How do I calculate rafter length in the UK?

Rafter length is calculated using the run (half the building span) and roof pitch. Formula: Rafter Length = Run ÷ cos(pitch angle) + Overhang. For UK buildings, standard calculations: Take half the building span (run), divide by the cosine of the pitch angle to get the rafter length to the wall plate, then add the eaves overhang (typically 300-450mm in UK). Example: 8m span building with 35° pitch and 400mm overhang: Run = 4m, Rafter = 4 ÷ cos(35°) = 4.88m, Total with overhang = 5.28m. Always add 100-150mm for cutting waste. UK Building Regulations require rafters to be sized according to span tables in Approved Document A.

What are standard UK roof pitch angles?

Standard UK roof pitches vary by roofing material and regional tradition: Slate roofs: Minimum 20-25°, optimal 30-45° for natural Welsh or Westmorland slate. Concrete interlocking tiles: Minimum 15-17.5°, common 22-35°, standard Marley/Redland tiles. Plain clay tiles: Minimum 35°, typically 40-50° for traditional appearance. Modern profiled metal: Minimum 5-10°, suitable for low-pitch applications. Thatch: 45-55° minimum required for proper water shedding. Regional traditions: Northern England/Scotland prefer steeper pitches (40-50°) for snow load and rain. South England traditionally lower (30-35°). Listed buildings must match existing pitch. Planning permission may restrict pitch changes. Standard new-build pitch in UK: 22.5-35° for modern tiles, 35-42° for slate aesthetic. Steeper pitches (40°+) provide more loft space but require more materials and stronger structure.

What timber sizes are required for UK roof rafters?

UK rafter timber sizes are specified by Building Regulations Approved Document A span tables. Common sizes for C16/C24 graded softwood: 47x100mm: Spans up to 2.0m at 400mm centres, 1.8m at 600mm centres. 47x125mm: Spans up to 2.5m at 400mm centres, 2.2m at 600mm centres. 47x150mm: Spans up to 3.0m at 400mm centres, 2.7m at 600mm centres. 47x175mm: Spans up to 3.5m at 400mm centres, 3.2m at 600mm centres. 47x200mm: Spans up to 4.0m at 400mm centres, 3.6m at 600mm centres. 47x225mm: Spans up to 4.5m at 400mm centres, 4.1m at 600mm centres. For longer spans, consider purlins with intermediate support. C24 grade timber allows slightly longer spans than C16. All structural timber must be strength graded and marked. Regularised (planed) timber is easier to work with but slightly smaller finished size. Always check local Building Control requirements.

What is a birdsmouth cut and how do I calculate it?

A birdsmouth (or bird's mouth) is a notch cut into the underside of a rafter where it sits on the wall plate, consisting of two cuts: the seat cut (horizontal) and the heel cut or plumb cut (vertical). This joint provides a solid bearing surface and prevents the rafter from sliding. UK calculations: Seat cut depth should not exceed 1/3 of rafter depth to maintain structural integrity (e.g., max 50mm on 150mm rafter). Seat cut length typically equals wall plate width (typically 100mm in UK). Plumb cut angle equals the roof pitch angle (e.g., 35° pitch = 35° from vertical). Heel cut angle equals 90° minus pitch (55° for 35° pitch). UK Building Regulations: The birdsmouth should leave at least 2/3 of the rafter depth above the notch. Wall plate must be adequately fixed to the wall with straps. Rafter should be fixed to wall plate with skew nails or framing anchors. For rafters over 47x150mm, consider engineered metal hangers instead of traditional birdsmouth.

How many rafters do I need for a UK roof?

Calculate the number of rafters by dividing the roof length by the rafter spacing, then adding one. UK standard rafter spacings: 400mm centres: Standard for tiled roofs, required for heavy coverings. 600mm centres: Acceptable for lightweight coverings, cheaper but needs thicker rafters. Formula: Number of rafters = (Roof length ÷ Spacing) + 1, for each side of a pitched roof. Example: 10m long building, 400mm spacing, pitched roof: Each side: (10,000mm ÷ 400mm) + 1 = 26 rafters. Both sides of roof: 26 × 2 = 52 rafters. Plus: 2 hip rafters per corner if hipped roof, jack rafters for hipped ends, valley rafters for valleys. Allow 10-15% extra for waste, damage, and cutting errors. UK Building Regulations: Spacing must be consistent, rafters must be evenly distributed, first and last rafters set in from gable by half the spacing. Consider: Regularised timber (47mm) vs sawn (50mm) affects actual centres.

