Our commercial flat roof systems

How to narrow six systems down to one

Buyers usually arrive at this page expecting to choose a material. In practice the roof chooses for you, and the sequence that gets to the right answer is the same every time.

Start with the deck. Profiled metal on purlins puts you in a different world from concrete or timber — it flexes, the gutters are internal, and over-sheeting becomes available as a route that simply does not exist on a solid deck. Then the falls. A level survey tells you whether the finished fall meets the 1:80 minimum BS 6229:2025 requires, or whether you need a tapered scheme to get there — which rules out anything that merely follows the existing surface. Then Part L. Renovating more than half the roof, or more than a quarter of the whole envelope, means upgrading to around 0.18 W/m²K, and a coating that adds no insulation may not satisfy that.

Then the constraints nobody puts in writing. Whether the building can close. Whether the insurer permits hot works. How much plant sits on the roof and how many penetrations that means. Whether the deck could carry a ballasted solar array later at roughly 15–25 kg/m². Each of those eliminates options faster than any comparison of membrane chemistry.

Only at the end does the material choice matter, and by then it is usually down to two. If you are weighing repair against replacement before any of this, start at repair or replace; if the building is industrial, industrial flat roofing covers the deck-first questions in more depth.