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Lean-to roof calculator: timber for a mono pitch roof
Work out rafters, ceiling joists, straps and fixings for a lean-to or mono pitch roof, with optional insulation. Enter the roof length and width, pitch, and rafter size and spacing.
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How the calculation works
The rafter length comes from the roof width, the pitch and the overhang at the eaves. The number of rafters comes from the roof length and your chosen spacing. Ceiling joists, trimmers, restraint straps and fixings are added from the same dimensions.
Covering materials such as tiles and battens are worked out separately in the roofing calculator.
Things to check
Rafter size depends on the span, pitch, spacing and roof covering, and is usually set out in the structural design or building control approval.
Where a lean-to meets an existing wall, it needs a wall plate fixed to the wall and a proper flashing detail to keep water out.
Worked example
A 4 m long, 3 m wide lean-to at 30° with 150 mm rafters at 400 mm centres:
| Material | Quantity | Price (ex VAT) |
|---|---|---|
| 47mm x 100mm x 4.8m Treated C24 Regularised Kiln Dried Sawn Timber (4 x 2) | 3 | £53.73 |
| 47mm x 150mm x 4.8m Treated C24 Regularised Kiln Dried Sawn Timber (6 x 2) | 4 | £116.32 |
| 47mm x 175mm x 4.8m Treated C24 Regularised Kiln Dried Sawn Timber (7 x 2) | 2 | £61.98 |
| Heavy Duty Engineered Bent Strap 1000mm | 7 | £50.61 |
| Heavy Duty Engineered Bent Strap 1500mm | 6 | £60.48 |
| Paslode Galvanised-Plus IM360 Collated Nails 3.1mm x 90mm 1100 Pack | 1 | £92.98 |
| Timco M10 x 100 Through Bolt Anchor BZP Pack of 2 | 10 | £37.00 |
| ForgeFix Multi-Purpose Screw 4.0 x 50mm ZYP Qty 200 | 1 pack of 602for 1 needed | £34.84 |
| Materials total | £507.94 | |
Today's prices, excluding VAT and delivery. Labour, plant hire and some items are priced when you request a quote. Use the calculator above for your own measurements.
Frequently asked questions
What pitch should a lean-to roof be?
It depends on the covering: each type of tile or sheet has a minimum pitch set by its manufacturer. Many tiled lean-tos are built at around 15° to 30°.
What size rafters do I need?
Rafter size depends on span, spacing and load, so check your structural design or building control approval. The calculator lets you choose the size and spacing.
Does this include tiles and battens?
No, this calculator covers the roof structure. Use the roofing calculator for tiles, battens, membrane and ridge.