"What is a mono pitch roof?"

"A mono pitch roof (also called a lean-to or single-pitch roof) has just one sloping surface, rather than two sides meeting at a ridge. It is common on extensions, garages, porches and modern builds, with the high side fixed to a wall plate or the main structure."

"How do I work out the rafter length for a mono pitch roof?"

"The rafter length is the span (horizontal run) divided by the cosine of the pitch angle. For a 3m run at 15°, that's 3 ÷ cos(15°) = 3.11m. Add an allowance for the eaves overhang at the low end."

"What is the minimum pitch for a mono pitch roof?"

"It depends on the covering. Most concrete interlocking tiles need at least 15–22.5°, single-ply membrane and EPDM can go as low as 1–5°, and standing seam metal works from around 3–5°. Always check the manufacturer's minimum pitch."

Mono Pitch Roof Calculator UK | Rafters, Battens & Materials

Enter span, pitch and length — get rafter lengths, batten runs, felt and fixings for a mono pitch (lean-to) roof instantly. Free UK construction calculator.

Results are estimates only and may vary depending on site conditions, material specifications and waste. EasyEstimate and its affiliates accept no responsibility for any loss or damage arising from reliance on these figures. Always verify quantities with your supplier before ordering.

How to Calculate a Mono Pitch Roof

A mono pitch (lean-to) roof has a single sloping plane. Because there is no ridge or second slope, the maths is simpler than a gable roof — you only need the span, the pitch and the building length.

The Formula

Rafter length = span ÷ cos(pitch) + eaves overhang
Roof area = rafter length × building length
Number of rafters = (building length ÷ rafter spacing) + 1
Materials = roof area × coverage rate (tiles, battens, felt) + waste

Worked Example: 3m span, 15° pitch, 6m long

Rafter length = 3 ÷ cos(15°) = 3 ÷ 0.966 = 3.11m (+ overhang)
Roof area = 3.11 × 6 = 18.7 m²
Rafters at 400mm centres = (6 ÷ 0.4) + 1 = 16 rafters

Enter your own dimensions in the calculator above for tiles, battens, felt and fixings with a waste allowance applied automatically.

Hand Cut Roof Calculator — traditional cut roof with ridge, hips and valleys
Trussed Roof Calculator — prefabricated truss roof quantities
Roofing Calculator — tiles, battens and felt for any roof

Frequently Asked Questions

What is a mono pitch roof?: A mono pitch roof (also called a lean-to or single-pitch roof) has just one sloping surface, rather than two sides meeting at a ridge. It is common on extensions, garages, porches and modern builds, with the high side fixed to a wall plate or the main structure.
How do I work out the rafter length for a mono pitch roof?: The rafter length is the span (horizontal run) divided by the cosine of the pitch angle. For a 3m run at 15°, that's 3 ÷ cos(15°) = 3.11m. Add an allowance for the eaves overhang at the low end.
What is the minimum pitch for a mono pitch roof?: It depends on the covering. Most concrete interlocking tiles need at least 15–22.5°, single-ply membrane and EPDM can go as low as 1–5°, and standing seam metal works from around 3–5°. Always check the manufacturer's minimum pitch.

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