What size timber is best for a lean-to roof?

For spans up to 3m, 4x2 inch (100x50mm) C16 timber is common. For larger spans or heavy roofing (tiles), 6x2 (150x50mm) or larger is required.

What pitch should a lean-to roof have?

A minimum pitch of 5 to 10 degrees is recommended for polycarbonate or corrugated roofs to ensure water runoff. Felt roofs may require a steeper pitch depending on the product.

Do I need planning permission for a lean-to?

In the UK, most lean-to structures are considered 'permitted development' if they are single-storey, under 4m high, and don't cover more than 50% of your garden area. However, always check with your local authority.

How far apart should lean-to rafters be?

Standard rafter spacing is 600mm centres. This provides a balance between structural strength for snow loads and efficient use of timber/roofing sheets.

Lean-To Project Estimator UK – Timber & Roofing Calculator

Lean-To Dimensions

Width (along wall) m:

Projection (from wall) m:

Wall Plate Height (m):

Front Post Height (m):

Roofing Material:

Timber Size (Rafters):

Material Estimate

Estimated Total Material Cost: £0

(Based on C16/C24 treated timber and standard roofing UK trade prices)

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Building a Lean-To: UK Structural Guidance

A lean-to structure relies on a strong "wall plate" bolted to your house or masonry, and a set of front posts supporting a "lintel" or "beam".

1. The Wall Plate

Use M10 or M12 masonry bolts spaced every 600mm. Ensure the plate is perfectly level and fixed into the brickwork (not the mortar joints).

2. Posts & Footings

Posts should be 4x4 inch (100x100mm) treated timber. For stability, these should be set 600mm into the ground with Postcrete or fixed to a solid concrete slab using bolt-down post shoes.

3. Rafter Spacing

Rafters are typically spaced at 600mm centres. If using lightweight polycarbonate, you might increase this to 700mm, but closer spacing is safer for snow loads in the UK.

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Permitted Development & Building Regs

In the UK, many lean-to structures fall under Permitted Development, but there are strict rules to follow to avoid a removal order:

Height Limits: The maximum height of a single-storey lean-to is 4m (or 3m if within 2m of a boundary).
Eaves Height: If within 2m of a boundary, the eaves height (the logic of the front post) should not exceed 2.5m.
Principal Elevation: No part of the structure can be in front of the wall that forms the principal elevation (front) of the original house.
Drainage (SUDS): You must ensure rainwater runoff does not flood a neighbour's property. Use guttering or drain to a soakaway/border.

Official UK Planning Guidance →

Lean-To Frequently Asked Questions

Do I need postcrete for a lean-to?

Yes, if you are burying posts into the ground, you should use roughly 1.5 to 2 bags of Postcrete per post. If you have an existing concrete slab, use heavy-duty bolt-down post shoes instead.

Can I use C16 or C24 timber?

For most domestic lean-to shelters, C16 treated timber is perfectly adequate. C24 is stronger and has fewer knots, but is usually more expensive. Always ensure the timber is 'Pressure Treated' (Green or Brown) for external use.

How many rafters do I need?

Professional builders recommend rafters every 600mm. This ensures the roof doesn't sag under snow loads. Our calculator follows this standard automatically.