Flat Roof Material Calculator

How the flat roof material calculator works

Enter the roof area, the coverage of one roll or kit from the product label, an allowance for laps and upstands, the membrane type and the price of one roll or kit. The calculator adds the allowance to the roof area to get the area to buy, divides by the coverage and rounds up to whole rolls or kits. It also shows the minimum fall a flat roof needs to drain.

The formula is N = ⌈A × (1 + o) ÷ C⌉, where A is the roof area, o the allowance as a decimal and C the coverage of one roll or kit. For example, a 215 ft² roof with a 10% allowance needs 215 × 1.1 = 236.5 ft² of membrane; with rolls covering 215 ft² that is 236.5 ÷ 215 = 1.1, so 2 rolls. In metric, a 20 m² roof with 10% needs 22 m², which is 2 rolls of 20 m².

Measuring the roof

Measure the flat area inside any upstands and add the upstands separately. An L-shaped roof is easiest split into rectangles: a 6 m by 4 m section and a 2 m by 3 m section make 24 + 6 = 30 m², and a 20 ft by 12 ft section with an 8 ft by 6 ft section make 240 + 48 = 288 ft². Measuring on a plan or from inside the room below avoids working at height.

The membrane types

EPDM is a synthetic rubber sheet and TPO a thermoplastic sheet; both are single-ply membranes. The US International Residential Code, as adopted in the Seattle Residential Code, chapter 9, covers them as thermoset and thermoplastic single-ply roofing, with EPDM to ASTM D4637 or D5019 and TPO to ASTM D6878 among others. GRP, glass-reinforced polyester, is built up on the roof from chopped strand glass-fibre mat wetted out with polyester resin and finished with a topcoat. As a supplier guide to GRP roof kits describes, it is sold as kits holding the mat, resin, topcoat and catalyst, so the result counts kits instead of rolls when GRP is chosen. For every type, the coverage figure to enter is the one on the product, since roll widths, lengths and kit sizes vary.

Laps, upstands and one-piece sheets

The allowance covers material that does not end up on the flat part of the roof: laps where two sheets join, and the membrane that turns up walls, kerbs and roof lights or dresses over the edge. The 10% default is a starting point for a simple rectangle. A roof with several walls, a roof light or a chimney needs more. To size the upstands directly, multiply the height the membrane turns up by the length of each wall: turning up 0.15 m against 14 m of wall adds 0.15 × 14 = 2.1 m² to buy.

Where a membrane is bought as one sheet cut to size, there are no laps to allow for. Enter the sheet area as the coverage and set the allowance to 0%, having already included the upstands and edge overhang in the sheet size.

Falls

A flat roof still needs a slope so that water runs off. The US code sets a design slope of at least 1/4 in per foot, a 2% slope, for both EPDM and TPO roofs (IRC R905.12.1 and R905.13.1). In the UK, BS 6229 defines a flat roof as one pitched no more than 10°, as the Liquid Roofing and Waterproofing Association guidance notes. The NHBC Standards 2025, chapter 7.1 suggest a design fall of 1:40 for membrane and liquid-applied waterproofing, to reach a minimum finished fall of 1:80. The LRWA explains the gap: structures deflect and building is never exact, so falls of 1:60 or 1:40 are designed to be sure of 1:80 on the finished roof.

These are the same kind of figure in different forms. A slope of 1/4 in per foot is 1 in 48, or 2.08%. A fall of 1:80 is 1.25%, about 0.15 in per foot, and 1:40 is 2.5%, about 0.3 in per foot. Across a 4 m roof, 1:80 is a drop of 4,000 ÷ 80 = 50 mm and 1:40 is 100 mm; across a 16 ft roof, 1/4 in per foot is a drop of 4 in.

Ordering

Rolls and kits are usually bought whole, so the count is rounded up. Because the number of rolls jumps in steps, check whether a different roll size gives a closer fit: two small rolls may cost less than one large roll plus an offcut. Adhesives, primers, trims, outlets and edge details are bought separately and are not included here.

What this tool does not do

It does not design the roof build-up, check insulation or ventilation, size outlets, or count accessories. Roof structure, falls and drainage on a new roof are design matters for a roofer or designer, and the membrane maker's instructions set the laps and details.

Using this calculator with other BuildMetricLab tools

The roof area calculator helps measure the roof, the gutter calculator sizes gutters for the water the roof sheds, and the liquid rubber calculator covers coatings for flat roofs. All BuildMetricLab tools run in your browser with no sign-up, and every formula is shown on the page so the maths can be checked.