Roofing Calculator
Calculate roof pitch, rafter length, roof surface area, quantities and complex pricing. No signup required - works on mobile and desktop.
Roof Area Calculator
Calculate actual roof surface area from plan dimensions and pitch
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Roofing calculation tips
How to measure roof pitch on-site
Use a digital level or smartphone app placed on the roof surface to get a direct degree reading. Alternatively, measure 1 metre horizontally from the roof edge, then measure the vertical rise at that point. The arctangent of rise / run gives you the pitch in degrees.
When to use rafter vs hip/valley pitch factors
Use the rafter pitch factor for simple gable or lean-to roofs where the slope runs in one direction. Use the hip/valley factor for hipped roofs where the slope changes direction - this includes the compound angle that increases the actual surface area.
Common waste percentages by material
Concrete tiles: 5-10%. Clay tiles: 10-15% (fragile, more breakage). Metal sheets: 5%. Asphalt shingles: 10-15%. Membrane: 5%. Add an extra 5% for complex roof shapes with many valleys, hips, or dormers that require numerous cuts.
Why plan area differs from actual roof area
Plan area is the footprint of the building viewed from above. A pitched roof covers more surface than the flat plan because it slopes upward. The pitch factor (1 / cos(pitch)) accounts for this difference. At 25 degrees, a 100 m² plan has about 110.3 m² of actual roofing surface.
Accounting for complex roof shapes
For roofs with multiple sections (L-shaped, T-shaped, with dormers), calculate each rectangular section separately using the same pitch, then add the areas together. For triangular sections (hips), use base × height / 2 with the pitch-adjusted height.
When to add extra material for cuts and overlaps
Beyond the standard waste percentage, add extra material for roofs with many penetrations (chimneys, skylights, vents), complex valley junctions, or when using materials that require specific overlap patterns. A rule of thumb: add 2-3 extra units per penetration or junction.
Formulas used
Rafter length
rafter = span / cos(pitch°)
Rafter pitch factor
factor = 1 / cos(pitch°)
Hip/valley factor
factor = sqrt((1/cos(pitch°))² + 1)
Roof surface area
area = plan_area × pitch_factor
Material quantity
quantity = (area × (1 + waste%)) / coverage_per_unit
Worked example: 10m × 8m garage roof at 35° pitch
- 1Plan area = width × length = 10m × 8m = 80 m²
- 2Pitch factor = 1 / cos(35°) = 1 / 0.8192 = 1.2208
- 3Actual roof area = 80 m² × 1.2208 = 97.66 m²
- 4Rafter length = span / cos(35°) = 5m / 0.8192 = 6.10m (half the building width as span)
- 5With 10% waste: 97.66 × 1.10 = 107.43 m² of material needed
- 6At £2.50/m² for concrete tiles: 107.43 × £2.50 = £268.58 material cost
Assumptions & limitations
- Calculations assume a single-pitch or gable roof. Complex roofs (hip, valley, dormer) require additional material for junctions.
- Pitch factor uses 1/cos(θ) for rafter-type slopes. Hip/valley factors use a compound angle formula.
- Waste percentages are guidelines only - always refer to manufacturer recommendations for your specific material.
- Batten calculations assume a mono-pitch roof and do not include extra battens for hips, valleys, or ridge lines.
- All measurements are estimates. Verify on-site before ordering materials.
When to ask a professional
This calculator provides estimates for planning purposes only. For structural roof design, load calculations, building regulations compliance, or roofs with complex geometry (multiple pitches, curved sections, or unusual materials), consult a qualified roofing contractor or structural engineer. Building control approval may be required for roof modifications in the UK.
Frequently asked questions
How do I calculate roof pitch?
Roof pitch is measured in degrees from horizontal. Use a digital level or smartphone app on the roof surface, or measure the rise over run ratio and convert to degrees using: pitch = arctan(rise / run).
What is a pitch factor?
A pitch factor converts flat plan area to actual sloped roof area. For a rafter-type roof, the factor is 1 / cos(pitch angle). At 25 degrees, the factor is approximately 1.103, meaning 100 m² of plan area has about 110.3 m² of actual roofing surface.
How is rafter length calculated?
Rafter length equals the span divided by the cosine of the pitch angle. The span is the plan-view distance from the wall to the ridge (one rafter). For a 10m span at 25 degrees: rafter = 10 / cos(25) = 11.03m.
What waste percentage should I add for roofing materials?
Typical waste percentages: concrete tiles 5-10%, clay tiles 10-15% (fragile), metal sheets 5%, asphalt shingles 10-15%, membrane 5%. Add more for complex roof shapes with many cuts.
What is the best free roofing calculator?
The best free roofing calculator handles pitch, rafter lengths, hip/valley lengths, surface area, waste, and material pricing in one place - exactly what this tool does, completely free with no signup. All calculations run in your browser and no data is sent anywhere.