Question answered
What fall does a flat roof need?
The short answer
A flat roof needs a finished fall of at least 1:80, the minimum in BS 6229, kept when the code was revised in 2025. To be sure of reaching it, the roof is designed at 1:40. That is a drop of 25 mm in every metre on the drawings and 12.5 mm in every metre once built, enough to keep water moving towards the outlet.
A flat roof is never meant to be level. The slope that carries rain to the outlet is small enough to miss by eye, which is exactly why it is often built too shallow.
The two figures: 1:80 finished, 1:40 designed
A fall of 1:80 means the roof drops 1 unit for every 80 units of run. In plain measures it is 12.5 mm for each metre, so a roof 4 m deep falls by 50 mm from its high edge to the low one. That is the minimum IKO gives for finished flat roofs, citing the 2018 edition of BS 6229 (since revised).
The same guidance wants the design to reach 1:40, or 25 mm per metre. The extra slope allows for the small errors and movement that every real building has.
Owners and builders sometimes mix the two figures up. The rule to hold on to is this: design at 1:40, and expect the finished roof to give you at least 1:80. A roof designed only to 1:80 has nothing in hand, and the first hollow in the boards will leave it holding water.
These are recommendations for the finished roof, not a promise about every flat roof already standing.
Why design for more than you need
A drawing is flat and exact. A roof is not. Timber joists are never perfectly even, a deck can deflect between its supports as soon as it takes a load, and the boards laid across it carry their own small errors. Add the thickness of the covering, which builds up at laps and around upstands, and a slope of exactly 1:80 on paper can vanish in places.
Deflection matters most. A deck that sags a few millimetres under weight or over the years creates a hollow, and a hollow on a gentle slope is enough to stop water moving at all. Designing at 1:40 leaves room for that movement while still ending with a usable fall.
This is the reason ponding is so common on extensions and garage roofs, where the drop was set by the height of a wall rather than by calculation. What it looks like once built is described in water pooling on a flat roof.
How a fall is actually made
There are three usual ways. The first is to lay the joists at a slope, either by setting them at different heights or by notching, so that the deck itself falls. The second is to fix tapered timber strips, called firring pieces, on top of level joists. The third, on a warm roof, is tapered insulation boards, cut so that the thick end sits at the high side and the board thins towards the outlet.
Which route suits depends on the build-up. A new extension can have the slope designed into the structure. An existing roof being re-covered often gets its fall from firring or tapered boards laid over the old deck. Both give the new waterproof layer a surface that sheds water on its own.
Plan shape matters as well. A large rectangle can take a single slope, while a roof with several levels may need falls running in different directions, with outlets or gutters positioned to catch each. Our flat roofing page lists the types of roof we work on, from small extensions to larger decks, and each needs its own fall plan.
If your existing roof is too flat
Checking is simple. Lay a long spirit level or straight edge along the roof, or look at where rain stays after a shower. A hollow that holds water on three separate rainy days is telling you the fall is lost there.
A new covering does not correct a poor fall. It follows the deck beneath it, so an overlay on the old roof carries the old shape forward; our page on overlaying an old flat roof says when that is still sensible. If the slope is wrong over the whole roof, re-covering is the moment to correct it, with tapered boards or strips under the new membrane.
Where the problem is local, a small hollow near an outlet, a smaller job may do. We decide that after a survey, photographing where the water collects, and we say plainly when a repair is enough. Where possible, measure again after the work and keep the figures with your invoice.
Roof falls, gutter falls and the covering
Do not confuse the roof's own slope with the fall of a gutter. Manufacturer guidance for gutters is far gentler, roughly 1:600 to 1:350 depending on the system, because a channel only has to move water gently along its length. A flat roof surface needs far more, because it is a wide area that must shed water in every direction it can.
The covering also has a say. The rules on slope for pitched coverings, such as tiles and slates, are set out in each maker's data sheet, and single-ply and liquid systems have their own instructions. The data sheet for the product being laid is always the final word.
Both EPDM and GRP perform best on a roof that drains. Neither is a cure for a roof that does not. If you are choosing a covering, fix the shape first, then compare materials, and ask that the fall be measured and photographed before the new layer goes on.
Roof falls: two more questions
Can a flat roof be laid perfectly level?
It can be built that way, but it will not drain. With no fall, every dip holds water and every seam stays wet. The recommended finished fall is 1:80, which is a small slope, but it is the difference between a roof that dries and one that stays damp.
How do I check the fall on my own flat roof?
From a safe position, look for where rain lingers after a shower, or lay a long straight edge and a spirit level along the surface. Divide the drop by the length: 12.5 mm per metre is 1:80. Photograph what you find and show a roofer.
Tell us about your roof
Describe what is happening and we will call you back to arrange a visit.
- Photographs of what we find
- A written quote with one total price
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