PROFESSIONAL ROOF ESTIMATOR
Calculate sloped area, covering, underlay, battens, ridge, fixings and drainage.
BUILD MEASURE HELPER
Calculation summary
Planning estimate only. Verify product specifications, site conditions and purchase quantities before ordering.
DIY complexity
Roofing combines significant fall-from-height risk with waterproofing that must be right the first time — a leak from a DIY mistake can cause hidden damage for months before it's noticed, and many regions require permits or professional sign-off.
This calculator estimates the covering, underlay, batten and accessory quantities needed for a pitched roof, from a simple mono-pitch or gable to a hip roof with valleys. It works from the plan area you measure on the ground and the roof pitch, because the actual sloped area a roofer covers is always larger than the flat footprint. Supported project types include re-roofing, new-build roofs and outbuilding or garage roofs, using concrete or clay tiles, steel or fibre-cement sheets, or slate. Beyond the covering material itself, the tool accounts for side and end overlaps, ridge and hip lengths, valley flashing, underlay membrane, battens and guttering. Results are given in both metric and imperial units, and the number of tiles, sheets or bundles is rounded up to whole purchase units after your chosen waste allowance. The calculator cannot measure irregular roof geometry for you, verify structural loading, or replace a roofer's survey — openings such as chimneys and skylights, and complex junctions, still need to be measured and entered manually.
Quick answer
For a 20 m² roof plan area at a 30° pitch with 10% waste, the sloped covering area is about 25.4 m² (20 ÷ cos 30° × 1.10). The final number of tiles, sheets or shingles to purchase depends on the effective coverage of the specific product selected, plus separate ridge, hip, valley and eave accessories.
sloped_area = plan_area ÷ cos(pitch)The sloped area is always larger than the flat plan area because the roof surface is tilted. Dividing by the cosine of the pitch angle converts a horizontal footprint into the true inclined surface a roofer measures.
Rounding rule: Sloped area itself is not rounded; rounding only happens once the area is converted into whole covering units.
Limitation: Assumes a uniform pitch per roof face; complex curved or multi-pitch faces must be measured and entered as separate zones.
units = ceil(sloped_area × (1 + waste) ÷ effective_coverage)The theoretical sloped area is inflated by the waste percentage, then divided by how much surface one tile, sheet or shingle effectively covers once overlaps are accounted for, and rounded up to a whole unit.
Rounding rule: Always rounded up (ceiling) — a roof cannot be covered with a fractional tile or sheet.
Limitation: Effective coverage varies by manufacturer and by minimum headlap required for the local wind and rain exposure; use the value from the selected product's data sheet, not a generic default.
A small garden outbuilding with a symmetrical gable roof: 20 m² plan area, 30° pitch, concrete tiles with 0.10 m² effective coverage per tile, no openings, 10% waste.
sloped_area = 20 ÷ cos(30°) = 23.09 m². With 10% waste: 23.09 × 1.10 = 25.4 m². units = ceil(25.4 ÷ 0.10) = 254 tiles.
23.09 m² is the theoretical sloped requirement; 25.4 m² is the quantity including waste; 254 tiles is the final purchase quantity. Ridge tiles for the ridge length and gable-end verge trim must be added separately.
⚠ Effective coverage of 0.10 m² per tile is an editable example, not a universal constant — always use the coverage stated on the selected product's data sheet.
A house re-roof: 60 m² plan area across four hip faces, 25° pitch, clay tiles at 0.09 m² effective coverage, one 2 m² skylight opening, 12% waste to allow for the extra hip cuts.
sloped_area = 60 ÷ cos(25°) = 66.2 m². Net after skylight: 66.2 − 2 = 64.2 m². With 12% waste: 64.2 × 1.12 = 71.9 m². units = ceil(71.9 ÷ 0.09) = 799 tiles, plus hip and ridge accessories calculated from the hip and ridge lengths.
The skylight is subtracted before waste is applied, but the extra waste percentage itself already accounts for the additional hip cutting this roof shape requires. Hip and ridge tiles are purchased as separate linear items, not folded into the 799 field tiles.
