Flat Roof Membrane Calculator
Last reviewed How we calculate
A flat roof is covered with a membrane rather than shingles, usually EPDM rubber, TPO or PVC. The membrane has to cover more than the roof itself, because it turns up walls and parapets and down over the edges. This works out how big a sheet that takes.
Membrane sheet size for a flat roof
The size of EPDM, TPO or PVC sheet a flat roof needs once the membrane runs up walls and over the edges.
This calculator needs JavaScript to work. The measurements below are the ones you would need to do the sum by hand.
Membrane needed As a single sheet. Seamed rolls need extra for every overlap.
- Sheet length
- Sheet width
- Flat roof area
- Roofing squares
- Perimeter, for edge trim
EPDM often comes in sheets wide enough to cover a small roof in one piece. TPO and PVC come in narrower rolls that are welded together, and every seam adds an overlap this figure leaves out. Drains, pipes and rooflights each need their own flashing pieces. Treat this as the sheet size to ask a supplier for, and let them confirm the layout.
How the calculation works
The edge allowance is added to both ends of the length and both sides of the width, and the two are multiplied. The allowance is the distance the membrane travels up a wall or over the edge trim, plus a little to trim. The perimeter is given as well, because that is the length of edge trim or termination bar the roof needs.
A worked example
A 30 ft by 20 ft roof is 600 sq ft. With 12 in extra at each edge, the membrane needs to be 32 ft by 22 ft, which is 704 sq ft, about 17 per cent more than the roof. The perimeter is 100 ft.
One sheet or several
EPDM is made in large sheets, and a small roof can often be covered in a single piece with no seams. TPO and PVC come in narrower rolls, so most roofs are made from several strips welded together, and each seam needs an overlap this figure does not include. The manufacturer details set the overlap and the way each edge is finished.
What a flat roof actually needs
No roof should be dead flat. Building codes typically require a minimum slope, often a quarter of an inch per foot, so water drains, and ponding water shortens the life of any membrane. If water sits on your roof for days after rain, the fall needs fixing before a new membrane goes on, and that is a question for a roofer rather than a calculator.
Frequently asked questions
Which is better for a flat roof, EPDM, TPO or PVC?
Each has its place. EPDM rubber comes in large sheets that can cover a small roof without seams; TPO and PVC come in narrower rolls with welded seams and light, reflective surfaces. Local installer experience matters as much as the material.
How much should membrane seams overlap?
The membrane manufacturer sets the overlap for each product and seam type. Add it to the sheet sizes for every seam, because the calculator figure assumes a single sheet.
Does a flat roof need any slope?
Yes. Building codes typically require a minimum slope, often a quarter of an inch per foot, so that water drains. Water that sits on a membrane for days shortens its life.