Question answered
What fall should a gutter have?
The short answer
A gutter should fall gently towards its outlet, and manufacturers publish figures from roughly 1 mm for every 600 mm of run to about 1:350 for some systems. This is manufacturer guidance, not a statutory standard. On a 6 metre run, that is a drop of 10 to 17 mm. Too little fall leaves standing water, while too much looks untidy and can make water overshoot.
A gutter has to slope gently towards its outlet or water lingers in it. The slope is small, so small errors matter, and it is set by the manufacturer's guidance rather than a legal figure.
Where the figure comes from
Brett Martin's installation guidance for rainwater systems gives a fall of 1 mm for every 600 mm of gutter. FloPlast's MiniFlo guide says 1:350 is recommended. Put side by side, the two show that makers of plastic systems settle on a very gentle slope somewhere between 1:600 and 1:350, and the maker of your own gutter profile has the final say.
It is worth being clear what this is. It is guidance from the people who make and test the gutters, not a British Standard figure that an inspector will measure with a level. The point is practical: a gutter that is dead level holds a shallow pool, and a pool left to sit gathers silt, grows algae and, in a hard frost, can strain the joints. A small, steady slope keeps water moving without being visible from the garden path. Profiles are sold with slightly different advice, so read the instructions that came in the box before you set a line.
What that means on a real run
Fractions are easier to picture as a drop. On a 6 metre run, 1 mm per 600 mm is a fall of 10 mm from one end to the other. At 1:350, the same run drops about 17 mm. Neither is much more than the width of a finger, which is why a sagging bracket or a slightly bowed fascia can wipe out the whole fall without anyone noticing from the ground.
Where the outlet sits matters. With an outlet at one end, the full fall runs along the length. With the outlet in the middle of a long stretch, the gutter slopes both ways towards it, and each half needs its own fall. Brett Martin also advises placing supports no further apart than 900 mm, because a channel that bows between brackets makes dips that hold water however good the overall slope. Our page on why a gutter sags deals with that side of the problem.
Signs the fall is wrong
The easiest check is after rain. If water stands in the gutter along part of its length, that stretch lies lower than the outlet side of it. Green growth, silt lines and a damp, dark gutter bottom all point to slow drainage. Water that spills over the front lip at one place often shows a dip there, not a blockage.
A gutter with too much fall at one end can throw water past the stop end in heavy rain. Staining on the wall below, or a brown streak under a joint, can come from either fault. Before you decide the slope is wrong, rule out a blocked outlet or downpipe, which gives the same signs. Our guide to gutter guards explains what they do and do not solve, and none of them corrects a fall that was laid wrongly. Leaves are only one cause of slow drainage; a flat spot is another.
Why the fascia decides whether the fall holds
Gutter brackets screw into the fascia, and the fascia in turn is fixed to the rafter ends. If the board is bowed, rotten or fixed to rafters that have dropped, the gutter follows it. We have to set the brackets to a line that gives the fall, which sometimes means packing a bracket out or correcting the board itself. See signs your fascias need replacing for how to tell.
Old roofs built or extended over many years can have eaves that do not run straight, and a straight gutter on a crooked board needs a decision about where to hide the difference. The wider layout of fascia, soffit and gutter is explained in the roofline explained, and our gutters, soffits and fascias page lists the work we do. A survey of the eaves with a level and a string line settles the question before any quote is written.
Setting the fall when a gutter is replaced
When we fit new gutter, the first job is to find the lowest sensible point for the outlet and the highest point of the run, then strike a line between them on the fascia. Brackets follow the line, not the board. If the board wanders, we pack the brackets so the channel keeps a smooth slope and the water does not meet a dip. On short runs, say a porch or a bay, the fall can be tiny and still work.
Long runs behave differently. A garage with a 9 metre front can either fall one way to a single outlet or split in two to a central one, and the two choices change where the downpipes land, so it is worth deciding at the start. Whichever is chosen, we check the finished fall by running water through before we leave.
Gutter slope: two more points
Is a perfectly level gutter wrong?
Level is usually not what manufacturers recommend. Without a gentle fall to the outlet, water stands in the channel, collects silt and can overflow in heavy rain. Some systems tolerate a very small fall, so check the instructions for your profile and the outlet position.
Can I check the fall myself from a ladder?
A spirit level and a string line can show a dip, but a ladder against a gutter is risky and bracket spacing is awkward to judge. After rain, look from the ground for standing water. If you see it, send us pictures and we can say where the dip is.
Tell us about your roof
Describe what is happening and we will call you back to arrange a visit.
- Photographs of what we find
- A written quote with one total price
- Or call 01903 299063
10-year workmanship guarantee, in writing.