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Mono-Pitch Roof Rise Calculator

Work out the rise of a mono-pitch (lean-to) roof from span and pitch in degrees. Returns rafter length, slope factor, and surface-area multiplier to BS 5534 and Approved Document A.

Mono-Pitch Roof Rise Calculator

Calculate from:
Rise (m)
1
Angle
14.04°
Rafter length (m)
4.432
Slope factor
1.031
Multiply footprint area by this for true roof surface area.

What this calculator does

This tool computes the vertical rise of a UK mono-pitch (lean-to or skillion) roof from the span and either the pitch in degrees or a target rise. It returns the rise, the slope angle, the rafter length (including eaves overhang), and the slope factor — the multiplier you apply to the plan area to get the true roof surface area for ordering tile, slate, or membrane.

Enter the span in metres, pick whether you are working from angle or rise, and supply the eaves overhang. The output is consistent with BS 5534:2014+A2:2018 for slated and tiled roofs and BS 6229:2018 for flat and low-pitch roofs.

How mono-pitch roof maths works

A mono-pitch roof is a right triangle in section. The horizontal leg is the span, the vertical leg is the rise, and the hypotenuse is the rafter along the slope. The relationships are:

  1. Rise = span × tan(angle) when working from degrees.
  2. Pitch = atan(rise / span) when working from rise.
  3. Slope factor = sec(angle) = 1 / cos(angle) — multiply plan area by this for actual roof area.
  4. Rafter length = (span + overhang) × sec(angle) — rafter material per piece.

The slope factor matters because slate, tile, and membrane orders are quoted per square metre of actual roof surface, not per square metre of footprint.

Reference test cases

SpanAngleOverhangRiseRafter lengthSlope factor
4.0 m14°0.3 m0.997 m4.43 m1.031
3.0 m5°0.15 m0.262 m3.16 m1.004
5.0 m22.5°0.4 m2.071 m5.85 m1.082
4.5 m17.5°0.3 m1.418 m5.04 m1.048

The first row is the canonical UK rear extension at 14° pitch — common because that angle accepts most reduced-pitch interlocking tiles (Marley Edgemere, Sandtoft Calderdale, Redland MockBond), provides positive drainage for natural slate at 22.5° with reduced headlap, or works as a low-pitch standing seam zinc roof.

Materials and minimum pitches

BS 5534 minimum pitch for slated and tiled roofs with standard 75 mm headlap:

  • 12.5° — Plain clay tiles, single lap with 75 mm headlap, sealed laps.
  • 15° — Reconstituted slate (e.g. Eternit Thrutone, Marley Edgemere) with manufacturer’s reduced-pitch detail.
  • 17.5° — Cement fibre slate with 75 mm headlap.
  • 22.5° — Natural slate, 100 mm headlap with sarking and underslate boarding.
  • 25° — Interlocking concrete tiles (Marley Modern, Redland Stonewold, Sandtoft Standard Pattern).
  • 30° — Pantiles and Roman tiles to BS EN 1304.

BS 6229 / BS 8217 minimum slope for low-pitch and flat roofs:

  • 1.5° (1 in 40) — Single-ply membrane (EPDM, TPO, PVC), mastic asphalt to BS 6925, hot-melt liquid systems.
  • 3° — Standing seam zinc, copper, lead to LSA Detail Sheet 3.
  • 4° — Profiled metal sheet to BS EN 14782 with sealed laps.
  • 6° — Profiled metal sheet with unsealed laps.

For a typical UK kitchen extension (4 m × 6 m) the cheapest compliant build is a 5° to 10° EPDM rubber roof on 18 mm OSB3 deck. Q1 2026 pricing from Travis Perkins, Wickes, and Selco runs £45 to £75/m² installed for a 1.2 mm Firestone RubberCover or Permaroof system, including 9 mm OSB3 firrings, perimeter trim, and gully outlets. A natural Welsh slate roof on the same span at 22.5° pitch runs £180 to £260/m² installed (NFRC 2026 trade rates).

