How to Measure Your Flat Roof for a GRP Kit
One of the most common questions we get asked at Roofing Kits Direct is: “What size kit do I need?” It’s a fair question, and getting it right matters — order too little resin and matting and you’ll be waiting on a top-up delivery halfway through the job; order too much and you’ve spent money on materials you didn’t need.
The good news is that measuring a flat roof for a fibreglass (GRP) kit isn’t complicated. You don’t need to be a professional roofer to get an accurate figure — you just need a tape measure, ten minutes, and to know which bits of the roof people usually forget to measure.
Here’s exactly how we’d talk you through it if you called us.
What You’ll Need
A tape measure (a long builder’s tape is easier than a small retractable one)
A ladder, if you need to get up and check details safely
A pen and paper, or the notes app on your phone
Ideally, a second pair of hands to hold the other end of the tape
If you can safely get onto the roof, measure directly. If not, measuring from ground level using the building’s dimensions as a guide works too — just be extra careful with anything that isn’t a simple rectangle.
Step 1: Measure the Main Flat Area
Start with the simplest bit — the main flat surface of the roof.
Measure the length in metres.
Measure the width in metres.
Multiply the two to get your area in square metres (m²).
For example, a roof that’s 4m long and 3m wide is 12m². This is the figure most people stop at — but it’s rarely the whole story.
Step 2: Don’t Forget the Upstands
This is the single biggest reason people under-order. An “upstand” (sometimes called a kerb) is any vertical surface the fibreglass needs to cover — where the roof meets a wall, a parapet, a rooflight kerb, or a door threshold.
GRP doesn’t just sit flat; it needs to run up these vertical edges to create a watertight seal, typically by around 150mm as standard. If your roof has upstands on all sides, that’s extra material you need to account for in your total area.
How to measure them: treat each upstand as its own small rectangle — length of that edge x height of the upstand — and add it to your main roof area.
Step 3: Account for Any Additional Features
Flat roofs are rarely just a plain rectangle. Look out for:
Roof lights or dormers — these usually need their own upstand allowance around the kerb
Chimneys or soil pipes — small penetrations still need fibreglass dressed around them
L-shaped or irregular roofs — split the roof into simple rectangles or triangles, measure each one separately, then add the areas together
Parapet walls — these often need a capping and an upstand on both the inside and outside face
If your roof is a genuinely tricky shape, sketching a rough plan on paper first — even if it’s not to scale — makes it much easier to keep track of each section as you measure.
Step 4: Add a Waste Allowance
Even the most careful installer ends up with some off-cuts and overlaps, particularly around trims and detailing. As a rule of thumb, we’d suggest adding 10% on top of your total measured area to cover wastage. It’s a small difference in cost against the hassle of running short mid-job.
Step 5: Use Our Free Kit Calculators
Once you’ve got your total area (including upstands, features, and waste allowance), you don’t need to work out resin ratios or matting layers by hand — that’s exactly what our online kit calculators are built for. Pop in your measurements and it’ll build a complete kit list for you, sized correctly, with nothing missing and nothing wasted.
We’ve got separate calculators depending on the system you’re using:
Fibreglass Restec 1010 Calculator
GRP Overlay Restec 2020 Calculator
Common Mistakes We See
Measuring only the flat top surface and forgetting upstands entirely — usually the cause of a mid-job resin shortage
Working in feet and inches, then converting incorrectly — always measure in metres to keep it simple
Forgetting trims and edge detail when totting up the shopping list — a kit needs more than just resin and matting
Guessing the shape instead of sketching it — a rough plan takes two minutes and saves a lot of head-scratching
Still Not Sure? We’ll Help You Get It Right
If you’d rather not do the maths yourself, or your roof has an unusual shape you’re not confident measuring, just ask us a question or request a free quote and send us your rough dimensions. We’ll help you work out exactly what you need before you order — that’s what we’re here for.
Once you’ve got your measurements sorted, browse our full range of fibreglass roofing kits and get everything you need in one order, with fast UK-wide delivery.
