A gutter needs a gentle fall towards the outlet so that water runs along it and away, not sits in it. Brett Martin, a gutter manufacturer, advises a fall of 1 mm for every 600 mm of gutter. That is a very slight slope: along a 6 metre run, the high end sits about 10 mm above the low end. You cannot see it by eye, and that is exactly why so many gutters are fitted flat or the wrong way round.
The consequences of a poor fall show up in familiar ways. Water that stands in the channel after rain grows moss and leaves a tide-mark. Gutters that are not draining properly overflow at the low spots, stain the wall below and soak the fascia. A gutter that is overloaded by standing water drags on its brackets, so it sags further, and the fall gets worse each year.
This guide explains what a fall is, the manufacturer's figure and the standard behind rainwater design, how a fall is set, the signs that a gutter's fall has gone wrong and how it is put right. It also explains why a gutter fall is not the same as the fall on a flat roof, which is a different figure under a different standard.
What fall means and why it matters
A fall is the slope of a gutter along its length. It is expressed as a drop over a distance, and it is always small. The purpose is to make water move to the outlet by gravity, so that it leaves the gutter and enters the downpipe, instead of lingering.
The effect of too little fall
A gutter with no fall, or a fall that has been lost, holds a thin layer of water along its length. The water does not run off in the minutes after rain stops. It stays and evaporates slowly. Moss, algae and silt build up in the standing water, and the gutter fills faster each time. In frost, the standing water freezes and can crack joints or push brackets.
The effect of too much fall
Too much slope can make water rush to the low end in heavy rain, so it overshoots the outlet and spills over the stop end. It also makes the front of the gutter look crooked against the straight line of the fascia, and it may need packing or brackets at different heights.
A balance
The right fall is a compromise between draining well and looking right. It has to be enough to carry the water, but not so much that it is visible or causes overshooting.
Where the water goes
With a good fall, rain from the roof is collected, carried to the outlet and sent down the downpipe to a drain or soakaway. Our answer on overflowing gutters explains how a blocked or flat run ends up spilling, and the clues to check first.
The manufacturer's figure and the standard
Two published references bear on gutter fall. One gives a number, and the other sets the framework for rainwater design.
| Reference | What it says |
|---|---|
| Brett Martin, installation guidance | A fall of 1 mm for every 600 mm |
| BS EN 12056-3:2000 | Roof drainage layout and calculation; listed as current by BSI |
Brett Martin's guidance
Brett Martin's guide to installing a rainwater guttering system gives 1 mm for every 600 mm. It is the manufacturer's figure for its own systems, so the maker's data sheet for your gutter is the first place to check. Other makers publish their own guidance, and the figures can differ a little.
BS EN 12056-3
BS EN 12056-3:2000 is the European standard for gravity drainage systems inside buildings, and its part 3 covers roof drainage layout and calculation. The British Standards Institution lists it as current. It deals with how much water a roof sheds and how gutters and outlets are sized to carry it. The fall figure in this guide comes from the maker's guidance, and the standard supplies the wider design framework.
Why both matter
The manufacturer's number gives a practical slope for fitting. The standard is the wider framework for whether the gutter, its outlets and its downpipes can carry the water from the roof. A gutter with the right fall but too few outlets can still overflow in a heavy downpour.
For a flat roof, falls are a different matter. A flat roof surface works to BS 6229:2025, with a finished minimum of 1:80, and that figure does not apply to a gutter.
How a fall is set during fitting
Setting a fall is a matter of measuring from a fixed point and spacing the brackets so that the gutter drops by the right amount across its length. It is not difficult, but it needs patience and a line.
Where the water leaves
The outlet, the opening to the downpipe, is the low point. Its position is fixed by the downpipe, so the first decision is where the water will go. On a long run, an outlet in the middle allows a fall in each direction, halving the drop needed at each end.
Marking the line
At the high end, the first bracket is fixed at a height that leaves room for the gutter to sit below the roof edge. The roofer then sets the low end using the fall: for a 6 metre run at 1 mm per 600 mm, about 10 mm lower. A string line pulled tight between the two points shows where each bracket goes, and a spirit level confirms it.