Code and authority references

  • BS 5534:2014+A2:2018 — Slating and tiling for pitched roofs and vertical cladding. Code of practice.
  • BS 6229:2018 — Flat roofs with continuously supported flexible waterproof coverings. Code of practice.
  • BS 8217:2005 — Reinforced bitumen membranes for roofing. Code of practice.
  • Approved Document A (England 2013, Wales 2014) — Structure. Loading and stability.
  • Approved Document C (England 2010 + 2013 amendments, Wales 2010) — Site preparation and resistance to contaminants and moisture. Weather resistance.
  • Approved Document L Volume 1 (England 2021, Wales 2022) — Conservation of fuel and power for dwellings. Roof U-values: 0.16 W/m²K for new dwellings, 0.18 W/m²K for renovation.
  • NHBC Standards 7.2 (2026 edition) — Pitched roofs. Eaves and verge details.
  • TRADA Eurocode 5 Span Tables (4th edition 2014) — Sawn lumber rafter span tables.
  • BS EN 1995-1-1 + UK National Annex — Eurocode 5 design of timber structures.

Practical notes for UK mono-pitch roofs

Permitted Development limits. Under Class A of the GPDO 2015 (as amended), single-storey rear extensions in England can be built without planning permission up to 4 m projection (detached) or 3 m (semi-detached and terraced), with eaves height under 3 m within 2 m of a boundary, and overall height under 4 m. The lean-to angle and rise determine whether you stay within the 4 m overall height limit — a 5 m projection at 22.5° pitch with a 2.4 m eaves height reaches 4.47 m at the abutment and is not PD compliant. Article 4 directions in conservation areas, AONBs, and World Heritage Sites typically remove PD rights altogether.

Wind zones and snow zones. UK Wind Zone 5 (north-west Scotland, Western Isles, Shetland, Orkney) per BS 6399-2 / BS EN 1991-1-4 NA requires increased fastener density and edge detailing per BS 5534 §5. Snow Zone 4 to 5 (Highland Scotland, Pennines above 300 m) per BS EN 1991-1-3 NA Fig NA.1 carry sk above 0.95 kN/m² and may require structural calculation per the Snow Load Calculator.

Lead flashing detail. Where the lean-to abuts an existing wall, BS 6229 and the LSA (Lead Sheet Association) detail manuals require a 150 mm step flashing chased into the brick mortar joint at no less than 25 mm depth, sealed with lead-friendly mastic, and dressed over the EPDM upstand or tile undercloak.

Insulation and U-values. Approved Document L Volume 1 sets a 0.16 W/m²K target for new lean-to roofs and 0.18 W/m²K for renovations. A typical warm-roof build is 18 mm OSB3 deck + 100 mm to 140 mm PIR (Celotex GA4000, Kingspan Therma TR27) above the rafters, achieving 0.18 to 0.13 W/m²K depending on thickness. The Attic Insulation Calculator handles the U-value math.

Sources and authority data

  • NFRC (National Federation of Roofing Contractors) — Technical Bulletin and member survey rates 2026.
  • Construction Skills / CITB — Slating and tiling guidance.
  • BBA Agrément — Lean-to system certifications (Solidor, Velux Cabrio, Marley reduced-pitch tiles).
  • Checkatrade 2026 — Lean-to extension cost data, £1,500 to £5,000 for a 3 m × 4 m kitchen extension flat roof; £180 to £260/m² for natural slate at 22.5°.
  • MyBuilder 2026 — UK trade rates for EPDM rubber roofs, £45 to £75/m² installed.
  • Travis Perkins, Wickes, Selco — Q1 2026 retail pricing for OSB3, EPDM membrane, lead flashing, and perimeter trim.

