The gutter overflows in heavy rain but not in light rain: what that means
A blockage does not care how hard it is raining. A gutter that copes with drizzle and fails in a downpour is telling you something else entirely.
A blocked gutter overflows in light rain as well, because the water has nowhere to go at any rate. If yours copes with drizzle and goes over in a downpour, the gutter is being asked to carry more than it can. That is a capacity, fall, outlet or overshoot problem, and clearing leaves will not fix it.
The short version
- A blockage does not care how hard it is raining, so a gutter that only goes over in a downpour is not blocked.
- Watch it from a window in the next downpour and note where it goes over. Mid-run, at the outlet, at a joint, at the end or over the front all along: each points somewhere different.
- A gutter looking half empty while water arcs over the front is overshoot. A bigger downpipe does nothing.
- Moving or adding an outlet often beats replacing the run.
- Water over the back edge feeds one strip of wall all winter, and that strip spalls at the first hard frost.
The distinction
Rainfall rate is the tell, and it rules a blockage out
A blockage is a fixed obstruction and it does not care about the weather. Water backs up behind it and tops the gutter at that point whatever the rate. In drizzle it takes longer, but it gets there, because nothing is leaving. A blocked gutter spills on an ordinary wet Tuesday, not only in a storm.
A capacity fault is a rate fault. Water arrives faster than the system can take it away. Below that rate everything works. Above it, the gutter fills faster than the outlet empties it and the surplus goes over an edge. Same gutter, same leaves, different weather.
So “only in heavy rain” narrows it to five things: the gutter is too small for the area draining into it, the fall is wrong, the outlet is in the wrong place or cannot swallow the flow, the water is overshooting the gutter entirely, or something is discharging into that run that was never allowed for.
One honest caveat. A gutter half full of silt copes with drizzle and not with a downpour, so a partial blockage behaves like a design fault. Clear it and look again first, and for when to do that see autumn gutter clearing and why timing beats frequency. If it still only goes over in heavy rain with a clean gutter, you are into the five.
The method
Watch it from a window during the next downpour
The diagnosis is only available while it is raining hard, which is exactly when nobody should be up a ladder. That is fine, because the view you need is from below and from the side.
Not from a ladder. The Health and Safety Executive says you should not consider going on any roof in poor weather conditions such as rain, ice, frost or strong winds, and notes that winds over 23mph affect balance. Heavy rain is the worst possible time to be on a ladder.
Pick an upstairs window giving a sideways view along the run. Ten minutes of proper rain is enough, and a phone on the sill lets you watch it twice. Note whether the gutter is brimming or half empty, whereabouts the water leaves, and whether it goes over the front lip and falls clear or over the back edge and down the wall.
| Where it goes over | What that suggests | What to check |
|---|---|---|
| Front lip, most of the run, gutter half empty | Overshoot: water leaves the tiles fast and clears the gutter | Pitch, tile projection, and the gutter’s height below the tile edge |
| Brimming the whole length, front and back | Under-capacity for the area draining in | Gutter size and profile against effective roof area, and downpipe count |
| At the outlet, deepest there | The outlet or downpipe cannot take the flow | Outlet size, the swan neck, the shoe and the gully |
| Mid-run, away from any outlet, standing water there on a dry day | Fall level or reversed, usually a dropped bracket | Sight along the front edge for a sag, then the brackets and fixings |
| At the far end, over a stop end | Too long a run for one outlet at the other end | Whether a second outlet, or moving the existing one, is possible |
| At one joint, and in light rain too | A failed union seal, or a bracket letting the joint open | That joint and the bracket either side |
| Where a valley, dormer or another downpipe discharges | Concentrated flow the run was never sized for | Everything draining in, and whether it was all there when the gutter was fitted |
What sets the capacity
A gutter is sized against area, pitch and one outlet
Rainwater is a building regulations matter. Schedule 1 of the Building Regulations 2010 says at requirement H3 that adequate provision shall be made for rainwater to be carried from the roof of the building, and Approved Document H shows how that is normally met: flow depends on the area drained and the rainfall intensity, read off a map of England and Wales, and capacity on the size and fall of the gutters and pipes.
