How to Calculate the Area of a Flat Roof in Square Metres

When you’re starting your project, knowing how to calculate the area of a flat roof accurately in square metres is perhaps the most important part of the process. Most of the materials will be ordered based on these vital measurements, so getting it right is key to the overall success or failure of the project.

At PermaRoof UK, we’re here to help – with practical, expert advice, tools and resources, kits and products developed to help your flat roof projects run smoothly from start to finish.

The importance of accurate measurements

Accurate measurements are central to the planning and execution of successful flat roofing. You use them to calculate the area of a flat roof in square metres, which then forms the foundation of the entire materials list. When you consider that some membranes are cut in a single piece and adhesives coverage is quantified in square metres, it is easy to see how getting the measurements wrong can have a detrimental knock-on effect.

Measuring at least twice, using our simple guidelines, can help to avoid under and over-buying mistakes. This is standard practice for trade roofers and for our operatives, who always measure twice – and then get a second opinion – before cutting EPDM for our roofing kits range.

We’ve produced a handy guide to help measure flat roofing accurately – download your free copy here.

Step one: How to produce accurate measurements

It is not essential to access the flat roof for measuring purposes. As long as you have ground access to the outside of the structure on two sides – width and length – you’re good to go without a ladder.

Having a second person present is the ideal approach; to hold tapes in place, double-check and write down the measurements. Use a long measuring tape, as measuring in one go can reduce the risk of miscalculations.

From ground-level and on the outside of the building, take your time and carefully measure the full length (longest side). It is important to measure the outer walls and not from the inside, where the wall depth is not considered. Measure twice, double-check and then write down this measurement.

Repeat on the shorter side to get the width measurement. Go back and check both width and length measurements – or at this point, get a second person to measure and compare the results. These two measurements are all you need to move on to calculating the area in square meterage and ordering your flat roof materials.

Step two: Calculating the area of the flat roof

What you do with your measurements next is up to you. You can manually work out the square meterage, use them in a flat roof calculator tool or use them to buy a roofing kit.

Manual calculation

To work it out manually, use a calculator or ready reckoner for accuracy. Simply multiply the length measurement in metres by the width measurement in metres and the resulting answer is your square meterage.

Example:

Length = 5 metres
Width = 4 metres

5 (length) x 4 (width) = 20 square metres

Using a flat roof calculator app

A flat roof calculator tool can be used to build a roof kit list and needs only the length and width measurements to do so. You can add specific details about the project, such as adjoining buildings, skylights or flues, gutters and drains and other details. This can help if you need to tailor a more comprehensive list of materials that includes edge trim and install accessories, for example.

Our PermaRoof Kit Builder app is a free tool, popular for trade roofers and available to use online with no sign ups or obligation to buy materials. You can choose whether you’d like to print, save or get a quote for your materials at the end. There are only four steps and it is fast and easy to use – give it a try here.

Buying flat roof materials

So, you have accurate measurements, have calculated the area of the flat roof, so no have everything you need to choose and buy your materials. Let’s look at a few options:

Rubber roofing membranes

EPDM membranes are sold typically by the metre and RubberCover EPDM can be cut to size, so that they can be installed in a single sheet. This is good news for DIY roofing projects as it cuts out the need for joins and makes the install literally seamlessly reliable.

It requires just a few simple tools to install and is straightforward for most domestic projects, such as DIY shed or garage roofing.

Liquid PU roofing kits

Liquid PU roofing is growing in popularity for a wide range of projects. In addition to seamless waterproofing, it provides the base for anti-slip surfacing and can be used to encapsulate almost any surface.

We sell our P100 roofing kits in two standard sizes for 5m2 and 10m2 projects in our online store.

Standard, off-the shelf EPDM kits

Both EPDM and liquid roofing systems can be bought in standard-sized kits that include everything in the right quantities. Our flat roof kits are among our most popular product ranges for both our trade and DIY customers – for this very reason. We also offer a bespoke design service for non-standard or more complex projects or you can build your own kit in our online store or through the Kit Builder App.

We can custom build complete kits to include a single sheet of RubberCover EPDM from 8m2 up to 900m2 (15m x 60m).

A note about buying adhesives and liquid roofing

Most roofing adhesives and liquid roofing system coverage rates are estimated in square metres and this is what you use to calculate how much you’ll need. You will find the estimated coverage rates in the product descriptions online or on the tins if you’re shopping instore.

Example: Based on an average coverage rate of 4.5 square metres per litre (Elevate RubberCover bonding adhesive) and our project measurement of 20m2 in total.

20 (area of the flat roof in m2) ÷ 4.5 (coverage rate per litre) = 4.44 (litres needed)

Related products to buy online:

We hope we have helped you with how to calculate the area of a flat roof accurately. We’re here to help with expert advice and are happy to answer any questions you might have 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.

This is an updated version of our popular blog post from 2022. Read the original version here.

Other articles about flat roofs:

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