What UK Building Regulations apply to roof rafters?

UK roof construction is governed by Approved Document A (Structure) of the Building Regulations. Key requirements: Timber grading: All structural timber must be strength graded (C16 minimum, C24 for longer spans) and carry appropriate marking. Span tables: Rafter sizes must comply with span tables based on pitch, loading, and spacing. These are in Approved Document A, TRADA publications, or can be engineer-calculated. Lateral restraint: Rafters must be tied to prevent roof spread, using ceiling joists, collar ties, or ridge beams. Roof straps: Required at intervals not exceeding 2m to restrain roof to walls. Wind bracing: Required to resist racking, especially for large roofs. Fixings: Specified nail sizes, patterns, and metal connectors required for structural integrity. Fire safety: Separation and fire stopping requirements in Approved Documents B. Party wall construction: Special requirements where roofs meet party walls. Building Control approval is required for all structural roof work. Trussed rafters must be designed by a structural engineer. Traditional cut roofs require either compliance with span tables or structural engineer calculations.

What tools do I need to cut rafters in the UK?

Essential tools for cutting roof rafters: Measuring and marking: Steel tape measure (8-10m), combination square, roofing square (or speed square), sliding bevel, carpenter's pencil, chalk line, spirit level (1.2m+). Cutting: Circular saw (185mm blade, 65mm+ depth of cut), mitre saw for accurate angles, handsaw (for finishing cuts), jigsaw (for curved cuts). Assembly: Claw hammer (20oz), nail gun (preferably framing nailer), drill/driver, scaffolding or sturdy trestles. Optional but helpful: Rafter template (first rafter as pattern), laser level, rafter calculator app, CAD software for complex roofs. UK safety requirements: HSE regulations require scaffold or working platform for roof work, PPE (hard hat, safety boots, eye protection), training for power tool use. All tools should be PAT tested if on commercial sites. Common UK brands: DeWalt, Makita, Milwaukee (professional), Bosch, Ryobi (DIY). Speed squares with rafter tables simplify angle marking without complex calculations.

What is the difference between common rafters and other rafter types?

UK roof construction uses several rafter types: Common rafters: Run from wall plate to ridge at right angles to both. Simplest and most numerous rafter type. Same length for each one on a rectangular roof. Calculated using span and pitch. Hip rafters: Run diagonally from corner of wall plate to ridge end on hipped roofs. Longer than common rafters (typically 1.4 times the common rafter length). Require compound angles for cuts. Usually one size larger timber than common rafters. Valley rafters: Run in the internal angle where two roof slopes meet. Similar calculation to hip rafters but sit in valley rather than on hip. Require different birdsmouth configuration. Jack rafters: Shortened rafters that run from wall plate to hip rafter (hip jacks) or from valley to ridge (valley jacks). Vary in length progressively. Same pitch angle as common rafters but with compound side cuts. Cripple rafters: Short rafters between valley and hip in complex roofs. Collar ties: Horizontal members connecting opposing rafters, not rafters themselves but often confused. For UK Building Regulations compliance, all rafter types must be sized appropriately for their span and loading.

Rafter Calculator UK 2026

Last updated: February 2026

Rafter Calculator UK 2025

Calculate rafter length, angle, birdsmouth cuts, and timber requirements for UK roof construction. All calculations comply with Building Regulations Approved Document A. Enter your roof dimensions to get accurate measurements for your project.

Calculate Your Rafters

Building Span (metres)

Total width of building (wall to wall)

Roof Pitch (degrees)

Eaves Overhang (mm)

Typical UK: 300-450mm

Roof Length (metres)

Length of building (gable to gable)

Rafter Spacing (mm)

Wall Plate Width (mm)

Typically 100mm in UK

Understanding UK Rafter Calculations

Rafter Length Formula

Basic Formula:

Rafter Length = (Span ÷ 2) ÷ cos(pitch) + Overhang

Run: Half the building span (to ridge centre)
Rise: Run × tan(pitch) = vertical height
Rafter: Run ÷ cos(pitch) = sloped length
Total: Add eaves overhang (300-450mm typical)
Order: Add 100-150mm for cutting waste

Birdsmouth Cut

The notch where rafter sits on wall plate:

Seat cut: Horizontal cut = wall plate width
Plumb cut: Vertical cut at pitch angle
Maximum depth: ⅓ of rafter depth
Example: 150mm rafter = max 50mm notch
Heel: Minimum ⅔ rafter remains above cut
UK standard: 50-75mm seat cut depth

