Flat Roof Membrane Calculator — m to rolls, adhesive, cost
How to use this calculator
Enter the flat roof footprint in metres — length × width, or switch to Area if you already have a take-off. Pick the membrane, the roll size your supplier stocks, and the seam overlap from the manufacturer's data sheet. The output drives your material order: rolls, pails of bonding adhesive, edge trim lengths, and the fastener count for mechanically attached or fully adhered details.
Membrane types — EPDM (rubber), TPO (heat-welded), PVC (chemical resistant), Modified bitumen (torch/cold-applied).
Overlap — 75–150 mm depending on membrane type. Select your currency for cost estimates.
Saved Calculations
| Time | Type | Area m² | Rolls | Adhesive | Edge | Total |
|---|
How to Calculate Flat Roof Membrane (Metric)
Single-ply membrane is supplied in rolls, so the cut count rarely matches a clean area divide. This tool lays the roof out as parallel strips across the short side: each roll covers its full length but only its width minus the side-lap, so a 3 m roll with a 100 mm lap nets about 2.9 m of usable width. End-laps, corners and upstands are covered by the waste slider — keep it near 10% for a plain rectangle and 15% for roofs with many penetrations or a tapered insulation scheme.
Adhesive is sized for a fully adhered build-up at roughly one litre per 10 m² of bonded surface; a contact-bonded EPDM job coats both faces, so raise the waste figure for that. Mechanically attached TPO/PVC uses far less adhesive but about ten fasteners-and-plates per m² across seams and field, so the fastener line tracks area. Edge trim (drip, gravel stop or coping) is counted along the perimeter in 2 m stock lengths, and prices accept any European currency.
How to use
Pick L×W for a plain rectangle or switch to the Area tab if you already have a take-off. Choose the membrane and the roll size your supplier stocks, set the seam overlap from the manufacturer's data sheet, then open the Cost and Insulation panels for a full material total. Size the roof surface first with the roof area and pitch calculator and detail the perimeter drainage with the gutter and downspout calculator.
Formulas
Effective coverage = (roll width − overlap) × roll length. Rolls = ⌈area × (1 + waste%) / effective coverage⌉. Adhesive = area / 10 m² per litre for fully adhered, in 20 L pails. Fasteners ≈ 10 per m² for mechanically attached systems. Edge trim = ⌈perimeter / 2 m⌉.
FAQ
What overlap for TPO/PVC? Hot-air welded seams typically require 75–100 mm overlap. Modified bitumen uses 100–150 mm. Always follow manufacturer specifications per EN 13956.
Do I need insulation? EN standards require thermal insulation below the membrane; typical boards are 1.2 × 2.4 m polyiso or EPS.
Pitched roof instead? This tool is for flat and low-slope decks. For pitched roofs use the metal roofing calculator or the shingles calculator, and frame the deck with the deck and joist calculator. Working in feet? Use the imperial flat roof membrane calculator.
On install day
Three things crews forget on a single-ply tear-off. Edge trim and termination bar first — order drip, gravel-stop or coping in 2 m stock with a matching cleat; budget €8–15/m installed in 2025 and double the perimeter quantity for parapet caps. Match the fastening method to the deck — fully adhered EPDM/TPO runs roughly €70–110/m² installed, mechanically attached €50–80/m², ballasted €40–60/m² but only on a deck rated for the 50 kg/m² stone load (EN 1991-1-4 wind uplift check still applies). Don't skip the cant strip and pipe collars — every penetration adds 15–30 min of detail work and a pre-moulded boot (€20–40 each) outlasts a field-welded patch. Code reference: EN 13956 (TPO/PVC) and EN 13707 (modified bitumen) set seam width and adhesion peel-strength — verify with the manufacturer's warranty sheet, not just the harmonised standard.
