Flat roof calculator: EPDM sheet size and felt rolls
The calculator needs JavaScript. The worked example, formula and figures below show exactly how it works, so you can do the sum yourself. A guide, not advice. Check your supplier's or maker's figures before ordering.
Short answer
Worked example
| You enter | Value |
|---|---|
| Roof covering | EPDM rubber |
| Deck length | 3 m |
| Deck width | 2 m |
| Upstand or turn-down at each edge | 150 mm |
| Extra at each edge for the drip or trim | 50 mm |
Membrane sheet: 3.40 × 2.40 m
| Result | Value |
|---|---|
| Membrane sheet size | 3.4 × 2.4 m |
| Membrane | 8.16 m² |
| Deck area | 6 m² |
| Edge trim | 10 m |
Show the working
- Each edge: 150 mm upstand + 50 mm drip = 200 mm
- Sheet: (3 + 2 × 0.2) × (2 + 2 × 0.2) = 3.4 × 2.4 m = 8.16 m²
- Edge trim: 10 m perimeter = 10 m
How it works
- Sheet length = deck length + 2 × (upstand + drip); sheet width = deck width + 2 × (upstand + drip)
- Membrane (m²) = sheet length × sheet width (× layers for torch-on)
- Adhesive tubs = deck area ÷ coverage per tub, rounded up
- Edge trim (m) = deck perimeter − edges against a wall
- Rolls = membrane area ÷ (roll length × (roll width − side lap)), rounded up
Upstand and drip allowances are illustrative: use the membrane maker's details for your edges and upstands.
Roll counts are by area; plan the strips across the roof and allow for end laps as the maker shows.
Our default (adjustable)
These are typical planning figures and our own allowances, not facts about your product. Change any of them in the calculator to match your bag, pack or tin.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Upstand or turn-down at each edge | 150 mm | Our assumption Not yet checked against the source | Our choice: change it to suit your job brief v2 R5 (test FR1, illustrative: 150 mm upstand all round). Researcher to source from membrane makers. |
| Extra at each edge for the drip or trim | 50 mm | Our assumption Not yet checked against the source | Our choice: change it to suit your job brief v2 R5 (test FR1, illustrative: 50 mm drip). Researcher to source from membrane makers. |
Regulations and standards
- BS 6229 (flat roofs): falls and drainage, with the membrane maker's minimum fall
Our checks use the guidance for England unless a badge says otherwise. Wales, Scotland and Northern Ireland have their own rules: check the source for where you live.