Pitch Angles

Common UK roof pitches by material:

15-17.5°: Modern interlocking tiles (minimum)
22.5-30°: Standard tiles, most common
30-35°: Traditional look, good water runoff
35-45°: Natural slate, Victorian style
45-55°: Thatch (minimum for water shedding)
Regional: Scotland/North steeper than South

UK Building Regulations: All roof construction must comply with Approved Document A (Structure). Rafter sizes are determined by span tables based on timber grade (C16/C24), spacing (400mm or 600mm centres), pitch, and imposed loads. Building Control approval is required for all structural roof work. For spans exceeding span table limits, or for complex roof designs, a structural engineer's calculations are required.

UK Rafter Timber Sizes (Approved Document A Span Tables)

The following span tables apply to C24 graded softwood rafters with roof pitch 15-45° and typical UK dead loads (tiles, felt, battens). Reduce spans by 10-15% for C16 timber or heavy coverings.

Rafter Size	Max Span @ 400mm	Max Span @ 600mm	Typical Use
47 × 100mm	2.00m	1.80m	Small extensions, porches
47 × 125mm	2.50m	2.25m	Small roofs, garages
47 × 150mm	3.00m	2.75m	Standard domestic roofs
47 × 175mm	3.50m	3.20m	Larger spans, traditional cut roofs
47 × 200mm	4.00m	3.65m	Larger domestic, with purlins
47 × 225mm	4.50m	4.10m	Maximum unsupported spans

Important Notes:

These are maximum spans - always check Building Regulations or consult a structural engineer for your specific situation
Snow load areas: Scotland, Northern England, and exposed areas require reduced spans or larger timbers
Heavy coverings: Natural slate, concrete tiles require reduced spans vs. lightweight tiles
Timber grade: C24 shown above; reduce spans by ~10% for C16 grade timber
Regularised timber: 47mm is regularised (planed); sawn timber is 50mm nominal

7 Essential Rafter Installation Tips

1. Create a Template Rafter First

Cut your first rafter carefully, checking all angles and dimensions. Test fit it in position before cutting any others. Use this first rafter as a template to mark all subsequent rafters, ensuring consistency. Mark the template clearly "TEMPLATE - DO NOT USE" to prevent accidental installation.

2. Check Wall Plate Level Before Starting

Before cutting any rafters, verify the wall plate is level along its entire length. Use a spirit level and string line. Variations of more than 10mm along the length will cause problems with rafter installation and ridge alignment. Pack or plane the wall plate as necessary to achieve level.

3. Mark Rafter Positions Before Installation

Mark rafter positions on both wall plates and the ridge board before installation. Start from one end and mark at your chosen centres (400mm or 600mm). Use a measuring tape and square to ensure marks are perpendicular to the wall plate. This ensures consistent spacing and proper alignment.

4. Install Rafters in Opposing Pairs

Always install rafters in opposing pairs (one each side of the ridge) to maintain balance and prevent the ridge from twisting. Temporarily brace the first few pairs until enough rafters are installed to provide stability. Use a prop to support the ridge until the roof is complete.

5. Use Proper Fixings Throughout

UK Building Regulations require specific fixings: Rafters to wall plate: 2× 75mm skew nails or framing anchor. Rafters to ridge: 3× 75mm nails or ridge hanger. Ceiling joist to rafter: 3× 75mm nails at overlap.

Consider using joist hangers for exposed or high-load applications. All nails should be galvanised for durability.

6. Install Lateral Restraint Straps

Building Regulations require lateral restraint straps at maximum 2m centres to tie the roof to the wall structure. These 30×5mm galvanised steel straps run from the rafters down the inner wall face, fixed with appropriate screws or nails. Essential for preventing the roof from lifting in high winds.

7. Consider Collar Ties or Ceiling Joists

Pitched roofs create outward thrust on walls. Prevent this by connecting opposing rafters with ceiling joists at wall plate level or collar ties at a higher level. Ceiling joists should be at every rafter or every other pair. Collar ties can be at maximum ⅓ down from the ridge, at every other rafter pair minimum.

Pro Tip: For spans exceeding span table limits, you'll need intermediate support such as purlins with struts to a load-bearing wall or steel beam. This is common in loft conversions where large clear spans are desired. Always consult a structural engineer for complex roof designs or when in doubt about loading requirements.