Brackets and support
Brackets must be spaced so that the gutter does not sag between them, as it would when full of water. The maker gives spacings for each system, and heavy materials like cast iron need closer support. Brackets fixed to the fascia need a sound board. If the fascia has rotted, the brackets will not hold, and our guide to eaves protection trays explains why the edge needs checking when boards are replaced.
Checking the result
Once fitted, the gutter is tested with water from a hose or a bucket. The water should run to the outlet without pooling. Any puddle that remains shows a low spot, and a bracket is adjusted.
The aim is a gutter that drains fully, so a hose test is worth the few minutes it takes.
Signs that the fall is wrong
You can often tell that a gutter has the wrong fall without a level. Water leaves its own evidence, and a few minutes of watching during or after rain is revealing.
What to look for
- Standing water in the channel some time after rain has stopped, especially in the middle or at the far end.
- Overflow at one spot while the rest of the gutter runs freely, usually at a low point.
- Tide marks of silt or moss along the bottom, showing where water sits.
- Green growth in one stretch of the gutter that is thicker than the rest.
- Stains on the wall or fascia below one section, where water spills or runs over.
- Sagging between brackets, or a gutter that has dropped from the fascia.
Checking safely
From the ground, look along the gutter in rain, or just after it. Watch where the water runs and where it pools or spills. Do not climb a ladder in wet weather. A dry day with a hose and someone holding a ladder at a safe, level position gives the same information more safely. Our answer on sagging gutters shows how to separate a lost fall from a failed bracket.
Not always the gutter
A gutter that overflows may be blocked, not flat. Clearing it first removes that cause. If it still pools after a clean, the fall is the problem. That is why we clean and test before we recommend changes.
On a survey we photograph the water standing in the gutter, where we find it, so you can see the evidence.
Why falls are lost over time
A gutter that was fitted with a good fall can lose it. The usual causes are worth knowing, because they point to the cure.
Failed brackets
Brackets are the main support. Corroded or bent ones, or ones that have pulled out of soft timber, let the gutter drop. A gutter full of water, leaves and sometimes ice is heavy, and the load on the brackets is large. A single failed bracket creates a low spot.
A rotten fascia
Where the fascia board is rotten, it cannot hold the fixings, and the whole run moves. The fall may be lost along the length, and the gutter may pull away from the roof edge. The fix is in the board and not in the gutter.
Movement of the structure
Buildings settle and move. Over decades, a long run of gutter on a wall or roof that has shifted may see its fall alter. Timber shrinks and swells, and so does the fascia.
Weight of debris and ice
Repeated loading of leaves, silt, snow or ice can bend a bracket or open a joint. A gutter that has been left uncleared for years will often have lost its line.
Poor original fitting
Some gutters were never right. Fitted by eye, or with the fall running the wrong way from the outlet, they never drained fully.
In each case, the fall is a symptom. Putting it right means finding the cause: replacing the brackets, repairing the fascia, or re-hanging the run from a fresh line. Doing it without finding the cause means the fall is lost again.
Putting the fall right
Correcting a gutter's fall is usually a matter of re-hanging the run, and it may be a small job or part of a larger one.
Re-hanging an existing gutter
Where the gutter is sound but its fall has been lost, the run is unclipped or unbolted, brackets are re-set along a fresh line, and any failed ones are replaced. The gutter is refitted with its joints checked and resealed. This is the most economical route, and it suits a gutter in good condition on a sound fascia.
Repair the fascia first
If the board is soft, it is repaired or replaced, then the gutter is hung. Boards that are only tired can sometimes be kept, and the survey shows which are sound.
Replacing the gutter
If the gutter itself is cracked, rusted or distorted, re-hanging cannot fix it. It is replaced, and the new run is fitted to a line from the start, with the brackets spaced to the maker's guidance and the outlet at the lowest point.
More outlets or a larger gutter
On a long roof, one outlet may not carry the flow, and an extra one or a bigger gutter profile helps. That is a design decision, drawn from the roof area and the standard.
What it costs
The price depends on length, access and what needs replacing. MyJobQuote gives gutter replacement at around £1,000 to £1,600 (Oct 2026), and Which? gives clearing at £75 to £350, ex VAT (Nov 2025). Re-hanging sits well below replacement, but the survey gives your own figure. For the work we cover, see our gutters, soffits and fascias page.
