The 4 Causes of a Leaking Gutter Joint (and How to Tell Which You Have)
Identifying the cause before touching anything prevents the most common mistake: applying sealant to a joint that needs something different.
Cause 1: Failed rubber gasket
The most common cause. The rubber or neoprene seal inside the union joint degrades over time, losing elasticity and the ability to create a watertight compression seal. Signs: water drips steadily from the joint during rain even when the gutter is clear of debris. The joint itself looks intact from outside. Fix: replace the gasket (£2 to £5).
Cause 2: Debris trapped in the seal
A small piece of grit, leaf fragment or roof granule lodged between the gutter section and the seal prevents full compression. Signs: the leak appeared shortly after a storm or heavy leaf fall. The gutter otherwise drains well. Fix: unclip, clean the seal face, refit.
Cause 3: Gutter out of alignment
If a gutter section has sagged or shifted, it puts lateral stress on the adjacent joint, pulling the seal out of its seated position. Signs: the joint leaks but also appears slightly misaligned or angled. There may be a visible gap on one side of the joint. Fix: rehang the adjacent gutter section and correct the fall before resealing.
Cause 4: Cracked union joint fitting
The plastic connector piece itself has cracked, usually from frost damage, UV embrittlement or physical impact. Signs: a visible crack in the connector plastic, often at the point where the clip engages. No amount of sealant fixes this permanently. Fix: replace the union joint fitting (£3 to £8).
Tools and Materials Needed
For a standard gasket replacement or debris-clean repair:
| Item | Approximate cost | Where to buy |
|---|---|---|
| Replacement rubber gasket (matching profile) | £2 to £5 | Builders merchant, online |
| EPDM gutter sealant (not silicone) | £6 to £12 | Screwfix, Toolstation, B&Q |
| Stiff brush or old toothbrush | £1 to £3 | Supermarket |
| Cloth and bucket of warm soapy water | Under £1 | At home |
| Flat-head screwdriver | Already owned | N/A |
| Replacement union joint (if cracked) | £3 to £8 | Builders merchant |
One critical note on sealant: use gutter-specific EPDM sealant, not standard silicone. Silicone does not bond well to uPVC and deteriorates quickly when exposed to standing water. The right product says "gutter and downpipe" on the tube and is typically grey or black. Cost difference: about £2. Performance difference: years.
Step-by-Step: Fixing a Failed Gasket (Most Common Case)
This repair works for both the gasket-only replacement and the debris-trapped-in-seal scenario. Safe for single-storey gutters or where a stable ladder gives secure access to a two-storey section.
- Dry the joint area — do not attempt this in rain. The gutter must be dry for sealant to bond. Wait at least 24 hours after rain if weather permits.
- Unclip the gutter section — press down on the front lip of the union joint clip and lever the gutter section out of the connector. On older fittings the clip may be stiff; a flat-head screwdriver used as a lever at the clip point releases it without damage.
- Remove the old gasket — pull it out of the channel inside the union joint. On older systems the gasket may have fused to the channel; work it free gently. Note its profile shape (the replacement must match).
- Clean all seal faces — scrub the inside of the union joint channel and the end of the gutter section with a stiff brush and warm soapy water. Remove all grit, debris and old sealant residue. Rinse and allow to dry completely.
- Fit the new gasket — press the replacement gasket firmly into the channel, ensuring it seats evenly with no twisting or bunching.
- Apply sealant if needed — for a new gasket in good condition, sealant is optional. For an older system where confidence in the seal alone is low, run a thin bead of EPDM gutter sealant around the inside of the channel before fitting the gasket.
- Refit the gutter section — push the section back into the union joint until the clip engages with an audible click. Check it is seated evenly on both sides.
- Test with water — pour a bucket of water into the gutter above the joint and watch the joint closely. A correctly fitted gasket produces zero drips.
The One Mistake That Makes It Leak Again
Applying new sealant without cleaning the joint face. This is the single most common cause of a repair that works for six months and then fails again.
Sealant applied over existing sealant residue, grit or biological growth (algae, moss film) does not bond to the gutter surface — it bonds to the contaminant layer. When that layer eventually breaks down or moves with thermal expansion, the new sealant moves with it and loses its seal.
Clean thoroughly, dry completely, then apply sealant. Skipping either step reliably produces a repeat repair within one to two seasons.
Second most common mistake: using the wrong sealant profile. If your gutters are square profile and you buy gaskets from a box of half-round spares, they will not sit correctly in the channel. Always take the old gasket to the merchants to match it, or photograph the union joint fitting and its moulded profile number (usually printed on the fitting).
When to Call a Professional Instead
Two situations make professional repair the right call even though the job is technically simple:
Two-storey or higher access: Resealing a joint requires working with both hands for several minutes while positioned accurately at the joint. This cannot be done safely from a ladder on a two-storey wall without a proper ladder system, standoff stabilisers and ideally a second person. The HSE guidance on working at height is clear: sustained work above 2 metres requires appropriate equipment, not just a ladder leaned against the wall. A professional visit costs £60 to £100 and takes 30 minutes. The equipment and time cost of doing it safely yourself is comparable.
More than two joints leaking: Multiple failing joints across the same gutter run indicate that the system is reaching end of life, not that there is a single point failure. Repairing three or four individual joints may cost more than a full gutter replacement and leaves you with a system that will continue to fail at other joints within the next two to three seasons.