How Much Fall Should a Gutter Have?

Brett Martin, a UK maker of plastic rainwater systems, advises setting a gutter with 1mm of fall for every 600mm of its length. Put another way, the channel drops less than two millimetres in each metre. On a 10 metre run to a single downpipe, that adds up to around 17mm between the high end and the outlet end.

That is a small slope, and deliberately so. A gutter is a shallow channel hung under the edge of a roof, and the aim is to move water gently towards the downpipe while keeping the line of the gutter looking level against the fascia from the garden. If the roofer sets it much steeper than that, the slope shows against the eaves line and looks wrong.

The British Standard for roof drainage layout and calculation, BS EN 12056-3, is the document designers use for the sizing of gutters and outlets. For the slope itself, the maker's figure is the one fitters work from in practice, and the fitting instructions for the particular brand on your house are the final word on any system.

With no fall at all, or a fall that has crept the wrong way, water cannot clear itself. It lies in the channel after every shower, picks up leaves, grit and roof moss, and slowly forms a silty crust. A gutter that holds standing water for days is also heavy, and that weight drags on the brackets until the run sags a little further, which flattens the fall even more.

You will see the signs from the ground. Look for green or black staining along the inside of the gutter, a line of dried silt, or puddles lying in the channel the morning after rain. Overflow at the joints in a dry spell of weather is another clue: when the gutter is full to the lip, it is the joints that give way first.

On a long front run, check whether the water collects at the middle rather than the ends. That usually means a bracket has dropped or the timber it is screwed to has gone soft. Our answer on a sagging gutter goes into the causes, and clearing the channel alone will not cure a bad fall.

Over-steep gutters are less common, but they cause trouble of their own. In heavy rain, water gathers speed down the slope and can shoot past the outlet, over the stop end or across the corner into the next section instead of dropping into the downpipe. The low end also collects most of the load, so the brackets there are working harder than the rest.

A steep fall usually comes from one of two causes. Either the gutter was set out badly against a fascia that is not itself straight, or the fascia has moved and the gutter has followed it. In the second case the real defect is behind the gutter, so it is worth knowing the signs of a rotten fascia board.

Flat roofs with internal gutters are a different matter. BS 6229:2025 keeps a minimum finished fall of 1:80 on a flat roof and in its built-in gutters, and that is a design rule for built-in channels, not a guide for a plastic gutter on a pitched roof. Do not mix the two numbers.

A simple check needs only a hosepipe and a helper. With the gutter clear, run water in at the far end and watch it travel. It should move steadily towards the outlet and leave nothing behind. If a pool stays behind after the flow has passed, that stretch is sitting too flat or has dropped. Do this from a safe position on the ground, with the hose set to a steady flow rather than full pressure, or from a first-floor window that opens to the eaves, and never lean from a ladder to watch.

A roofer correcting the fall will usually reset the brackets, and may need to pack out or replace part of the fascia if it is the cause. Ask whether the work covers a full re-hang of the run or only the visible low spot, because a run that has settled in one place often has other weak points.

If you are weighing new gutters against a re-hang, our answer on gutter replacement cost gives national figures to compare against, and the gutters, soffits and fascias page lists everything we do on the roofline. Send us a photo of the gutter while it is raining, with the lowest stretch in frame, and we will say whether the fall is to blame or something else is.