The short answer
Most flat roofs need a finished fall of at least 1 in 80 towards their outlets, with no backfalls anywhere, under the British Standard for flat roofs, BS 6229, revised in December 2025. The design fall is set higher than that, from the structure and a survey, so the 1 in 80 survives building tolerances and deflection. Zero-fall roofs are only acceptable for specific certified systems.
On this page
“Flat” is a figure of speech. A flat roof that is truly level holds water, and water left sitting on a roof finds every weak lap, ages the membrane faster and adds weight the structure was never designed for. Every commercial flat roof should fall towards its outlets, and the question for anyone commissioning a new roof or a refurbishment is how much, and how the contractor will achieve it.
The minimum: 1 in 80, finished
The British Standard that governs flat roof design in the UK is BS 6229, the code of practice for flat roofs with continuously supported flexible waterproof coverings. Its 2025 revision, which came into effect in December 2025, keeps the familiar rule that the minimum completed fall on most flat roofs is 1 in 80. That is a drop of 12.5 mm for every metre of roof.
Two words matter there. “Completed” means the fall measured on the finished roof, after the deck has deflected under load and after normal building tolerances have taken their share. And “most” acknowledges that some certified systems are designed differently (see zero falls below).
The standard is also blunt about backfalls: they are not acceptable. A backfall is any area where the roof slopes away from its outlet and traps water. On older roofs, backfalls are often where the trouble starts.
Design fall: why 1 in 40 is no longer automatic
For years, designers were told to set out a flat roof at 1 in 40 so that the finished roof would still achieve 1 in 80 once everything had moved. The 2025 revision removed that single rule. The design fall now has to come from structural analysis or survey data, with guidance that 1 in 60 is an appropriate starting point in many cases and 1 in 40 needed where the deck deflects more.
In practice, for a building owner, that means two things:
- On a new roof or a full replacement, ask the contractor or designer what design fall they have used and why. “It’s what we always do” is no longer the answer the standard expects.
- On a refurbishment, the existing deck has already moved as much as it is going to. A survey of the current levels tells the designer what fall the new build-up needs to add.
How falls are created
There are three common ways to give a flat roof its fall:
| Method | How it works | Where it is used |
|---|---|---|
| Structure laid to falls | The deck or joists are set at a slope | New buildings, timber decks with firrings |
| Screed to falls | A sand and cement or lightweight screed laid at a slope on a concrete deck | Concrete roofs, older buildings |
| Tapered insulation | Insulation boards cut to a slope, laid in a pattern across the roof | Most refurbishments and re-roofs |
Tapered insulation has become the standard answer on commercial re-roofs because it fixes two problems at once: it creates the fall and it brings the roof up to the current insulation standard. Board suppliers design a layout for each roof, with crickets and valleys steering water to the outlets.
Zero falls
Some roofs are deliberately designed with no fall: blue roofs that hold rainwater back on purpose, and some ballasted or inverted systems. BS 6229 allows zero falls only for specific systems with third-party certification for that use, and only where there are no backfalls. If a contractor proposes a zero-fall roof, ask to see the certificate that covers it.
What ponding tells you
Standing water a day or two after rain is a sign that the fall is not doing its job. Small puddles at laps are common and usually harmless. Large ponds that stay for days point to one of these:
- blocked or too few outlets;
- deflection in the deck since the roof was built;
- backfalls from poor original setting out;
- a roof refurbished without correcting the levels.
Ponding is a good reason to commission a roof survey before choosing a repair. Patching the membrane under a pond deals with the symptom, not the cause.
Outlets, overflows and upstands
Falls only work if the water has somewhere to go. A good flat roof design includes enough outlets for the area, set at the low points, and overflows or emergency outlets so that a blocked outlet does not flood the roof. Upstands at walls, rooflights and plant are usually detailed at 150 mm or more above the finished roof surface, which becomes harder when thicker insulation raises the roof level. Thicker tapered insulation on a re-roof often means outlets, upstands and rooflight kerbs have to be raised too, and that work should be in the price.
Questions to ask before you sign
- What finished fall will the roof achieve, and how has the design fall been worked out?
- Is there a tapered insulation layout drawing, and does it remove every existing backfall?
- How many outlets are there, are any new ones needed, and where are the overflows?
- Will upstands, outlets and rooflight kerbs be raised to suit the new roof level?
- If the roof is zero-fall by design, what certificate covers the system?
Falls are invisible on a finished roof until the first heavy rain, which is exactly why they belong in the specification. Our commercial flat roofing page compares the systems that go on top of them, and the cost calculator shows what a re-roof with new insulation typically costs.
A final note on safety: checking falls and ponding means going onto the roof. Do not send staff up to look. HSE guidance is to treat any roof as fragile until a competent person has confirmed otherwise, and rooflights on flat roofs are easy to miss.
If your flat roof is holding water, the form below gets it looked at and priced by a contractor who covers your area; the Commercial Roofing UK home page compares the routes from repair to replacement.