Gutter fall
What fall should a gutter have?
The short answer
A gutter needs only a very slight fall towards the outlet. Brett Martin's installation guidance gives 1 mm of drop for every 600 mm of run, FloPlast's MiniFlo guide gives 1:350, and a safe summary is roughly 1:600 to 1:350 depending on the system. You will not see that from the ground. Too little leaves water standing; too much looks crooked and pushes water past the outlet. Follow the maker's guide for the gutters on your house.
How much slope a gutter run needs to drain, how to check it from the ground, and what happens when the run is wrong.
The small fall that keeps a gutter draining
A gutter is a long, shallow channel, and it only has to lean gently towards the outlet, the point where water drops into the downpipe. Manufacturers publish the figure for their own systems. Brett Martin gives 1 mm of fall for every 600 mm of run, and FloPlast's MiniFlo guide gives 1:350. Both are manufacturer instructions rather than a British Standard, so the leaflet that came with your gutters is the document to follow.
In plain terms, that is a drop of a few millimetres along a typical front elevation. The eye will not pick it out. Water finds it anyway, because it runs to the lowest point and the plastic profile is smooth. Anything steeper than the maker asks for gains you very little, and it tends to show: the run looks as if it is sliding off the house, and the gap between the gutter and the roof edge opens up at one end.
What a level or backwards run does to the roofline
A run with no fall holds a shallow pool along its length. Leaf mulch, moss crumbs and grit settle in that pool instead of washing to the outlet, and over a season they build into a damp bed that feeds more moss. In a downpour the water has no other way out than over the front lip, or back against the fascia board, the painted or plastic board that the gutter hangs from.
That is why a slow, steady drip from the joints or a dark tide mark on the wall under the run so often traces back to the fall rather than to the gutter itself. If you are seeing overspill in heavy rain, our answer on why a gutter overflows covers the other likely causes, such as a failed seal at a union. A backwards run is rarer than a level one, but it sends water away from the downpipe and needs the same cure.
Checking the fall without climbing a ladder
Stand well back at one end of the house and look along the gutter. Compare it with something that should be level, such as the course lines in the brickwork or the edge of a window cill. The gutter should drift very gently towards the downpipe and sit evenly away from the roof edge along its whole length. A belly in the middle, or a visible kink at a bracket, tells you the run has dropped.
The better test is a wet day. Watch from a dry spot indoors or under cover as the rain starts. Water should move steadily towards the outlet. If it lingers at the far end, spills over the middle, or lies in a long stripe after the rain has stopped, the fall is wrong or a section has sagged. Please do not lean a ladder on the gutter itself, because plastic runs flex and snap under that load. A roofer working from the proper access can check it with a line and a level in a few minutes.
Resetting a gutter whose slope has gone
Gutters hang on brackets screwed to the fascia or clipped to the rafter ends. To reset one, the roofer finds the highest point, which is usually the end furthest from the outlet, and stretches a line from there down to the outlet, allowing the drop the manufacturer gives. Each bracket is then moved to meet that line, and the joint seals are checked or replaced as the sections are relifted.
The line only holds if the board behind it is sound. Brackets screwed into soft or rotten timber will pull and let the run sag again within months, so the fascia has to be right first. Our answers on rotten fascia boards and gutter bracket spacing explain the board and the fixings. If the gutter is old, brittle or split along the back, resetting it may cost more than it saves, and a plain verdict on that comes after we have looked at it, with photographs of the cracks, the brittle plastic or the split back so you can judge for yourself.
Flat roof and internal gutters follow a different rule
Everything above is about the gutters hung on the outside of the roofline. Gutters formed inside the roof, such as those behind a parapet or between a flat roof and a wall, are built from lead, single-ply membrane or GRP (glass-reinforced plastic), and they are laid to a standard of their own. BS 6229:2025, the current code for flat roofs, keeps its 1:80 minimum finished fall, for ordinary roof areas and internal gutters alike.
Roofers usually design steeper, around 1:40, so water still clears once the build-up has bedded down and the deck has taken its final shape. That is a long way from the slim fall of a hung gutter, and the two should not be confused when you are reading a quote. If your roofline includes a flat roof section that ponds, our flat roofing page explains the repair options, and the guttering, soffits and fascias hub covers the outside runs.
Related questions
Gutter slope queries
Should a gutter be perfectly level to look neat?
No. A perfectly level run will hold water, because nothing pulls it to the outlet. It is better for the gutter to follow the maker's slight fall and sit evenly beneath the roof edge, which looks tidy from the street and drains properly in a downpour.
Can a house settle enough to change the fall?
The usual culprits are smaller. Loose brackets, a warped or rotten fascia board and a gutter weighed down with silt all pull a run out of line. We check the board and the fixings first, and the price that follows is a single figure on paper.