A gutter does not see the footprint of your roof. It sees an effective area, and a steeper roof presents a bigger one from the same plan area.
| Surface | Effective design area |
|---|---|
| Flat roof | Plan area of the relevant portion |
| Pitched at 30° | Plan area x 1.29 |
| Pitched at 45° | Plan area x 1.50 |
| Pitched at 60° | Plan area x 1.87 |
| Pitched over 70°, or any wall | Elevational area x 0.5 |
A steep slate roof hands its gutter close to half as much again as a shallow one of the same footprint, and the standard domestic half round is the same size on both. Nothing is broken there. The run was never big enough.
The outlet does more work than the gutter size
Approved Document H’s table of gutter and outlet sizes carries an assumption printed underneath it that matters more than the numbers. Those sizes are for a gutter laid level, half round in section, with a sharp edged outlet at one end only, and past a stated distance from the stop end the capacity should be reduced. A long run with one end outlet is the worst case it knows about.
Move the outlet and the arithmetic changes. Where the outlet is not at the end, the gutter is sized for the larger of the two areas draining into it, so an outlet in the middle turns one long run into two short ones. Gutters should also fall towards the nearest outlet, which is why a level or backwards-falling run ponds mid-span and tops the front edge before the water reaches the pipe.
The design route beyond that is BS EN 12056-3, on roof drainage layout and calculation, whose scope applies where the outlets are large enough not to limit the flow capacity of the gutter. The calculation assumes the outlet is not the bottleneck. If yours is restricted, by a tight swan neck, a leaf guard or a silted gully, the run behaves as though it were undersized when it is not.
The one people misdiagnose
Overshoot: the water never lands in the gutter at all
Tell overshoot apart by where the water lands, and you can do it after the rain has stopped. An overshooting gutter throws water clear, leaving a splash line on the path a foot or more out from the wall with dry brickwork above. A gutter spilling over its back edge does the opposite: a dark wet strip down the wall and a dry path. Wet wall, dry path is capacity or fall. Dry wall, splash line is overshoot.
Overshoot comes from water leaving the roof too fast, not from there being too much of it. A steep pitch flings water off the bottom course, and if the gutter sits too low or too far out from the tile edge the stream clears the front lip. Three things put it there: a fascia renewal that changed the bracket height, tiles projecting too little, and a valley or hip concentrating a sheet of water onto one short length of eaves.
The fixes are position and shape: raising the gutter or bringing it in, a deeper profile, or an eaves tray that turns the flow downward before it reaches the lip. A bigger downpipe will not help, and a quote offering one comes from somebody who has not watched it in the rain.
The commonest cause on an extended house
Something is discharging into a gutter that was never sized for it
If your house has been added to, start here. A gutter is sized for what drained into it on the day it was fitted, and adding a conservatory, a garage roof, a dormer or a loft conversion does not make anyone resize the run below.
Approved Document H is direct about the worst version. Where a roof with an effective area greater than 25 square metres discharges through a single downpipe onto a lower roof, a distributor pipe should be fitted to the shoe so the flow spreads wide enough at the receiving gutter not to over-top it. The Approved Document expects a single pipe emptying onto a lower roof to overwhelm the gutter beneath unless the flow is spread deliberately. Where a main roof downpipe ends at the extension gutter, the fix is often a shoe and a distributor rather than new guttering.
Valleys do it more quietly. Two slopes feed one channel and the whole of both arrives at one point in the gutter. Where a design has valley or parapet gutters and over-topping would mean water entering the building or wetting insulation, the Approved Document says the design should follow BS EN 12056 and that additional outlets may be needed. A choked valley is also where the expensive roof leaks start, traced in what a brown stain on the ceiling means.
So, on a dry day: what drains into this length of gutter, and was all of it there when the gutter went up?
Why it is worth fixing
It is a masonry problem, not a rainwater problem
Approved Document H assumes gutters will overflow sometimes and cares where the water goes when they do: any overflow in excess of the design capacity, caused by conditions such as above normal rainfall, should be discharged clear of the building. Over the front lip onto a path is the overflow the regulation tolerates. Over the back edge, between gutter and fascia and down the wall face, is the one it exists to prevent.
