Rafter Length Calculator

Last updated: June 2026

Horizontal run

Roof pitch

degrees

Overhang allowance

Rise

1.73 m

Rafter before overhang

3.46 m

Cut length estimate

3.76 m

Formula

rafter length = horizontal run / cos(pitch angle). Rise = tan(pitch angle) x run. Add overhang allowance separately.

About this calculator

The Rafter Length Calculator estimates roof rafter length from horizontal run, rise, pitch angle, and overhang. It is useful for checking small roofs, sheds, pergolas, and early roof-material estimates.

Rafter Length Calculator method

The calculator uses metric project dimensions, applies the material formula shown below, and then adds any waste or allowance entered. Quantities are rounded up where materials are normally purchased as whole boards, sheets, blocks, rolls, bags, or packs.

rafter length = square root(run squared + rise squared)
rise = tan(pitch angle) x run
total length = rafter length + overhang allowance

How to use the Rafter Length Calculator

Enter the horizontal run from wall plate to ridge or support point.
Enter rise or pitch angle depending on the information you have.
Add eaves or overhang allowance if required.
Review the slope length before selecting stock timber lengths.
Compare with roof pitch and roof area calculations for covering materials.

Worked examples

Lean-to roof

Input: 2.4m run, 0.8m rise, 0.25m overhang

Calculation: sqrt(2.4 squared + 0.8 squared) + 0.25

Result: The calculator gives the sloping rafter length including overhang.

Pitch angle known

Input: 3m run, 30 degree pitch, 0.3m overhang

Calculation: Rise is derived from tan(30 degrees) x 3m before calculating length.

Result: The result helps select a practical timber length before cutting.

When this calculator is useful

Use it before ordering materials, comparing supplier pack sizes, or checking whether a first estimate is realistic. Keep the result with your measured dimensions so you can update the calculation if the project size changes.

Inputs that affect the result

Run vs span: Run is normally half the total span for a simple pitched roof, not the full building width.
Overhang: Eaves, birdsmouth cuts, fascia details, and trimming can change the final cut length.
Structure: Roof framing can be structural and may need proper design or building control input.

Common mistakes to avoid

Check 1: A complex roof with hips, valleys, dormers, or unequal pitches needs more detailed setting out.
Check 2: Timber sizes, spacing, and fixings are not calculated here.
Check 3: Measure along the correct roof plane before ordering materials.

Material planning notes

Planning point	Why it matters
Measure finished area	Use the actual finished dimensions after allowing for openings, edges, posts, slopes, or returns.
Allow for waste	Cuts, breakages, joins, pattern matching, awkward shapes, and site handling usually mean ordering more than the exact calculated amount.
Round purchase quantities up	Most materials are bought as full boards, sheets, bags, blocks, rolls, packs, or bulk deliveries.
Check supplier specifications	Coverage, density, sheet size, block size, and pack size vary by product and supplier.

Limitations and safety

This calculator is for general project planning only. It does not replace a structural design, building regulations advice, manufacturer instructions, supplier specifications, or a competent tradesperson. Projects involving structure, roofs, retaining walls, foundations, drainage, electrics, gas, or safety-critical work should be checked properly before buying materials or starting work.

Frequently asked questions

Should I order exactly the calculated quantity?

Usually no. Order in the nearest practical pack, board, sheet, bag, roll, or delivery quantity, and keep a sensible waste allowance for cuts and mistakes.

What waste allowance should I use?

Simple rectangular jobs may only need 5-10%. Patterned, angled, fragile, or irregular projects may need more. The right allowance depends on the material and layout.

Why might my supplier quantity differ?

Suppliers use specific product sizes, densities, coverage rates, and pack quantities. Use the calculator as a planning estimate, then compare it with the product label or datasheet.

Can I use this for professional building work?

It can help with a rough take-off, but professional work should be checked against drawings, specifications, site conditions, and relevant UK rules or standards.

What measurements should I double-check?

Check length, width, depth, height, spacing, openings, pitch, and units. A small unit mistake can change a material order by a large amount.

Related calculators

roof-pitch-calculator
roof-area-calculator
roofing-material-calculator

What does this mean?

This calculator is designed to help you understand the likely number before you make a decision or start an application.

Your result should be checked against official UK guidance, especially if your circumstances include dependants, exemptions, prior leave, or a complex immigration history.

Treat the figure as a planning tool rather than legal advice. Where the answer affects an application deadline or major payment, speak to an authorised adviser.