Frequently asked questions

What is a typical mono-pitch (lean-to) roof angle in the UK?
Most UK mono-pitch roofs sit between 5° and 25°. BS 5534:2014+A2:2018 sets a minimum pitch for slated and tiled roofs of 12.5° (about 1 in 4.5) for plain clay tiles, 22.5° for natural slate, 25° for interlocking concrete tiles, and 17.5° for reconstituted slate, all with the standard 75 mm headlap. Below those pitches you must use a fully supported sheet covering — single-ply membrane, mastic asphalt, lead, copper, or zinc — or specify the manufacturer's reduced-pitch product (Marley Edgemere reduced-pitch is approved to 15°). For a typical garden room or kitchen extension lean-to, a 5° to 10° EPDM roof is the most common build.
How do I calculate the rise on a mono-pitch roof from span and angle?
Rise = span × tan(angle). For a 4 m span at 14° pitch, the rise is 4 × tan(14°) = 4 × 0.2493 = 0.997 m. To convert pitch to angle the inverse holds: angle = atan(rise / span). The rafter length along the slope is span / cos(angle), and the slope factor — the multiplier you apply to the plan area to get true roof surface area — is sec(angle) = 1 / cos(angle), or equivalently sqrt(1 + (rise/span)²). For 14° that factor is 1.031, so a 4 m × 6 m extension has a roof surface of 24 × 1.031 = 24.7 m².
What is the minimum pitch for a mono-pitch roof under UK Building Regulations?
Approved Document C2 covers weather resistance generally; the actual pitch limits live in BS 5534 for tiles and slates, and BS 8217:2005 + BS 6229:2018 for flat and low-pitch roofs. The absolute minimum after deflection is 1° (1 in 60) for fully adhered single-ply membrane per BS 6229, but 1.5° (1 in 40) is the practical design pitch to avoid ponding. EPDM, TPO, PVC, and mastic asphalt all work down to 1.5°; profiled metal sheet to BS EN 14782 needs 4° (1 in 14) sealed-lap or 6° (1 in 10) unsealed; standing seam zinc, copper, or lead needs 3° (1 in 19) per LSA Detail Sheet 3. For tiles and slates, follow BS 5534 minimum pitch.
How does the eaves overhang affect the rafter length?
The overhang adds to the horizontal span, so the rafter material grows in proportion. For a 4 m span at 14° with a 300 mm eaves projection, the rafter length is (4 + 0.3) × sec(14°) = 4.3 × 1.0306 = 4.43 m. NHBC Standards 7.2 require a minimum 50 mm overhang at the eaves to throw water clear of the wall face, and PAS 2030/2035 deep-retrofit insulation specifications often demand a 200 mm minimum to clear external wall insulation. BBA Agrément certificates for prefabricated lean-to systems (Velux Cabrio, Solidor lean-to, etc.) typically specify 150 mm to 250 mm.
What is the slope factor and why does it matter for mono-pitch roofs?
The slope factor is sec(angle) — the ratio of true roof surface area to plan footprint. For 5° it is 1.004, for 10° it is 1.015, for 15° it is 1.035, for 22.5° it is 1.082, for 30° it is 1.155, and for 45° it is 1.414. Multiply the plan area of the lean-to by the slope factor to get the actual area of EPDM, slate, or tile to order. A 4 m × 6 m kitchen extension at 10° pitch has 24 m² of footprint and 24.36 m² of roof surface — round up to the nearest standard EPDM sheet size (typically 4.57 m or 6.10 m wide membrane rolls).
Can I have a lean-to roof with one wall higher than the other?
Yes — that is the defining feature of a mono-pitch (lean-to or skillion) roof. The high wall is taller than the low wall by exactly the rise. For a 4 m span at 14° pitch attached to a 2.4 m high outer wall, the inner abutment wall meets the existing house at 2.4 + 4 × tan(14°) = 2.4 + 0.997 = 3.4 m. Permitted Development for rear extensions under Class A of the Town and Country Planning (General Permitted Development) (England) Order 2015 limits the eaves height to 3 m within 2 m of a boundary, and the overall height to 4 m for single-storey rear extensions (3 m for terraced and semi-detached, 4 m for detached). Always check the local LPA's Article 4 directions before building under PD.
What rafter size do I need for a mono-pitch roof span?
TRADA Eurocode 5 Span Tables 4th edition give for C16 grade rafters at 400 mm centres under 0.6 kN/m² imposed load: 47×100 to 1.99 m, 47×125 to 2.55 m, 47×150 to 3.06 m, 47×175 to 3.55 m, 47×200 to 4.05 m, 47×225 to 4.55 m. C24 grade adds about 12 percent to those spans. For sites with significant snow load (Scotland Highland zones 3 to 5 per BS EN 1991-1-3 NA), reduce by 10 to 25 percent or upsize one section. The [Roof Rafter Calculator](/calculators/roof-rafter-calculator/) handles species, grade, spacing, snow zone, and bearing length together.
How do I find the height of the high wall for a lean-to extension?
The high wall is the existing house wall and you do not build it from scratch — the lean-to abuts an existing flank or rear wall and the rafter ridge plate (or wall plate fixed to the existing wall) sits at the calculated rise above the eaves plate. For a 4 m span at 14° lean-to attached at 2.4 m eaves height, the wall plate against the house sits at 2.4 + 0.997 = 3.4 m AGL. The wall plate is typically a 100 × 75 mm C16 timber bolted to the brickwork through DPC and a 6 mm lead flashing dressed under the slates or tiles of the existing roof — see NHBC 7.2.10 and BS 6229 for the chase-and-cover detail.

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