That spill goes down the same narrow band of wall every time, because the fault does not move. Brick and mortar are porous by design, and Approved Document C, on resistance to moisture, is written around a wall that takes a wetting and then dries out. A strip fed repeatedly through a wet autumn never gets the drying half. Then it freezes, the wet face has nowhere to put the expansion, and it lifts off in flakes, which is why frost damage to brickwork and paving shows up after the thaw. Clearing an outlet is a morning. Repointing a panel of wall is another job entirely.
Indoors, a damp patch high on a wall appearing after a long soaking rather than a cloudburst is the signature of water coming over a gutter rather than through the roof: compare it against water coming in only during driving rain. Low down, where a downpipe discharges badly, you get the misread covered in damp at the bottom of a wall, and a path or drive falling back towards the house makes it worse.
Next steps
What to do about it, in order
An overflowing gutter is diagnosed before it is quoted: clear it on a dry day, watch it from an upstairs window in the next heavy rain, and read the splash line once it is dry. The first three cost nothing and none involves height in bad weather.
- Clear the gutter, the outlet and the gully on a dry day, so the next reading is clean. If you would rather not go up, that is what gutter and exterior cleaning is for.
- On the next heavy rain, watch from an upstairs window for ten minutes and compare what you see against the table above.
- Once dry, look for the splash line. Wet wall and dry path is capacity or fall. Dry wall and a line on the path is overshoot.
- Sight along the front edge from one end. A sag or reversed fall shows up against the line of the fascia, and standing water confirms it.
- Count what drains into that run, count the downpipes serving it, and note where each outlet sits.
- Only then take quotes, and ask each contractor which of the five it is and how they know. Somebody who has watched it answers specifically.
A fair share of “you need new guttering” quotes are really “the outlet is at the wrong end” or “a bracket has dropped”. If the wall underneath has had a winter of it, the brickwork is the other half of the job, worth looking at together rather than two trades pointing at each other. See roofing, gutters and rainwater goods, or ask for a free site visit. Thomas comes out himself and will tell you if it can wait.
Questions
Common questions
Does a gutter overflowing in heavy rain mean it is blocked?
Usually not, a blockage stops the water leaving at any rainfall rate, so a blocked gutter spills in light rain as well. Overflowing only in a downpour points instead to capacity, fall, outlet position, overshoot or something extra discharging into that run.
Will a bigger downpipe stop the gutter overflowing?
Only if the outlet or the pipe is the bottleneck. Approved Document H says a rainwater pipe should be at least the size of the outlet it serves, so an undersized pipe is worth fixing. If water is arcing over the front of a half-empty gutter, a bigger pipe changes nothing.
How do I tell overshoot from an overflowing gutter?
Look at where the water lands. Overshoot throws it clear of the wall and leaves a splash line on the path a foot or more out, with dry brickwork above. A gutter spilling over its back edge leaves a dark wet strip down the wall and a comparatively dry path.
Do I need the whole run of guttering replaced?
Often not, approved Document H sizes a gutter against the largest area draining to any one outlet, so adding or moving an outlet turns one long run into two short ones and can be the whole answer. Ask a contractor which fault they have found before agreeing to a full replacement.
Can I go up and look while it is raining hard?
No, HSE guidance is not to go on a roof in poor weather including rain and gusting winds, and it notes that winds over 23mph affect balance. Everything you need to see is visible from an upstairs window, and the view from below is better than the view from the gutter anyway.
Read next
Related advice
The guides below pick up where this one leaves off, and the roofing page covers the work itself and the places we cover, if you would rather have the job looked at.
Who wrote this
Thomas Hannen
Thomas owns Bournemouth Roofing & Building and prices every job himself instead of sending somebody else. The company is Bournemouth Roofing & Building Ltd, number 15471719, registered in England and Wales. If you want to talk something through before you commit to anything, the site visit and the written itemised quote are both free.
Free quote
Gutter going over the wall every time it rains hard?
Thomas comes out himself, looks at the run, the outlet, the fall and the wall underneath, and puts in writing what he found and what it would take to put right.