How to Measure a Flat Roof for EPDM: A Step-by-Step Guide
Ordering the correct size EPDM rubber membrane is one of the most important steps in any flat roofing project. Accurate measurements help reduce waste, save money, and ensure your installation goes as smoothly as possible.
Whether you're a DIY homeowner replacing a garage roof or a professional contractor working on a larger project, this guide explains exactly how to measure a flat roof for EPDM.
Why Accurate Measurements Matter
EPDM rubber roofing is often supplied as a single, seamless sheet. This minimises joins and creates a durable, watertight finish that can last for decades.
Ordering a membrane that's too small can delay your project, while ordering one that's too large may increase costs unnecessarily. Taking a few extra minutes to measure correctly ensures you receive the right size first time.
What You'll Need
Before you begin, gather the following:
- Tape measure (at least 5m, or longer for larger roofs)
- Notebook or measuring sheet
- Pencil or pen
- Calculator
- Ladder (if required)
- A second person for larger roofs
Always follow appropriate safety precautions when accessing any roof.
Step 1: Measure the Length
Measure the longest distance from one end of the roof to the other.
Take your measurement from edge to edge rather than the size of the building underneath, as roof decks often extend slightly beyond the walls.
Write this measurement down in metres or millimetres.
Example:
Roof length: 6.20 metres
Step 2: Measure the Width
Next, measure the widest point across the roof.
Again, measure the roof deck itself rather than internal building dimensions.
Example:
Roof width: 4.10 metres
Step 3: Allow for Upstands and Edge Details
EPDM doesn't stop at the flat surface. The membrane usually turns up walls, around rooflights, or over parapets and edge trims to create a fully waterproof seal.
You'll need to add enough material for these details.
Typical allowances include:
- Upstands against walls
- Parapet walls
- Fascia edges
- Drip trims
- Rooflight kerbs
- Skylights
A common allowance is 150mm–300mm per edge, depending on your roof design and detailing.
For example:
Roof length: 6.20m
Add 300mm at each end
Final membrane length:
6.80m
Repeat this calculation for the width.
Step 4: Check for Obstacles
Make a note of anything that passes through the roof, including:
- Roof vents
- Soil pipes
- Extractor fans
- Air conditioning units
- Rooflights
- Solar cable entries
These won't normally affect the membrane size, but they are useful information when ordering accessories such as pipe boots or flashing materials.
Step 5: Sketch the Roof
A simple hand-drawn plan can be extremely helpful.
Include:
- Overall dimensions
- Drainage outlets
- Wall locations
- Rooflights
- Pipes
- Changes in level
- Any unusual features
This makes it easier to double-check your measurements before placing your order.
Measuring Irregular-Shaped Roofs
Not every roof is a perfect rectangle.
If your roof has:
- L-shaped sections
- Multiple levels
- Extensions
- Angled corners
Break the roof into smaller rectangles.
Measure each section individually before calculating the overall membrane requirements.
If you're unsure, contact our technical team who can help determine the most suitable membrane size.
Measuring Around Parapet Walls
Parapet walls require extra material so the membrane can be dressed up and over the wall where required.
Measure:
- Roof deck
- Height of parapet
- Width of parapet (if wrapping over)
Always allow sufficient material for secure termination.
Measuring Roofs with Internal Gutters
Internal gutters should always be measured separately.
Record:
- Length
- Width
- Depth
These areas often require additional detailing and accessories.
Common Measuring Mistakes
Avoid these common errors:
· Measuring the Building Instead of the Roof: The roof deck is often larger than the building footprint.
· Forgetting Upstands: Many people only measure the flat surface, leaving insufficient material for walls and edge details.
· Rounding Measurements Down: Always round up rather than down to ensure adequate coverage.
· Ignoring Roof Features: Vents, skylights and outlets should all be noted before ordering.
What Information Should You Provide When Ordering?
Having the following information ready will help ensure you receive the correct products:
- Roof length
- Roof width
- Upstand heights
- Number of corners
- Rooflights
- Pipes or penetrations
- Internal gutters
- Drainage outlets
- Edge trim requirements
Photographs can also be useful if you need technical advice.
Need Help? Use Our Roof Calculator
If you're unsure about your measurements, don't worry.
Our online roof calculator can help estimate the correct EPDM membrane size, or our experienced team can review your measurements and recommend the most suitable products for your project.
Taking advantage of expert advice before ordering can help prevent costly mistakes and give you confidence that you have everything you need.
Why Choose EPDM?
Once you've measured your roof correctly, EPDM offers a range of long-term benefits:
- Lifespan of over 50 years
- Excellent weather resistance
- UV stable
- Low maintenance
- Flexible in all seasons
- Suitable for garages, extensions, garden rooms, sheds and commercial buildings
When installed correctly, EPDM provides a seamless, durable waterproof barrier that protects your property for decades.
Final Thoughts
Learning how to accurately measure a flat roof is the first step towards a successful EPDM installation. By measuring the roof deck, allowing for upstands and edge details, and noting any roof features, you'll ensure you order the right membrane and accessories for your project.
If you're ever unsure, it's always worth asking for advice before placing your order. At Flexible Roofing Supplies, we're here to help with expert guidance, quality EPDM products and all the accessories you need to complete your roofing project with confidence.
