Roof Pitch Calculator UK

Calculate roof pitch angle, slope ratio, and rafter length for UK roofing projects. Includes Building Regulations guidance and material recommendations.

UK Building Regulations Note

Roof pitch affects material selection, insulation requirements (Part L), and structural design. Minimum pitches vary by material: tiles typically 15°+, slates 20-25°+, and flat roofs need special waterproofing below 10°. Always check BS 5534 and manufacturer specifications.

Calculate Pitch from Rise & Run

Enter the vertical rise and horizontal run of your roof to calculate the pitch angle.

Vertical Rise

Height from wall plate to ridge

Horizontal Run

Distance from wall to under ridge

Unit

Calculate from Angle

Enter the roof pitch angle to calculate rise, run ratio, and slope percentage.

Roof Pitch Angle (°)

Typical UK range: 22.5° - 45°

Horizontal Run

To calculate actual rise

Unit

Rafter Length Calculator

Calculate the length of common rafters for your roof, including optional overhang for eaves.

Building Span (Full Width)

Total building width (run = span ÷ 2)

Roof Pitch (°)

Eaves Overhang

Typically 300-600mm in UK

Unit

Convert Pitch Ratio

Convert between different roof pitch notation systems (degrees, X:12, percentage).

Rise Value

Run Value

Usually 12 (US system) or 10

Common UK Roof Pitches

Pitch Angle	Ratio (1:X)	Common Use	Notes
10° or less	1:5.7+	Flat roofs	Requires specialist membrane (EPDM, GRP)
15°	1:3.7	Low-pitch tiles	Minimum for some interlocking tiles
22.5°	1:2.4	Modern housing	Common for cost-effective builds
30°	1:1.7	Standard UK homes	Works with most tiles and slates
35°	1:1.4	Traditional UK homes	Popular, good loft space
40°	1:1.2	Steeper traditional	Excellent weather shedding
45°	1:1	Period properties	Easy for roof conversions
50°+	1:0.8	Gothic, Victorian	Dramatic appearance, specialist work

UK Roofing Materials by Minimum Pitch

Material	Min. Pitch	Typical Pitch	Notes
EPDM / GRP / Felt (flat)	0°	1-3°	1:80 fall minimum for drainage
Single-ply membrane	1.5°	3-5°	Mechanically fixed or adhered
Metal standing seam	3°	5-15°	Zinc, copper, aluminium
Profiled metal sheets	5-6°	10-20°	Industrial/agricultural buildings
Fibre cement slates	15-20°	25-35°	Depends on headlap and exposure
Interlocking concrete tiles	15°	22.5-35°	Some low-profile at 12.5°
Interlocking clay tiles	15°	30-40°	Premium option
Natural slate	20-25°	30-45°	Welsh slate heritage material
Plain clay/concrete tiles	35°	40-50°	Traditional double-lap method
Thatch	45°	50-55°	Traditional, listed buildings

* Minimum pitches may vary by manufacturer, exposure zone, and local conditions. Always check BS 5534 and manufacturer specifications.

7 Tips for UK Roof Pitch Calculations

Measure accurately: Use a spirit level and tape measure for rise/run measurements. Small errors compound over roof length—1° error over 6m span = 100mm ridge height difference.
Account for overhang: UK eaves typically extend 300-600mm beyond the wall. Add this to your run when calculating total rafter length and material quantities.
Consider exposure zones: Scotland, coastal areas, and high ground may require steeper minimum pitches. BS 5534 exposure maps should be consulted for slate and tile specifications.
Check loft conversion potential: Planning a future loft conversion? 2.2m head height is needed for habitable rooms. Steeper pitches (40°+) provide more usable space.
Solar panel optimisation: For maximum solar energy capture in the UK, south-facing roof pitches of 30-40° are ideal. Flat roofs need angled mounting systems.
Structural loading: Steeper roofs shed snow but create more wind uplift. Building Regulations and structural engineer input may be needed for pitches over 40°.
Use the 3-4-5 rule for checking: On site, a 3m horizontal, 4m vertical, and 5m hypotenuse confirms a perfect right angle (53.13° pitch). Scale proportionally for different pitches.

7 Common Roof Pitch Mistakes

Confusing pitch with slope: Pitch (degrees or ratio) differs from slope (percentage). A 45° pitch = 100% slope, but a 22.5° pitch ≠ 50% slope (it's actually 41.4%).
Using US vs UK notation: American specifications use X:12 ratios (e.g., 6:12 = 26.57°). UK often uses 1:X ratios (e.g., 1:2 = 26.57°). Confirm which system your plans use.
Ignoring building regulations: Changing roof pitch affects ridge height and may need planning permission. Conservation areas and Article 4 areas have stricter controls.
Wrong material for pitch: Installing tiles below their minimum pitch leads to water ingress, blown tiles, and voided warranties. Always verify material suitability.
Forgetting structural implications: Steeper pitches increase roof weight and wind loading. Existing walls and foundations may need assessment for pitch changes.
Not accounting for dormer windows: Dormers reduce effective pitch on that section. Ensure minimum pitch is maintained where materials are actually fixed.
Measuring from wrong reference point: Rise should be measured from the top of the wall plate to the apex, not from ground level or external wall face.

Frequently Asked Questions

What is the minimum roof pitch for tiles in the UK? +

The minimum roof pitch for standard interlocking concrete tiles is typically 15° (about a 1:3.7 ratio). For plain tiles, it's 35° due to their double-lap design. However, some modern low-pitch tile systems can work at 12.5° or even 10° with appropriate underlay and sealing. Always check the manufacturer's specifications and ensure compliance with BS 5534 (Slating and Tiling) and Building Regulations Approved Document C. The pitch affects waterproofing requirements, so lower pitches may need enhanced underlay or fully-supported roofing systems.

