How do I fix a leaking gutter joint?

Why gutter joints leak, cleaning and reseating a union properly, and why sealant is usually the wrong answer.

Difficulty: beginner
Time: 1 hr
Read: 4 min
Safety: warning

Short answer

Most gutter joints leak because the rubber seal is dirty, distorted or has rolled out of its groove. Take the joint apart, clean everything, check the seal, lubricate it and reassemble to the correct insertion depth. Sealant applied over the outside almost never lasts.

A gutter union is a mechanical seal that relies on a clean rubber gasket and the right insertion depth. When one leaks, one of those two things is wrong, and adding silicone on the outside treats neither.

Safety

Ladder work at eaves height. Set the ladder at about 75 degrees on firm level ground, tie or foot it, and use a stand-off so it does not rest on the gutter. Never overreach — move the ladder. If the gutter is above single-storey height or the ground is uneven, get someone with a tower to do it. Never stand on a gutter or lean on a downpipe.

Setting the fall of a gutter, and the joint that has to move

Step by step

Confirm it is the joint that leaks.Pour water in above the joint and watch. Drips at the union are a joint problem; water running over the front edge is a blockage or a fall problem; water at the back is more likely a missing drip edge on the roof felt.
Empty and clean the run first.Silt and moss sitting in the gutter hold water permanently against the joint. A gutter that only leaks when it is dirty does not need a new seal.
Release the union.Most have a clip that levers up with a screwdriver, letting the gutter ends come out of the union. On a cold day, warm the plastic first — it is brittle and snaps.
Inspect the seal.Look for grit, a rolled edge, a flattened section or perishing. A seal that has taken a permanent set no longer springs against the gutter and has to be replaced.
Clean every mating surface.The seal groove, the seal itself, and the gutter ends. A single piece of grit under the seal is enough to leak. Wipe with a cloth and clean water.
Check the gutter ends are cut square and deburred.A rough or angled cut end drags the seal out of its groove when it is pushed in. File the end smooth and square.
Lubricate and reassemble to the mark.Silicone lubricant or washing-up liquid on the seal, then push the gutter in to the insertion line moulded inside the union — not fully home, so there is room for expansion.
Test and check the bracket support.Pour water through. Also check there is a bracket within about 150mm of each side of a union — an unsupported joint flexes and leaks however good the seal is.
Replace the union if the seal cannot be sourced.A new union is a few pounds and takes ten minutes. It is nearly always a better outcome than a patched seal.

Tips

Replace the whole union rather than hunting for a matching seal. Seals are brand-specific and often not sold separately.
If several joints leak on the same run, the problem is usually the fall or the bracket spacing, not the seals.
Do gutter work in mild dry weather. Cold plastic snaps and a wet ladder is a bad idea.

Common mistakes

Sealing over the outside of a leaking joint with silicone — It bridges a joint that is designed to move with temperature, so it tears within a season and leaves a mess that makes a proper repair harder. Fix the seal instead.
Pushing the gutter fully home when reassembling — It removes the expansion allowance, so the run pushes the joints apart in summer heat. Insert only to the moulded line.

If it doesn't work

Joint leaks only in heavy rain

Cause: Gutter running full because of a blockage or insufficient fall — Fix: Clear the run and check the fall to the outlet. The seal is not designed to hold back a gutter running at capacity.

Leak returns within weeks of resealing

Cause: Grit under the seal, or a distorted seal — Fix: Take it apart again, clean everything, and fit a new union rather than reusing a set seal.

Water dripping behind the gutter at the wall

Cause: Roof felt not lapping into the gutter — Fix: Fit an eaves protector or felt support tray so water from the roof underlay discharges into the gutter rather than behind it.

Joint pulled apart completely

Cause: Thermal expansion with no gap, or a bracket has failed — Fix: Refit to the insertion mark and add a bracket close to each side of the union.

Questions people ask

Can I use silicone to fix a leaking gutter joint?

As a genuine emergency measure for a few weeks, yes. As a repair, no — the joint is designed to move as the plastic expands, and sealant across it tears. Reseating or replacing the union is quicker anyway.

Why does my gutter leak at the same joint every year?

Usually because it is unsupported, or because the run has no expansion allowance so that joint takes all the movement. Add a bracket either side and reset the insertion depths along the run.