What size gutter do I need?

It follows from the effective roof area draining into it — plan area plus half the vertical projection of the slope — and your local rainfall intensity. Approved Document H publishes the pairings: roughly 37 square metres for a 100mm half-round, 53 for 115mm and 103 for 150mm, each with a matching outlet. A larger gutter on an undersized outlet does not help.

I added an extension and now the gutter overflows — connected?

Almost certainly. Adding roof area to an existing run increases the flow it has to carry without changing its capacity. This is one of the most common reasons a gutter that worked for decades suddenly does not.

Would a second downpipe fix it?

Often yes, and it is usually cheaper than replacing the whole run with a bigger profile. Splitting a long run so water travels a shorter distance to an outlet increases the flow the system can shift. Where it goes matters — it needs a gully or a proper connection, not a discharge onto the ground.

Why does water shoot straight over the gutter?

Because the gutter is set too low or too far out relative to the tile edge, so fast run-off from a steep roof clears the front lip instead of dropping in. Re-setting the height and projection fixes it; a bigger gutter on its own may not.

Caution

Updated 30 August 2026

Gutter Overflowing But Not Blocked

You have cleared the gutter and it still spills over in heavy rain, or it only ever overflows in the worst downpours.

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What's happening?

A clear gutter that still overflows is a capacity or geometry problem, not a cleaning problem. Approved Document H sizes gutters against the effective roof area draining into them, which is the plan area plus half the vertical projection of the slope — so a steep roof collects more wind-driven rain than its footprint suggests. Its table pairs a nominal 100mm half-round gutter with about 37 square metres of effective area, 115mm with about 53, and 150mm with about 103, with the outlet sized to match. Rainfall intensity also varies across England and Wales, roughly 0.016 to 0.022 litres per second per square metre depending on where you are. A gutter that was marginal when it was fitted overflows once an extension, a converted garage or a re-roof adds catchment to it.

What usually causes it?

Gutter genuinely undersized for the effective roof area draining into it, often original to the house and never reviewed
An extension, conservatory, dormer or converted garage roof added into an existing run without upsizing it
Outlet or downpipe too small for the gutter, or only one downpipe on a long run and set at the wrong end so water travels too far — a correctly sized gutter feeding an undersized outlet still backs up
Gutter laid to no fall or a reverse fall, so water sits rather than moving to the outlet
A roof valley discharging its whole catchment into one short section of gutter, overwhelming it locally
Deep-flow or square profile replaced with a shallower half-round on appearance grounds, losing capacity
Gutter set too low relative to the tile edge, so fast run-off shoots straight over the front lip in a downpour

Is it dangerous?

The consequences are the same as any overflow and they are cumulative: saturated walls, penetrating damp inside, perished pointing, frost damage to brick, and wet fascia, soffit and roof-edge timber. Because an undersized gutter overflows in the same place every heavy rain, the damage concentrates — you often see a green algae stripe down the wall marking years of it. There is no direct injury risk from the gutter, but do not investigate from a ladder in the rain, which is precisely the combination that produces falls.

Can I fix it myself?

Measure and observe, which is the useful part here. Note whether the overflow happens only in exceptional rain, which points at marginal capacity, or in any moderate shower, which suggests fall or outlet geometry. Measure the roof footprint draining into that gutter, count the downpipes, and photograph the profile and the tile-to-gutter relationship. If the gutter overflows the day after you cleared it, say so when you call — it stops you paying for another clean. Do not try to re-hang a gutter run yourself from a ladder; getting the fall right along several metres needs level access and two hands.

When should you call a fascia, soffit and guttering installer?

Call within weeks. This is a design fix rather than an emergency, and it wants doing before another winter of soaking the same patch of wall. Bring it forward if damp has already appeared inside on that elevation, or if the fascia below is soft. Mention any extension or re-roof done since the gutter was fitted, because that is very often the whole explanation and it changes the quote from a clean to an upsize.

What will the fascia, soffit and guttering installer actually do?

Calculate the effective roof area draining into the run, allowing for pitch, and compare it against Approved Document H gutter and outlet sizes for the local rainfall intensity, which varies materially across the country
Check the existing fall with a level along the whole run and measure the gutter-to-tile relationship
Recommend upsizing the profile, adding a second downpipe, or repositioning the outlet — whichever the numbers actually call for
Re-hang the run to a consistent fall towards the outlet, with brackets at the correct centres for the profile
Set the gutter height and projection so run-off from the tile edge lands in the gutter rather than over its front lip
Separate a valley or a large roof area onto its own outlet where one point is being overwhelmed

Frequently asked questions

We’re happy to answer all your questions.

Sources & References

Gutter Overflowing In Heavy RainCaution Sagging GutterCaution Roof Valley LeakingHigh risk

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