What is the standard roof pitch in the UK? +

The most common roof pitch in UK residential construction is between 30° and 45°, with 35° being particularly popular for traditional tiled roofs. Modern housing developments often use 22.5° to 30° pitches for cost efficiency, while period properties (Victorian, Edwardian) typically feature steeper pitches of 45-60°. The pitch you choose affects aesthetics, material costs, loft space usability, and must comply with local planning requirements. In conservation areas, you may be required to match existing roof pitches.

How do I calculate rafter length from roof pitch? +

Rafter length is calculated using the Pythagorean theorem: Rafter = √(Rise² + Run²). Alternatively, if you know the pitch angle and run: Rafter = Run ÷ cos(angle). For example, with a 4m run (half-span) and 35° pitch: Rafter = 4 ÷ cos(35°) = 4 ÷ 0.819 = 4.88m. Remember to add 300-450mm for eaves overhang when ordering timber, and allow for plumb cuts at the ridge and bird's mouth at the wall plate. For trussed rafters, contact a truss manufacturer who will calculate exact specifications.

What roof pitch is needed for slate in the UK? +

Natural slate typically requires a minimum pitch of 20-25° depending on slate size, exposure zone, and headlap used. Smaller slates and high-exposure areas (Scotland, coastal regions, above 90m altitude) need steeper pitches—often 25° or more. Welsh slate on heritage properties commonly uses 35-45° pitches. BS 5534 provides detailed guidance based on exposure calculations. Artificial (man-made) slates can sometimes work at 17.5° with appropriate underlay and reduced headlap, but always verify with the manufacturer.

What is the difference between roof pitch and roof slope? +

Roof pitch is typically expressed as an angle in degrees (e.g., 35°) or as a ratio of rise to run (e.g., 7:12 in the US meaning 7 inches rise for every 12 inches horizontal run, or 1:1.4 in UK notation). Roof slope or gradient is expressed as a percentage (rise ÷ run × 100). A 45° pitch equals a 100% slope because the rise equals the run. A 30° pitch equals a 57.7% slope. In UK construction, degrees and ratios are most commonly used in specifications, while percentage gradients are more common in civil engineering for roads and drainage.

Do I need planning permission for roof pitch changes in the UK? +

Changing roof pitch almost always requires planning permission because it alters the building's external appearance and usually increases ridge height. It's not covered by permitted development rights. You'll also need Building Regulations approval for: structural changes to support the new roof, insulation requirements (Part L requires U-values of 0.16 W/m²K for new roofs), and fire safety provisions. In conservation areas or for listed buildings, additional heritage consents may be required. Some Article 4 directions remove even more permitted development rights. Always consult your local planning authority before beginning any work.

How does roof pitch affect energy efficiency? +

Roof pitch impacts energy efficiency in several ways. Steeper pitches (35°+) create larger loft spaces that are easier to insulate effectively and ventilate properly, improving thermal performance. A 35-45° south-facing pitch is optimal for solar panel installation in the UK, maximising energy capture. Lower pitches may suffer from thermal bridging at eaves where insulation is squeezed. Building Regulations Part L sets U-value requirements of 0.16 W/m²K for new build roofs regardless of pitch. Steeper roofs also shed snow faster, preventing ice dam formation, but may create more wind turbulence affecting heat loss.

What is a 4:12 roof pitch in degrees? +

A 4:12 pitch (4 inches of rise per 12 inches of horizontal run) equals 18.43°. This US-style notation is common in imported materials and specifications. Common conversions: 3:12 = 14.04°, 4:12 = 18.43°, 5:12 = 22.62°, 6:12 = 26.57°, 7:12 = 30.26°, 8:12 = 33.69°, 9:12 = 36.87°, 10:12 = 39.81°, 12:12 = 45°. UK specifications more commonly use degrees directly or decimal ratios like 1:2.4 (which equals 22.62°, the same as 5:12). Always clarify which notation system is being used in specifications.

Official UK Resources

Building Regulations

Approved Document A (Structure) and C (Resistance to weather) cover roof construction requirements.

GOV.UK Approved Documents →

BS 5534:2014+A2:2018

British Standard for slating and tiling, covering design and fixing requirements for pitched roofs.

BSI Standards →

NHBC Standards

National House-Building Council technical standards for new homes, including roof construction guidance.

NHBC Standards →

NFRC

National Federation of Roofing Contractors - technical guidance and competent roofer scheme.

NFRC Guidance →

TRA

Trussed Rafter Association - technical guidance for prefabricated roof trusses.

TRA Technical →

Historic England

Guidance on roof repairs and alterations to listed buildings and conservation areas.

Historic England Roofing →

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About This Roof Pitch Calculator

This comprehensive UK roof pitch calculator is designed for homeowners, builders, architects, and DIY enthusiasts planning roofing projects. It calculates pitch angle from rise and run measurements, converts between different notation systems (degrees, X:12 ratio, UK 1:X ratio, percentage slope), and determines rafter lengths including eaves overhang.

The calculator includes UK-specific guidance on Building Regulations requirements, minimum pitches for different roofing materials (tiles, slates, membranes), and structural considerations. All calculations use standard trigonometric formulas and are suitable for preliminary planning, material ordering, and DIY estimation.

Created and maintained by Mustafa Bilgic, this tool is part of the UK Calculator suite—free, privacy-focused calculators designed specifically for United Kingdom residents. All calculations are performed locally in your browser; no data is sent to any server.

Disclaimer: This calculator provides estimates for planning purposes. For actual construction, consult a qualified structural engineer, architect, or roofing contractor. Roof work must comply with Building Regulations and may require planning permission. Professional surveys should be obtained for all structural modifications.

Roof Pitch Calculator UK | Angle, Slope & Rafter Length