7 Common Rafter Mistakes to Avoid

1. Cutting Birdsmouth Too Deep

The mistake: Cutting the birdsmouth notch deeper than ⅓ of the rafter depth, weakening the timber at the critical bearing point. This can cause the rafter to crack or split under load.

The fix: Maximum birdsmouth depth = ⅓ of rafter depth. For a 150mm rafter, maximum notch is 50mm. If you need a deeper seat, use a larger rafter size instead. Always leave at least ⅔ of the rafter depth above the birdsmouth.

2. Using Undersized Timber for the Span

The mistake: Selecting rafters based on what's available rather than what's required for the span. Undersized rafters will sag over time and may fail under snow loads or during strong winds.

The fix: Always consult Building Regulations span tables or get structural engineer calculations. Factor in roof covering weight, snow loads (especially in Scotland/Northern England), and spacing. When in doubt, go one size larger.

3. Forgetting to Allow for Overhang

The mistake: Calculating rafter length to the wall plate only, forgetting the eaves overhang. This results in rafters that are too short and no protection for the wall from rain.

The fix: Always add the eaves overhang (typically 300-450mm for UK) to your calculated rafter length. Then add 100-150mm extra for cutting waste. Better to have slightly long rafters that can be trimmed than too short.

4. Not Using Strength-Graded Timber

The mistake: Using general construction timber rather than strength-graded (C16 or C24) timber for structural rafters. Ungraded timber may have hidden defects that reduce its load-bearing capacity.

The fix: UK Building Regulations require strength-graded timber for structural applications. Look for the grade stamp (C16, C24) on the timber. C24 is stronger and allows longer spans. Purchase from reputable timber merchants, not general DIY stores for structural work.

5. Omitting Lateral Restraint Straps

The mistake: Failing to install lateral restraint straps that tie the roof to the walls. This can allow the roof to lift in high winds, particularly on exposed sites.

The fix: Install 30×5mm galvanised straps at maximum 2m centres. Straps run from rafters down the inner wall face and are fixed with at least 5 fixings each end. This is a Building Regulations requirement that Building Control will check.

6. Ignoring Roof Spread

The mistake: Not providing adequate resistance to roof spread (the outward thrust that pitched roofs exert on walls). This can cause walls to bow outward over time and in extreme cases, structural failure.

The fix: Install ceiling joists connecting opposing rafters at wall plate level, or collar ties at a higher level. Ceiling joists should be at every rafter or every other pair. For rooms-in-roof construction without ceiling joists, use a ridge beam supported at gable ends to eliminate thrust.

7. Getting Angle Cuts Wrong

The mistake: Confusion between plumb cuts and level cuts, or mixing up the angle directions. This results in rafters that don't fit properly at the ridge or wall plate.

The fix: Plumb cut = parallel to the plumb line (vertical when rafter is in position) = pitch angle from the square face. Level cut (seat cut) = horizontal when rafter is in position = 90° minus pitch from the square face. Use a speed square with rafter tables to mark angles correctly. Always cut one test rafter first.

Safety Warning: Roof construction is dangerous work at height. UK law requires proper scaffolding, edge protection, and PPE. Falls from height are the leading cause of construction deaths.

If you're not experienced with roof construction, consider hiring a qualified roofer or carpenter. Building Control approval is required for all structural roof work - getting it wrong is not just dangerous, it's also illegal.

6 Official UK Roofing Resources

Planning Portal - Building Regulations

Official UK government guidance on Building Regulations Approved Document A (Structure). Contains span tables, loading requirements, and structural requirements for domestic roofing. Essential reading before any structural roof work.

Visit Planning Portal →

TRADA (Timber Research & Development)

Industry body for timber construction. Publishes definitive span tables and guidance documents for timber roof construction. Their "Roofing" publications are the industry standard reference. Members-only detailed guidance, but free summaries available.

Visit TRADA →

NFRC (National Federation of Roofing Contractors)

Trade body for UK roofing contractors. Provides technical guidance, best practice guides, and a "Find a Roofer" directory. Their technical department can answer complex roofing questions. Essential for finding qualified contractors.

Visit NFRC →

HSE (Health & Safety Executive)

Official UK health and safety guidance for working at height, including roof construction. Covers legal requirements for scaffolding, edge protection, PPE, and safe working practices. Falls from height are the leading cause of construction deaths - essential reading.

Visit HSE Roofwork →

CITB (Construction Industry Training Board)

Provides training and qualifications for construction trades including roofing. Their NVQ Level 2/3 in Roofing covers cut roof construction techniques. Site cards (CSCS) may be required for site work. Also funds apprenticeships in roofing trades.

