Ask the Experts: How to Calculate a Flat Roof Area

There is a 2024 updated version of this popular blog post, read it here.

‘How to calculate a flat roof area in square metres” is a commonly asked question at PermaRoof UK. This is one of the most important steps to get right in any roofing project. We’re here to help from the very start with advice about measurements, preparation and installation of your flat roof, in addition to a high quality range of products.

The importance of accurate measurements for flat roofing

When you begin planning any flat roof project, making sure your measurements are accurate is a vital part of the process, as getting this part wrong can have a knock-on effect on the entire project.

Most kits and rubber membranes are sold by the metre and adhesive coverage estimated in square metres – and you’ll ideally have both these sets of measurements to order your materials. To avoid over- or under-estimating your kit list and buying the wrong quantities, measure twice, following our guidelines below. You may even want a second opinion from someone else – just to be certain!

EPDM rubber membranes can be installed in a single sheet, which minimises joins and makes the system fast and easy to install for a DIY enthusiast. This is another big reason to get the measurements right first time.

Before we get into the basics, you can read or download a copy of our handy guide below (PDF)

Further reading:

Step 1: Measuring the flat roof

Tip: Use a long measuring tape – measuring in one go with a single tape can minimise the risk of miscalculating the area

You do not have to access the flat roof to measure accurately. On the ground, carefully measure the full length of the structure from the outside. Measuring from inside can give a false, too small measurement, particularly if your structure has brick or stone-built walls.

Make a note of the length measurement and then measure the width of the structure, noting that down too. Go back and measure both length and width for a second time, comparing your noted measurements for accuracy. Once you have two, double-checked numbers, you can move on to calculating the total roof area.

Step 2: Calculating the roof area in square metres

Now you have your accurate measurements, you can manually work out the flat roof surface area. This is a simple calculation that you can do on paper before you order your EPDM rubber membrane. If you would like to use your measurements to calculate a flat roof kit, which includes all the installation essentials, you can use an app to do it for you – more about that a little bit later…

Manual working out

Use a calculator or ready reckoner for accuracy and multiply the length measurement by the width measurement in metres.

Example

• Length = 4 metres
• Width = 3 metres

4 (length) x 3 (width) = 12 square metres

Using a flat roof calculator

A flat roof calculator is a fast and simple way to calculate a flat roof area, while simultaneously building a materials list. In addition to measurements, you can input specific project details into the PermaRoof Kit Builder, which helps to tailor a more comprehensive list. These can include adjoining walls or buildings, extrusions (skylights, flue pipes), gutters, drainpipes and other detailing.

You can choose whether you’d like to print, save or get a quote for your completed list at the end (no obligation to purchase).

Try the free PermaRoof UK Kit Builder

How to buy flat roof materials

As we’ve previously mentioned, EPDM rubber membranes are typically sold by the metre for projects. We stock both standard (1.14mm) and premium (1.52mm) Firestone EPDM membranes and SealEco fleece backed EPDM in our online store by the metre or by the roll.

For ease of ordering, our online store offers our customers the option to add your width and length measurements on the product page. We can custom-cut Firestone EPDM in sheets measuring from 8m2 up to 60m x 15m, which means that most average domestic flat roofing projects can be typically completed without joins, so this is why your measurements need to be accurate.

Square meterage is the common estimate for adhesives and liquid roofing coverage rates. You can refer to this estimate when buying adhesives – compare with your project measurement (square metres) to determine how much you will need.

Example:

We stock Firestone water based adhesive for EPDM roofing onto timber decks. The average coverage rate is up to 4.5 square metres (m2) per litre.

My project = 12m²

12 (m2) ÷ 4.5 (approx. coverage per litre) = approximately 2.66 litres needed

Related products to buy online:

Buying a flat roof kit

A flat roof kit is ideal for DIY projects, simplifying the materials order with everything included. With several types of kits available in standard sizes for sheds and garages, for example, you only need your measurements to order everything.

We stock a wide range of standard roofing kits and have an option for you to enter your own size (again using length and width measurements). For more complex flat roof projects, we’d recommend using the Kit Builder App online or calling us for help with your order.

We hope we have helped you with how to calculate a flat roof area accurately. If you have a further question about preparation, installation or any other aspect of your flat roof project, give our expert team a call on 01773 608 839 or use the live chat for a quick query.

Other articles about flat roofs:

PermaGroup has established links with some of the biggest names in our industry thanks to our reputation of excellence. Read more about becoming a supplier or stockist here.