The Ultimate Flat Roof Area Calculator

The working out can often be the hardest part of a DIY flat roof project. Accurate measurements are key to quantifying and whether you have a simple rectangle or a complex design, we’re here to help with our ultimate flat roof area calculator guide.

Typically, EPDM is quantified and purchased by project dimensions in metres and liquids by square meterage, calculated in conjunction with coverage rates. In this guide, we’ll cover multiple scenarios and help you quantify for EPDM membrane, adhesive and our liquid roofing system.

Measuring tips

Before starting, gather a few essentials:

• Long tape – the longest you can find, so that your measurements are accurate in a single take.
• Notebook and pencil – we recommend you do it the old-fashioned way and write the measurements down – it’ll be easier to work out.
• Ladder – access to the roof isn’t always necessary, but keep one ready, just in case.
• Ideally, another person to help with the tape, foot a ladder if needed and to double-check the measurements.

Access to the roof itself should only be necessary in some cases, for example:

• If the roof design is complex with multiple angles that cannot be accounted for on the ground.
• If access to the base of the building (on at least the longest and the shortest sides) proves difficult or impossible on the ground.
• If the flat roof has a projection that is wider than the building itself, in which case, the actual building perimeter will be smaller than the area of the flat roof.

If you’re planning the waterproofing on a new build project, then there may be architects’ drawings to refer to with accurate measurements already provided. If that’s the case, then you may want to skip the measuring part – but we’d always recommend a quick double check to be on the safe side if the building is up, safe and accessible.

How to calculate the flat roof area for EPDM membrane

Most of the measuring is done at ground level around the outside of the project building. Always measure externally to ensure the depth of the wall is accounted for. Measure and record measurements in metres and millimetres.

Note that standard, off-the-shelf shed and garage roof kits often also display the metric>imperial conversion in the UK for buying convenience.

Standard square and rectangular flat roof area calculator

Standard square and rectangular buildings are the most common in the UK and straightforward to measure, so we’ll start with those. Write down each exact measurement, then measure again to ensure accuracy.

• Square projects – a building that is square and equal on all sides with 90-degree angles needs to be measured on just one side only (if you’re confident that it is truly square).
• Rectangular roofing – from ground level, carefully measure the external length of the longest side of the building. Then measure the width (the shortest side).

These two measurements – length and width – will be all that you need to order EPDM membrane or an EPDM flat roof kit for square and rectangular projects.

Measuring a more complex flat roof

When dealing with a flat roof that has a complex design (e.g., multiple sections, irregular shapes), measuring its area accurately for waterproofing can be challenging. At this point, it will be necessary to decide whether you are happy to join pieces of EPDM where necessary or if want a seamless installation.

Calculating the roof area for coverage with a single sheet of EPDM and no seams

If truly seamless coverage is required, you will need to measure the project as if it were a simple square or rectangle. Even if the roof shape is broken down into triangles, you still need to cover the whole roof with a single membrane sheet. For this, you should imagine the smallest rectangular or square boundary that can encompass the entire roof, also known as the roof’s bounding box.

To find this:

• Length: Measure the longest distance across the roof, which could be the span from one end to the furthest opposite point.
• Width: Measure the widest point across the roof, perpendicular to the length.

Add an allowance for overhang

It’s a good idea to include an overlap allowance for errors, seams, or adjustments. A typical overlap is between 5-10% of the total membrane size. Add this percentage to the calculated dimensions for safety. This approach ensures that the membrane sheet will be large enough to cover the entire roof without needing to make multiple joins.

Note that for some complex designs, this will inevitably mean that there will be membrane wastage. EPDM is high quality membrane and can be rolled and stored for future use. Rolling the membrane is better than folding it for storage, as there will be fewer creases.

Using triangulation to minimise wastage

If you are confident using seams in the installation, triangulation is an effective method to calculate the area of irregular shapes by breaking them down into smaller, manageable triangles. To do this accurately, and especially if the building is not the same shape as the roof, access to the roof deck may be the best approach.

Take all the necessary safety precautions and here’s how you can do it:

Tip: If there is a square or rectangle section within the project outline, mark that out first and measure it as above. Then break the rest of the design into triangles as outlined below and add the two measurements together.

Step 1: Break the project down into triangles

Start by visually or physically dividing the complex flat roof into several triangles. You’ll need to ensure that every section of the roof is covered by a triangle. This method works because the area of a triangle can be calculated easily using basic geometric formulas.

Step 2: Measure the dimensions of the triangles

For each triangle, you’ll need to measure the base and the height:

• Base: This is one side of the triangle, preferably along the roof’s edge or a section that’s easy to access.
• Height: This is the perpendicular distance from the base to the opposite vertex (the peak of the triangle).

Step 3: Calculate the area of each triangle

Once you have the base and height for each triangle, use the following formula to calculate the area of each one:

• Area of a triangle = ½ x Base x Height

Perform this calculation for each triangle, and ensure the areas are noted in square metres (m²).

Step 4: Add up the areas

After calculating the area of each triangle, add them up to get the total area of the roof. This will give you the square meterage (m²) required for waterproofing.

Example Calculation

Imagine you’ve divided the roof into three triangles:

• Triangle 1: Base = 4 metres, Height = 3 metres
  • Area = ½ × 4 × 3 = 6 m²
• Triangle 2: Base = 5 metres, Height = 4 metres
  • Area = ½ × 5 × 4 = 10 m²
• Triangle 3: Base = 6 metres, Height = 2 metres
  • Area = ½ × 6 × 2 = 6 m²

Total area of the roof = 6 m² + 10 m² + 6 m² = 22 m²

As previously mentioned, membrane is sold in sheets, so use the following method to calculate the dimensions of the membrane sheet you need to buy after determining the total flat roof area through triangulation:

Calculate the required membrane size

Once you have the length and width of the bounding box, use these values to determine the actual size of the membrane sheet you need to order:

For example, if the total roof area is 22 m²:

• The longest distance across the roof (length) is 6 metres.
• The widest distance across the roof (width) is 4 metres.

This gives a base requirement of a 6m x 4m membrane sheet.

If you add 10% for overlap:

• Length: 6m + (10% of 6m) = 6.6 metres
• Width: 4m + (10% of 4m) = 4.4 metres

So, the total membrane sheet size to buy would be 6.6m x 4.4m. Double check to ensure that all parts of the roof will be covered.

Quantifying adhesives and liquid roofing

To quantify the required adhesive and also for liquid roofing systems, the square meterage is typically needed. For even squares or rectangular projects, to find the square meterage, simply multiply the length measurement by the width measurement:

Example:

• 5m (length) x 4m (width) = 20 square metres (20 m²)

This is then compared with the coverage rates of the products. It is important to check the manufacturer’s coverage rates carefully – coverage rates can and often do differ depending on the substrate, the application method (whether spray or liquid) and the applicator.

Using our example, if the total roof area is 20 m²:

Elevate’s water-based adhesive has a coverage rate of up to 4.5m2 per litre, so to find out how much you need in total, divide the total flat roof area by the coverage rate:

• 20 ÷ 4.5 = 4.44 litres

Simplify with a flat roof area calculator app

The free PermaRoof EPDM Kit Builder App does exactly what it says on the tin. If you’re here because you need to quantify materials for the flat roof, this is the fastest and easiest way to do it if you’re not buying one of our standard roof kits.

There will still be measuring to do, but adhesive, edge trim and all other materials are automatically quantified for you once you’ve added the measurements and project details.

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.