The short answer
IKO, citing BS 6229:2018, gives the recommended minimum finished fall for a flat roof as 1:80, about 0.72 degrees. To still reach that after tolerances, deck deviation and deflection, the design should aim for 1:40, about 1.44 degrees. So the design figure is 1:40 and the finished minimum is 1:80, never the reverse.
The two numbers, and which is which
A fall of 1:40 means the roof drops 1 unit for every 40 units of run. Over a 2 metre run, that is 50 mm. A fall of 1:80 drops 25 mm over the same 2 metres. The first is steeper, and it is the figure a designer works to.
IKO's guidance, which cites BS 6229:2018, the British Standard code for flat roofs whose waterproofing is supported along its whole length, puts the recommended lowest finished fall at 1:80. It then advises designing for 1:40, so that when the real roof is built, with its small errors, it still ends up at 1:80 or better.
| Stage | Fall | Angle |
|---|---|---|
| Design fall | 1:40 | about 1.44 degrees |
| Minimum finished fall | 1:80 | about 0.72 degrees |
A common slip is to write the design fall as 1:80. It is the other way about, and the difference is the whole point of the margin.
Why design for steeper than you need
A deck is never perfectly laid. Joists vary, boards sit slightly differently, and a long span deflects a little under load once weight is on it. Each of these eats into the fall. If the roof is designed only to 1:80 and a couple of those errors land the wrong way, parts of the roof end up flatter than the minimum and water stops draining.
Designing for 1:40 builds in margin. It is a cushion against construction tolerances, deck deviation and deflection, and that is precisely what IKO says it is for. The spare fall is used up on site, and the finished roof still meets the minimum. A roof designed to the bare minimum has nothing left to lose.
The same thinking applies to a re-cover. When an old roof is stripped, the deck is measured again, because years of load and damp may have changed the slope. If the surface now falls less than the old drawings suggest, extra tapered insulation or new firrings bring it back. Guessing from the previous roof is how a replacement inherits the same puddles as the one before it.
How the fall is actually made
There are three common ways. On a cold roof, the joists can be cut or laid at a slope, or a firring, a tapered timber strip fixed to the top of each joist, makes the slope. On a warm roof, the insulation can be cut as tapered boards, which makes the fall without touching the structure. A third method is a screed laid to falls, which is more usual on concrete decks.
Each suits different buildings. A new extension can be built with firrings from the start. An existing roof with a flat deck often uses tapered insulation when it is re-covered. To understand how the layers differ, read our guide to warm and cold flat roofs.
What goes wrong when there is no fall
Where the slope is lost, water settles. Standing water does not always cause a leak straight away, but it speeds ageing, grows algae, carries silt that wears the surface and finds small flaws. Over time it can penetrate a lap or a trim that would stay dry on a properly draining roof.
Approved Document A, which covers structure under the Building Regulations, even mentions ponding as a consequence of unacceptable deflection in roof members. Where a deck has sagged, the fault may be structural rather than a matter of the covering. We explain what we look for in a spongy flat roof, and in the difficult case of a roof that has been blistered by trapped moisture, flat roof blisters.
Fall, walkways and the finished surface
Where a roof is meant to be walked on, the fall still matters. A paved or decked surface laid on supports sits above the waterproofing, and the waterproofing beneath still has to drain. A walk-on roof also loads the deck more, so deflection is a bigger risk. See our page on a walk-on flat roof.
Gutters are a separate matter. Rainwater gutters are laid to a far gentler tilt than a deck, with makers suggesting about 1:600 to 1:350 depending on the system, which is flatter again than a roof deck. Do not confuse the figures: a gutter fall and a roof fall are different things, set for different jobs.
What we do about it on a survey
At survey we look for where water sits and for signs that the deck has moved. If a roof has too little fall, the right cure may be new tapered insulation or firrings as part of re-covering, rather than a thicker layer of the same covering. Where the fall cannot be put right without work to the deck, we explain that and why.
You can do a first check yourself. Lay a long straight board on the roof, put a spirit level on it, and note the gap at one end when the bubble centres. Only do this where the roof is safe to reach, such as a low garage you can step onto with care. Otherwise leave it to the survey.
Pictures taken on site travel with the written price. If you are weighing the options, our page on repairing or replacing a flat roof helps, and the main flat roofing page describes the work we do.