Lean-To Project Estimator
Estimate timber, roofing sheets, fixings and post footings for a lean-to shelter, carport or garden room. Calculates slope length, pitch angle and all structural quantities. 2026 UK pricing.
Height where roof meets house wall
Must be less than wall plate height
Enter dimensions above to see calculated pitch and slope length.
Quick Reference
Lean-To Roofing — Minimum Pitch & Span Guide
Construction Guide
Building a Lean-To
Wall plate fixing
The wall plate is the horizontal timber bolted to the house wall at the top of the lean-to. Use 150×50mm or 100×75mm C24 treated timber. Fix with M10 or M12 stainless steel masonry bolts (Rawlbolts or similar) at 600mm centres — never into mortar joints. The plate must be perfectly level. Seal between the wall plate and masonry with a flexible mastic to prevent water ingress behind the plate.
Install lead flashing or a proprietary stepped flashing at the wall plate to channel water away from the junction — this is the most common point of failure on lean-to roofs.
Posts and front beam
Front posts are typically 100×100mm (4×4 inch) pressure-treated C16 or C24 timber. If burying in ground: minimum 600mm depth with 20kg bags of Postcrete (approximately 2 bags per post). Tamp firmly and brace until set (30–60 minutes). If fixing to an existing concrete slab: use heavy-duty bolt-down post shoes with M12 anchor bolts set in resin.
The front beam spans between posts and supports the rafter feet. Size it to match or exceed the rafter depth — typically 100×75mm or 150×50mm. Notch the top of each post or use joist hangers to locate the beam securely.
Roofing materials compared
Polycarbonate sheets: Lightweight, quick to install, translucent (lets in light). 10–16mm twin-wall sheets are good thermal insulators. Fix with glazing bars at rafter centres; use foam closure strips at eaves and ridge. Minimum 5° pitch. Clear or opal finishes available.
Corrugated bitumen (Onduline/Coroline): Lightweight, easy DIY installation, good value. 2m × 0.95m sheets with 150mm side and end overlaps. Nail through the ridges directly into rafters with specialised nails. Minimum 10° pitch. Available in green, brown, red and black.
Timber deck + mineral felt: Most weatherproof, traditional finish. 18mm OSB boards fixed to rafters; 3-layer mineral felt laid in strips with 75mm overlaps and bonded with cold adhesive or torch-on. Minimum 12° pitch. Adds significant weight — ensure posts and rafters are adequately sized.
FAQ
Common Questions
Most lean-to structures fall under Permitted Development rights. Key rules in England: maximum height 4m (3m if within 2m of a boundary); eaves must not exceed 2.5m within 2m of a boundary; must not be forward of the principal elevation; total outbuildings must not exceed 50% of the curtilage. Check with your local authority before starting — PD rights can be removed by Article 4 directions or conditions on your planning permission.
At 600mm centres with standard UK loads: projection up to 2.5m — 100×50mm; up to 3.5m — 150×50mm; up to 4.5m — 200×50mm. These are indicative span tables. For heavier roofing (felt on OSB), use the larger size. Always use pressure-treated C16 or C24 timber for external use. For projections over 4.5m, consult a structural engineer.
Front posts should be spaced no more than 1.8–2.4m apart along the width of the structure. Our estimator places posts at maximum 1.8m centres plus one at each end. For example, a 4m wide lean-to needs 3 posts (at 0m, 2m and 4m). Posts should be 100×100mm treated timber.
Pitch angle = arctan((wall plate height − front post height) ÷ projection). Slope length = √(projection² + (wall height − post height)²). For example: wall plate 2.4m, post height 2.0m, projection 3m: rise = 0.4m, pitch = arctan(0.4/3) = 7.6°, slope = √(9 + 0.16) = 3.03m. Our calculator does this automatically.