The short answer
BS 6229:2025 sets a minimum finished fall of 1:80, which is about 12.5 mm of drop for every metre, on the main roof and in internal gutters. Designers aim for around 1:40 so the roof still reaches 1:80 once the deck deflects and the layers settle. Different systems and manufacturers add their own requirements, so the figure for your roof also depends on the covering chosen.
The figure written into the standard, the steeper design fall aimed for on paper, and why those two numbers are not the same.
The 1:80 finished fall
Despite the name, a flat roof has to slope a little. That slope is called a fall, and it sends rain towards an outlet or gutter instead of letting it sit. A fall is written as a ratio. A fall of 1:80 means that for every 80 units along the roof, it drops by 1, which works out at 12.5 mm over each metre.
The code of practice that covers this is BS 6229, written for roofs with flexible waterproofing laid over a continuous deck. The 2025 edition appeared on 16 December 2025, the day the 2018 edition was withdrawn at BSI. It deals with roofs pitched at no more than 10 degrees. A published summary of the update says BS 6229:2025 holds on to a minimum 1:80 finished fall on the main roof and in internal gutters.
The word finished is important. It means the fall that is actually there once the roof is built, loaded and settled, measured on the surface. It is the number the roof must achieve, not the number drawn on a plan. A roof that holds standing water after the covering is down has usually missed it, which our page on flat roof ponding water looks at.
Why designers aim for 1:40
If the target finished fall is 1:80, why do designers talk about 1:40? Because a roof shifts between the drawing board and the day it is put to use. Joists deflect under weight, the deck can bow slightly, insulation compresses, and the covering is laid with tolerances. Each of these eats into the slope. A roof designed at the bare minimum can easily end up with a stretch that is flat or even dips backwards.
The roofing maker IKO, quoting the 2018 edition, says to design the roof with a 1:40 fall. That is twice the minimum slope on paper, and the extra acts as a cushion. If the deck sags a little, the finished fall can still reach 1:80 where it counts. It is a fair point that 1:80 is the finished minimum; the design figure is a different thing, and the two should not be mixed up.
On many existing roofs the fall was never designed at all. A garage or extension roof may have been laid on flat joists with a thin tapering layer, and the real fall is closer to nothing. That is the main reason for puddles, and why correcting the fall is often part of a good replacement.
How the fall differs by system
The standard sets the baseline for felt and similar coverings, but each system also has its own rules. Single-ply membranes such as EPDM, liquid-applied coatings and GRP all come with manufacturer guidance, and the installer should follow it. Some products tolerate a shallower fall, while others need a clear slope for the warranty to stand, so always check the data sheet for the product proposed.
The fall must also work with the drainage. A fall towards a gutter or an outlet is no use if the outlet is too small or sits proud of the surface. Gutters have their own rule of thumb: Brett Martin's installation guidance says 1 mm for every 600 mm along the run. Our page on a blocked flat roof outlet shows how drainage problems build up.
Construction method matters too. A cut-to-falls insulation board, tapered timber firrings laid on the joists, or a screed laid to falls are three ways to build the slope. Which suits you depends on the structure, the height of the upstands and your door thresholds. Our page on warm roof against cold roof shows how insulation fits into this.
What this means when you replace a roof
When an old roof is stripped, the fall is the first thing to measure. A roofer lays a long straight edge and a level across the deck, or runs a string line, to see where the slope runs out. If the existing fall is short, the options are to raise one side of the joists, lay tapered firrings on top of the deck, or use tapered insulation. Each changes the height at the edges, so door thresholds, window sills and wall flashings must be checked before work starts.
It is worth asking whether the proposal achieves the finished fall, and how it will be shown. Our single written price comes after the survey, alongside a note of the fall we measured. Insulation matters here too, so ask your building control body how Part L applies to the new build-up.
Do not accept a roof that lies flat on the grounds that it will be fine. A flat roof that holds water ages faster and leaks sooner. For a wider look, see our flat roofing page and the guide to Building Regulations for flat roofs.
More on this in Flat Roofing in Crawley
Two more fall questions
Can a roof with no fall at all be built?
A roof with truly no fall is a risk, because every dip holds water and the covering ages faster. Some systems tolerate very shallow slopes, but the standard still expects water to drain. A roofer should show how the fall will be achieved with tapered boards, firrings or a screed, and what the data sheet requires.
Is there a simple check for whether my roof drains properly?
Look at it after rain. Water still lying there a day or two later shows the fall is poor or the outlet is blocked. A spirit level on a long straight board gives a rough check. On a 3 metre run, 1:80 is roughly 37 mm, so a bubble that sits level is a poor sign.