How to Measure a Flat Roof Accurately

When you begin any roofing project, making accurate measurements is a necessity, to avoid buying the wrong quantities of materials.

When these types of mistakes are made, it can lead to overbuying or underbuying – and neither scenario is a good one in roofing.

It is therefore critical to ensure you measure your flat roof accurately, and once you are satisfied you have the right measurements, go ahead and measure again to be absolutely sure!

You could also have another person measure too, just to be certain.
So, how do you measure a flat roof accurately? It really isn’t that difficult once you know how…

Using a long measuring tape, carefully measure the full length of the structure from the outside of the building. Then measure the full width.
NOTE: you don’t have to be on the flat roof to do this accurately, you can measure on the ground.

Measure from outside the building to give an accurate representation. Depending on the thickness of your walls, measuring from the inside could give you a false measurement that is too small.

Once you have your two accurate (and double-checked) measurements, you are ready to move on to calculating the size of membrane you will need.

Measure a flat roof: calculating the surface area

There are a couple of ways you could tackle this calculation of surface area and membrane size.

Manual working out

Using a calculator or ready reckoner, multiply the length measurement you took (in metres), by the width measurement (in metres).
E.G. length 4m x width 3m = 12 square metres.

This will give you the total square meterage of the flat roof, which will enable you to order your membrane.

NOTE: most single-ply flat roof membranes are sold in square metres.

Permaroof UK Ltd sell a wide range of flat roof kits available in standard sizes for sheds, garages and other domestic buildings.

Buying a standard shed roof kit is easy, just using the measurements taken.

Bespoke kits can also be tailored to your requirements. The quantity of adhesive is calculated for customers, based on the size of the EPDM flat roofing membrane required.

Use a flat roof app

A flat roof app is probably the simplest way to calculate roof materials from accurate measurements. Using an app gives the additional benefit of working out adhesives and accessories, based on specific details about your project, such as:

  • Gutters
  • Drainpipes
  • Roof extrusions – skylights, flue pipes etc
  • Edge trims
  • Upstanding and adjoining walls
  • Adjoining buildings


Permaroof offer free tools that also enables trade roofers and homeowners to send off the materials list via the portal or email to receive a quotation.

The Permaroofkit app is a free, online easy-to-use EPDM kit builder that works out the quantities and builds your EPDM kit for you in just four steps. All you need is your measurements and details about the flat roof, such as whether it adjoins another building, or has any features (skylights, flue or drain pipes).

Try the Permaroof Kit Builder app online now at:

If you weren’t confident before, simply by following the instructions above and using an app for your materials list, finding how to measure a flat roof should now be easy.

Permaroof UK Ltd offers industry-leading EPDM rubber roofing application training courses for trade flat roofers and homeowners at our purpose-built training centre located at our headquarters in Derby.

Find out more about EPDM training at: and book your training course online at:

Buy EPDM rubber roofing in our online store at:

You can download a comprehensive installation guide and individual tutorials in the resources section of our site at: