How much EPDM do I need? Measuring your flat roof

One of the best things about EPDM is that most domestic flat roofs can be covered with a single sheet, with no joins for water to find. To get that, you need to order the right size. Measure carefully, add the right amount for edges and walls, and the rest is simple.

Step 1: Measure the roof deck

Measure the length and width of the deck at its widest points, edge to edge. Take each measurement in two or three places, as older roofs are rarely square, and use the largest figure. Work in metres.

While you're up there, note:

• Which edges run to a wall (these need an upstand), and which are open edges with a drip or gutter.
• Any pipes, vents, rooflights or outlets coming through the roof.
• The deck material (OSB3, ply, or something else), as it affects which adhesive you need.

Step 2: Add 300mm to every edge

The membrane needs to be bigger than the roof so it can wrap over the edge trims and turn up the walls. Our rule is to add 300mm to every edge, which means adding 600mm to both the length and the width.

Example: a roof measuring 10m × 3m needs a membrane of 10.6m × 3.6m.

That allowance comfortably covers a standard 150mm upstand against a wall and the turn over a drip or gutter trim. Extra is trimmed off once the membrane is bonded, so it's always better to have a little too much than too little. If an upstand needs to go higher than usual, for example under a door threshold or to clear a raised wall detail, add the extra height on that edge.

Step 3: Match it to a roll width

EPDM comes in fixed roll widths and is cut to length. Pick the smallest width that covers your membrane width, so the narrower dimension runs across the roll. ClassicBond® 1.2mm comes in widths of 3.05m, 3.66m, 4.57m, 6.10m, 7.62m and 9.15m.

In the example above, a 3.6m width fits a 3.66m roll, cut to 10.6m long. One sheet, no seams.

Step 4: Work out the trims and accessories

Measure the run of each type of edge:

• Drip trim along open edges where water runs off.
• Gutter trim along edges that drain into a gutter.
• Wall trim or termination bar where the membrane turns up a wall.
• Corners: count internal and external corners separately.

Then the adhesives. Water-based deck adhesive covers the main area on a timber deck, and contact adhesive goes around the perimeter and on upstands. We'll explain the difference in a separate guide.

1.2mm or 1.5mm?

1.2mm is the standard choice for most roofs: garages, extensions, dormers, sheds, garden rooms and site cabins. 1.5mm is the heavier-duty option for commercial roofs and roofs that will see regular foot traffic. If you're not sure, ask us.

The quick way: use the calculator

Put your roof dimensions into our EPDM roof calculator and it works out the membrane size and lists every adhesive, trim and corner, with delivery to your postcode. If your roof is a standard size, our ready-made kits for porches, sheds, garages, dormers, extensions, garden rooms and site cabins have everything in one order.

Common measuring mistakes

• Measuring the inside of the room, not the deck. Always measure the roof itself, including any overhang.
• Forgetting the upstand. A membrane cut to the exact deck size can't turn up the wall.
• Measuring once. Out-of-square roofs catch people out. Measure both ends and the middle.
• Ignoring what comes through the roof. Pipes and outlets need pipe seals or outlet fittings, so list them before you order.

Got an odd-shaped roof or not sure about a detail? Send a sketch with measurements to hello@supraseal.com and we'll work it out with you.