EPDM rubber roofing is becoming increasingly popular due to its durability, ease of installation and efficiency in protecting your flat roof from the elements.

And if you too are a recent convert who’s seriously considering laying EPDM on your flat roof, you’ll want to know how to measure up, so you get just the right amount of EPDM materials.

You’ve come to the right place!

At Roofing Superstore, we’re going to walk you through calculating your roof area and getting ready to fit your new EPDM roof.

Let’s jump in.

Table of contents:

How do you calculate the area of a flat roof?

The first thing you need to do before buying your rubber roof kit is to carry out accurate roofing measurements.

It’s quite simple, actually. All you’re going to need is a ladder, a long tape measure and your phone (or a calculator, if you’re old school) for the final roof area calculation.

Here’s how you go about it:

Roofing Superstore EPDM

Take out your trusty ladder and carefully get up on the roof. Exercising full caution, measure the full length and full width of the roof from the outside of the walls.

This last bit is absolutely crucial, especially if you have thick exterior walls. If you don’t measure from the outside, you’ll get a smaller area and will probably end up buying insufficient roofing EPDM membrane. It’s best to measure twice, just to be on the safe side.

Once you have the accurate length and width of your flat roof in metres, simply multiply the two to get the total area.

For instance, if the full width is 5.5 metres and the full length is 4.5 metres, the total area would be 5.5m x 4.5m = 24.75m².

You want to use metres as your unit of measurement because most rubber roof membranes in the UK are sold in square metres.

Now that you know the total area of your flat roof, you’re ready to move on to the next step.

How is EPDM calculated?

If you’ve never worked with EPDM roofing membrane before, you might be tempted to think that you only need as much of it as is sufficient to cover the roof itself, right?


The membrane needs to overhang over the roof and gutter edges on all sides so that rainwater can flow freely across the roof and make its way straight to the drainage system.

The question is:

How much should a rubber roof overhang?

As with many DIY projects, the answer varies depending on whom you ask. Some manufacturers say as little as 75mm might be sufficient, while others suggest you go much higher at 200mm.

Unless a specific figure is given in the manufacturer’s fitting instructions, we suggest that you go for 100-125mm, which should be more than enough for most EPDM roof systems.

Once you’ve decided on the exact length of the overhang, you can use the following simple formula to calculate how much EPDM membrane you’re going to need:

(Full length of roof + overhang) x (full width of roof + overhang) = square meters of membrane materials needed

Let’s do a quick example, using the measurements we had earlier and the 125mm overhang we suggested.

(5.5m + 0.125m) x (4.5m + 0.125m) = 26.015625m²

So, for a roof with a total area of 24.75 square metres, you’re going to need about 26 square metres worth of EPDM membrane. You might want to buy a bit more, though, just in case.

Estimating other EPDM roof materials

The amount of EPDM membrane is the most important measurement for your project.

And if you’re buying a full kit – which is certainly the easiest way to go – everything else you need, such as bonding and contact adhesives, will be provided in the appropriate quantities.

If you’re buying adhesives separately, we recommend that you purchase from the same supplier and brand as the EPDM membrane, as they’re designed to work together.


Once you’ve settled on an option, make sure you check the product description carefully. That’s because the total coverage rates are likely to vary depending on the smoothness of your roof’s surface and the application method you’re going to choose.

Here’s a specific example to make things clearer:

Start right away with 4 months to pay Enjoy 0% over 4 monthly instalments* PayPal Credit * Subject to status. Minimum spend required. T&Cs apply. "Very easy and good to deal with." Angus MacDonald Contact Bonding Adhesive for Firestone

The solvent-based Bonding Adhesive BA-2012 by Firestone has a coverage rate of 2.35 to 3m² of surface (two sides) / litre if applied with a roller compared to a coverage rate of 2.75 to 3.50m² of surface (two sides) / litre if applied with SuperSpreader and sprayer.

See the difference? That’s why you always must read the small print.

Once you have the coverage rate, you can easily calculate how much adhesive will be required. Use the same method for any other EPDM roof materials you intend to use.

Ultimately, the best way to keep your rubber roof cost low and to avoid any unnecessary hassle is to buy a full kit. The same goes if maths isn’t your best suit.

The bottom line

Once you’ve decided to go down the increasingly popular EPDM rubber roofing route, you need to ensure you’re getting as many EPDM materials as you need – no more and no less.

This way, your next flat roofing project won’t break the bank.

The good news is:

Calculating the roof area is a breeze. And from there on, you only need to factor in the overhang to figure out how much roof membrane you’re going to need.

Make sure your measurements are accurate, and pretty soon you’ll have this covered! Yes, quite literally.

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