What is the minimum fall for a flat roof?

A fall is the slope that carries water off a roof, written as a ratio. A fall of 1:80 means the surface drops one unit for every 80 units of run. In practice that is 12.5 mm over a metre, a gradient you could not see with the naked eye. On a roof four metres deep it comes to about 50 mm from the high edge to the low.

BS 6229 is the code of practice for flat roofs finished in membranes that are supported along their whole length. BS 6229:2025 is current, published in December 2025, and it replaced the 2018 edition. It applies to flat or curved roofs pitched at up to 10 degrees. On falls, the 2025 edition holds to a minimum finished 1:80 across the main roof area and in internal gutters, so the figure survived the rewrite. Anyone still quoting the withdrawn 2018 document is working from an out-of-date source.

The two terms are easily mixed up. The finished fall is what the completed roof must achieve, measured on its surface. The design fall is what the roofer builds in, allowing for timber that moves, boards that dip and layers that add thickness. If you design for the minimum, any small shortfall in the build leaves you under it.

IKO, a membrane maker, wrote in 2021 that the recommended minimum fall is 1:80 and that the roof should be designed for 1:40, citing the earlier 2018 edition. GreenSpec says at least 1 in 40. Our approach follows that logic: aim for around 1:40 so the finished roof drains whatever happens in between. A fall of 1:40 is 25 mm in every metre, double the minimum, and the extra is a margin rather than a luxury. Timber dries and shifts, boards flex under load and foot traffic, and a membrane never lies perfectly smooth, so a roof designed at the bare limit has nothing in hand.

A fall comes from the structure or is added on top of it. On a new build, the joists can be set to a slope. On an existing flat roof, the usual routes are firring strips, which are tapered lengths of timber laid across the joists to make the slope, or tapered insulation boards that form the fall as part of the insulation layer. Each has its own cost and its own effect on the height of the roof. Where the existing joists already run in the right direction, the job is simpler; where they run across the fall, the slope has to be built up above them.

Getting the fall right matters at the edges, where the roof meets doors, walls and gutters. A steeper slope raises one side, and upstands, thresholds and window sills all have to work with the finished height. That is why we settle the plan at survey and not on the day. Our guide to flat roof falls and drainage goes through the options in more depth.

The 1:80 figure belongs to the roof surface and to internal gutters, the kind that sit within the roof's own build-up. A rainwater gutter on the outside of a building is a different product, with guidance that comes from the maker rather than from the standard. Brett Martin gives a fall of 1mm for every 600mm, with brackets no more than 900mm apart, while FloPlast quotes 1:350 for its MiniFlo system.

A fair summary is somewhere between 1:600 and 1:350, depending on the system. Follow the instructions for the gutter you actually have. If a gutter holds water along its length, the fall is wrong or a bracket has dropped. Our answer on standing water on a flat roof explains what happens when a roof surface suffers the same fault.

You can judge a poor fall without tools. After rain, see where the water sits. Water still lying there hours after the shower has ended points to a shallow fall, a sagging deck or a blocked outlet. A roof that clears cleanly towards its outlet probably has an adequate fall, whatever the number on paper. Look in the lowest corners and along the line between the outlet and the furthest edge, because that is where trouble gathers first.

If you want a measurement, a survey can take one with a level and a straight edge. Our photographs and measurements show whether the fall can be corrected as part of a repair or needs a new build-up, and the answer is set out in a single written figure once we have measured up. Our flat roofing page covers the system, and the Building Regulations guide covers the paperwork.