⚠ Hip and valley junctions consume more cut tiles than a straight gable; if the calculator's default waste feels low for a complex roof shape, increase it rather than trusting a single universal percentage.
A 6 × 4 m mono-pitch roof (24 m² plan area) at 12° pitch, steel sheets 1.8 m² effective coverage each, sold individually, plus underlay sold in 75 m² rolls at a local price, 8% waste.
sloped_area = 24 ÷ cos(12°) = 24.5 m². With 8% waste: 24.5 × 1.08 = 26.5 m². units = ceil(26.5 ÷ 1.8) = 15 sheets. Underlay: ceil(26.5 ÷ 75) = 1 roll (covers the whole roof with surplus).
Purchasing 1 whole underlay roll for a 26.5 m² requirement leaves roughly 48.5 m² unused — this is a normal consequence of buying in whole rolls, not a calculation error. The main cost driver here is the 15 sheets; the underlay roll and battens are comparatively minor line items.
⚠ Changing the selected currency only changes the display symbol — it does not convert the price you entered. Enter a current local supplier price in the currency you have selected.
The result panel separates the plan area you measured, the theoretical sloped area, the sloped area including waste, and the final whole-unit purchase quantity — each figure is larger than the last for a reason, not a rounding inconsistency. Accessory quantities (ridge, hip, valley, underlay rolls, battens, gutters) are shown as their own line items because they follow different formulas and purchase units than the field covering. Where a price is entered, the material cost and accessory cost are totalled separately before a combined estimate, so it stays clear which part of the budget covering versus accessories represent.
Roof covering dimensions, minimum pitch and required overlaps vary significantly by manufacturer and country — always take effective coverage and minimum pitch from the selected product's technical data sheet rather than a generic assumption. Metric markets typically size tiles and sheets in millimetres and coverage in m² per unit; imperial markets often price roofing in squares (100 sq ft). Local wind and snow-load rules can require additional fixings, higher headlap or different battens than a mild-climate default. Gutter and downpipe sizing follows local rainfall intensity standards, not a single international figure. Selecting a different currency changes the displayed symbol only — enter a current local supplier price for an accurate cost estimate, and confirm applicable sales tax separately since it is not built into the calculator's cost output.
This calculator produces a planning estimate, not a structural design, building-permit submission or contractor quotation. It does not verify rafter or truss loading, wind uplift resistance, fire performance, condensation control or compliance with local building regulations. Complex geometry — curved roofs, multiple intersecting pitches, dormers — should be broken into separate measured zones or verified with a roofer's survey. Always confirm final quantities, minimum pitch and fixing specification against the selected product's current technical data sheet before ordering.
Last reviewed: 2026-08-24
Because the roof surface is tilted. Dividing the flat plan area by the cosine of the pitch angle converts it into the true inclined surface a roofer actually covers — the steeper the pitch, the bigger the difference.
The sloped-area formula applies per roof face; hip and valley lengths and their accessory quantities are calculated separately from the geometry of the selected roof type, not folded into the field-tile count.
It is the usable surface one tile, sheet or shingle contributes once required overlaps are removed — always smaller than its physical size. Using the physical size instead of effective coverage will understate the number of units needed.
Roofing waste also has to cover cuts at hips, valleys, verges and around penetrations, which consume more material per metre than the cutting waste on a flat floor or wall, so roofs typically use a higher default percentage.
No. They are purchased as separate linear items calculated from ridge and hip length, because they are different products with their own unit coverage.
Yes, as long as you enter the correct effective coverage, overlap and minimum pitch for the selected material from its manufacturer data sheet — the geometry formulas are the same regardless of covering material.
No. Changing currency only changes the display symbol and formatting; enter a current supplier price in the currency you select.
No. It is a planning estimate. Verify minimum pitch requirements, wind and snow loading, local building regulations and the manufacturer's installation instructions before ordering or applying for approval.