Visit CITB →

Local Building Control

Your local authority Building Control department must approve all structural roof work before and during construction. They inspect work at key stages and issue completion certificates. Contact them early to understand requirements. Alternatively, use an Approved Inspector.

Find Local Building Control →

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How the Rafter Calculator Works

This calculator provides quick, accurate results for your calculations. Simply enter your values into the fields above and the tool will compute the result using the standard formula. All calculations are performed in your browser for instant results with no data sent to any server.

While this tool is designed to be as accurate as possible, results are estimates and should be verified against official sources where decisions depend on precise figures. The calculator is free to use and works on all devices.

Key Information

This tool uses established formulas and current UK-specific data where applicable. Results are rounded appropriately and clearly labelled. For financial calculations, all figures use the 2025/26 tax year unless otherwise stated. For measurement conversions, standard international conversion factors are applied.

Example Calculation

Enter your values in the calculator above to see a personalised result. The tool handles the mathematical computation automatically, saving you time and reducing the risk of manual calculation errors. Results can be used for planning, comparison, or educational purposes.

Source: Based on standard formulas and official UK data where applicable. Last updated March 2026.

Frequently Asked Questions

Rafter length = (Span ÷ 2) ÷ cos(pitch angle) + overhang. For an 8m span building with 35° pitch and 400mm overhang: Run = 4m, Rafter = 4 ÷ cos(35°) = 4.88m, Total = 5.28m. Add 100-150mm for cutting waste. Always verify with Building Regulations span tables.

A birdsmouth is a notch cut where the rafter sits on the wall plate, consisting of a horizontal seat cut and a vertical plumb cut. Maximum depth is ⅓ of rafter depth to maintain structural integrity - e.g., max 50mm on a 150mm rafter. The seat cut typically equals the wall plate width (100mm).

For a 4m rafter span with C24 graded timber at 400mm centres: 47×200mm rafters will span up to 4.0m. At 600mm centres, you'd need 47×225mm. For C16 grade, reduce spans by approximately 10%. Always verify with Building Regulations span tables for your specific loading conditions.

Yes, all structural roof work requires Building Regulations approval in the UK. You must submit a Building Notice or Full Plans application before starting work. Building Control will inspect at key stages (before covering rafters) and issue a completion certificate. Without this, you may face problems selling your property.

The most common UK roof pitch for new builds is 22.5-35° for modern interlocking tiles. Traditional pitches are steeper: 35-45° for slate, 45-55° for thatch. Minimum pitches vary by material: 15-17.5° for profiled metal/modern tiles, 20-25° for slate, 35° for plain tiles. Regional traditions also influence pitch.

Roof spread (outward thrust on walls) is prevented by connecting opposing rafters with ceiling joists at wall plate level, or collar ties higher up. Alternatively, use a ridge beam supported at gable ends to eliminate thrust entirely. This is especially important for loft conversions where ceiling joists are removed.

Standard UK rafter spacing is 400mm centres for most tiled roofs, especially with heavier coverings like concrete tiles or slate. 600mm centres is acceptable for lightweight coverings and reduces material cost but requires larger timber sizes. 400mm is preferred as it provides better support for battens and felt.

Essential tools: Circular saw (185mm blade), roofing square or speed square, combination square, sliding bevel, steel tape (8-10m), chalk line, spirit level, claw hammer or nail gun. A mitre saw is helpful for accurate angle cuts. Safety equipment is mandatory: scaffolding, hard hat, eye protection, and non-slip footwear.

About This Calculator

Created by UK construction and carpentry specialists with combined 35+ years experience in residential roofing, timber frame construction, and Building Regulations compliance. Our team includes CITB-qualified carpenters, roofing contractors, and building surveyors who have worked on projects ranging from domestic extensions to large commercial developments.

All calculations comply with Building Regulations Approved Document A (Structure) and are based on TRADA span tables for strength-graded softwood timber. Timber sizes and spans verified against C16 and C24 grading specifications. Loading assumptions based on typical UK conditions including snow loads and wind exposure.

While this calculator provides accurate results for standard roof configurations, always consult Building Control or a structural engineer for complex roofs, unusual loadings, or spans exceeding table limits. Safety guidance aligned with HSE Working at Height Regulations.

Last updated: 14 February 2026 with current Building Regulations requirements, timber specifications, and span table data.

Reviewed by: UK Calculator, Founder & Developer

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