The short answer
Guttering needs a slight slope towards the downpipe, so water keeps moving. Manufacturer guidance from Brett Martin is 1mm of fall for every 600mm of gutter. That is a very gentle slope, barely visible to the eye. Too little and water stands and overflows. Too much looks wrong and can send water past the outlet in heavy rain.
The right fall is tiny. Getting it slightly wrong is what leaves a gutter holding water for days.
The figure and where it comes from
Brett Martin, a gutter manufacturer, advises 1mm of fall for every 600mm of gutter run in its installation guidance. Over a six-metre length that comes to ten millimetres, about the thickness of your finger. It is deliberately small.
Is there a British Standard for gutter fall?
BS EN 12056-3 is the standard for the layout and calculation of roof drainage, and BSI lists it as current. The fall figure here comes from the manufacturer's guidance rather than the standard. Different gutter systems can carry different instructions, so check the maker's guide for the product on your house.
Do not mix this up with flat roofs. BS 6229:2025 keeps a minimum finished fall of 1:80 on flat roofs with flexible waterproof coverings, a different surface with a different figure. A gutter and a flat roof are separate drainage jobs, each with its own guidance.
For gutters, the number that matters is the maker's, and the principle is simple: a slight, steady slope towards the outlet. It should be steady enough that you can hardly see it from the ground, and consistent from one end of the run to the other.
What happens with too little fall
A gutter fitted dead level, or one that has sagged, holds water in its low points. That water has nowhere to go until the level rises enough to spill.
Why does my gutter hold water after rain?
Look for a puddle that sits in one section when the rain has stopped. It usually means a sag between brackets, a bracket that has dropped, or a run that was never fitted with enough slope. Standing water carries silt and leaf mould, which settle, rot and make the problem worse. In winter it can freeze and push joints apart.
Standing water also puts extra weight on the brackets, and a gutter that is heavy with silt sags further. The cycle is slow, and many homeowners only notice it when the edge starts to overflow in a downpour or a green stain appears on the wall below.
Often the cause is not the fall at all but a blocked outlet, so check that first. Our answers on overflowing gutters and unblocking a downpipe run through the simpler causes before you look at the slope.
What happens with too much fall
A steep gutter looks odd, because the run visibly slopes against the straight line of the roof edge. The deeper concern is how water behaves in it.
Can a gutter slope too much?
It can. In a heavy shower the water rushes towards the outlet, builds up at the low end and may overshoot or spill over the front of the gutter beside the downpipe. The low end also has to carry all the flow from the whole run. A gentle fall spreads the load better.
A steep fall can also mean the fascia is out of line or the brackets were fitted unevenly. If the fascia is the problem, straightening the gutter alone will not last. The gutter and the board belong in the same visit, because brackets can only hold the line the fascia gives them.
On a long run, an outlet part-way along lets the gutter fall to it from both ends, which keeps each slope short. That is one reason the position of downpipes is decided before the brackets go on, and why moving a downpipe later can mean refixing the whole length.
How to check your own gutters
You can get a rough idea from the ground, but do not climb a ladder to measure.
Can I check the fall without going up?
Watch the gutter during or just after steady rain. Water should run towards the downpipe and clear. Look for a section where it pools, drips from the underside of a joint, or overflows at one spot. After the rain, a puddle left in the gutter, or a patch of green wall below one point, marks the low spot.
From a safe position at a window, or with binoculars, you can sometimes see a sag between two brackets. A bracket spacing that looks wide, or brackets tilting forward, is another clue.
A roofer checks the fall with a level or a string line, and fixes it by moving brackets, adding brackets or refitting the run. Where the gutter has worn out, replacing it is often better than adjusting it; our page on repairing or replacing guttering helps you decide. Our visit takes in the gutter from close range and the board behind it. You are sent photos of any dips, and one written price follows. If the run needs refixing rather than replacing, we say which brackets are at fault and why, so you can see where the water is being held. The main gutters, soffits and fascias page lists the rest.
More on gutter fall
Do all gutters need a fall?
Gutters are normally fitted with a slight fall, but check the maker's instructions for your system. Brett Martin's guidance is 1mm for every 600mm, so a gutter that holds water where the maker expects it to drain probably needs its brackets adjusting, or the run refitting.
Can I adjust the fall myself?
Small adjustments to brackets are possible, but they involve working at height and handling a full gutter, so a tower or scaffold is safer than a ladder. If the board behind is rotten, a new bracket will not hold. A survey tells you whether it is a quick fix or a wider repair.
The full picture: Gutters, soffits and fascias in Taunton ยท More short answers