin, or a 100 mm steel C purlin. A 6 m slope needs 6 purlins a side.
Purlin size chart
1.2 m apart, light roof. Search for a span.
| Span (m) | Timber (mm) | Steel (mm) |
|---|---|---|
| 2 | 100 | 100 |
| 2.5 | 125 | 100 |
| 3 | 150 | 100 |
| 3.5 | 175 | 100 |
| 4 | 200 | 100 |
| 4.5 | 225 | 125 |
| 5 | 250 | 125 |
| 6 | 300 | 150 |
mm deep.
Span and purlin
Timber, light roof. Slide the span.
The roof section shows the purlins.
How to size a purlin
Span to depth. Step through it.
Span
Between supports.
Fix purlins with cleats or hangers at every support.
Timber or steel purlins
Purlin materials. Compare them.
Traditional.
Light steel.
Lapped.
Steel purlins span further and stay straight, timber is easier to cut and fix.
Before you build
Purlin checks. Tick each item.
Tick each item as you go.
Brace long purlins with sag rods to stop them twisting.
What you get
Purlin sizing. Each tile explains one result.
mm.
Size.
Per side.
Weight.
Timber or steel.
Roof sheet makers list the most purlin spacing for each sheet.
Frequently Asked Questions
No question matches that search.
What size purlin do I need?
About the span divided by twenty for timber or by forty for steel.
What size timber purlin for a four metre span?
About two hundred millimetres deep.
How far apart should purlins be?
About one to one point five metres for most sheet roofs.
Are steel purlins better than timber?
They span further for their depth, but need bolted fixings.
What is the difference between a purlin and a rafter?
Rafters run down the slope, purlins run across them.
Do heavy roofs need bigger purlins?
Yes. Tiles and slate need a deeper purlin than metal sheet.
How many purlins do I need?
Divide the slope length by the spacing and add one.
A roof that stays straight
Size the purlins for the span and the roof and the covering stays flat and secure.
More size calculators
Each one has its own calculator, worked examples and answers to common questions.