The short answer
Manufacturers recommend a small fall towards the outlet. Brett Martin says 1 mm for every 600 mm of gutter, and FloPlast's MiniFlo guide recommends 1:350 where a fall is used. In practice that is roughly 1:600 to 1:350 depending on the system, which is under 3 mm per metre. Too little holds water, and too much can send it past the outlet.
What gutter fall means, in plain figures
Fall is the downward slope of a gutter along its length towards the outlet, where water drops into the downpipe. It is expressed as a ratio. A fall of 1:350 means that for every 350 mm along the gutter, it drops by 1 mm. Brett Martin gives it another way: 1 mm of fall for every 600 mm of run.
Turn those into something you can picture. On a 6 metre run, a fall of 1 mm per 600 mm gives a total drop of 10 mm from one end to the outlet. At 1:350 the same run drops by a little over 17 mm. Neither is much, and that is the point. The slope is far too slight to see by eye, which is why it often goes wrong. A gutter that looks level to the eye may be level in fact, or may slope the wrong way.
These are manufacturers' recommendations, not building standards. A product's own installation guide is the document to follow, and the range across systems runs from about 1:600 to 1:350. Some systems are designed to be fitted level and rely on their shape to move water, so check the guide for yours.
Why too little fall makes a gutter overflow
Water finds its own level. In a gutter with no fall, or with a fall away from the outlet, water settles in the low sections and stays there. That is a series of small ponds along the run, and each has consequences.
- Standing water carries silt and leaf litter, which drop out of suspension and build a layer of sludge.
- Sludge narrows the channel, so the next rain fills it faster.
- Water sitting against the joints finds every weak seal and begins to drip.
- In frost, standing water freezes, expands and can split a joint or crack the gutter.
- The added weight pulls on the brackets and loosens them, which makes the sag worse.
All of this is why a gutter can overflow during a modest shower even when the downpipe is clear. The outlet is not the problem. The water is not reaching it. Our page on sagging gutters explains how a run loses its fall over time, and it is the commonest way a good fall turns into a bad one.
Can there be too much fall?
Yes, within reason. A steep fall sends water quickly towards the outlet, which sounds good, but the water arrives faster than a small outlet can accept and may carry on past it, or leap the gutter at the corner. A steep slope also looks wrong against the roofline, because the gutter visibly runs away from the eaves line, and it can pull the downstream end of the run too close to the roof edge or too far from it.
The aim is a modest slope that keeps water moving without letting it race. In practice, fitters set the high end of the run and the outlet end to the maker's figures, then check with a string line or a water level. The gap at the high end between the gutter and the fascia top should look much the same as the low end, because only the gutter drops, not the whole roofline.
Where a fascia itself runs level and the gutter drops against it, brackets at the outlet end sit slightly lower on the board. That is correct, not a mistake, and it is why one set of brackets does not simply sit in a straight line.
How to check your own gutter without a ladder
Plenty can be read from the pavement. In heavy rain, watch the gutter. Water should move towards the outlet and the front lip should stay clear. Where water pools, hangs at the lip or overshoots a section, the fall is suspect. After the rain stops, a gutter with the right fall empties itself within a short time, apart from a film of water. A gutter that still holds a few centimetres in places is not draining.
Another check is the stain pattern below. Green or dark streaks under a joint or between brackets show where water has been standing or spilling. Our guide to fascia capping explains that the board behind a gutter that has leaked for years is often not in a fit state to be covered.
The final check needs a stable ladder and a helper, or better a survey: a tape and a spirit level on the front of the gutter at each end. Please keep off the roof and avoid reaching from a ladder at first-floor height. If you suspect a problem, it is safer to have it checked from a tower.
Setting the fall right, and what it takes
Correcting a gutter's fall means re-fixing the brackets so the run slopes the right way. If the fascia is sound, it is a quick job. If it is rotten, the board comes first; see our page on signs that a fascia needs replacing and the materials covered in fascia and soffit materials. Fixing a gutter to a soft board is the surest way to lose the fall again within a season.
Gutter replacement is priced nationally by MyBuilder, in its guide updated in August 2026, at £30 to £70 per metre, with scaffold extra. Whether a job needs full replacement or a reset is settled at survey, and the written quote afterwards is a single total, so ask any quote you receive whether it includes VAT and scaffold.
When we survey, we photograph the run from each end, check the slope against the maker's guidance for that system and say whether the fix is a reset or a renewal. We fit the remedy to the fault and no more. What we fit is guaranteed in writing for 10 years, and the rubbish goes with us. Call 01793 626096, message on WhatsApp or use the form on our gutters, soffits and fascias page.