Gutter Joint Leaking? How to Fix It Properly
To fix a leaking gutter joint, remove the union clip, clean both gutter ends and the fitting, replace the rubber seal if it has degraded, refit the union with the correct expansion gap, and apply a bead of silicone sealant as extra insurance. The whole repair takes 30–60 minutes per joint and costs virtually nothing if you already have sealant. Most leaking joints are caused by thermal movement displacing the seal, debris trapped in the joint, or a hardened seal that no longer flexes. This is the most common gutter repair in the UK and one of the easiest.
A dripping gutter joint might seem trivial, but left unfixed it causes disproportionate damage. The constant trickle saturates the fascia board directly behind the joint, promotes rot, stains the brickwork below, and can lead to penetrating damp in the wall. One leaking joint on a north-facing elevation — where it never dries out — can cause hundreds of pounds of damage in a single winter.
Why Gutter Joints Leak
1. Thermal Expansion Has Displaced the Seal
PVC-U expands and contracts with temperature. A 4-metre length of gutter can move 3–4 mm between a cold winter night and a hot summer day. If the expansion gap inside the union joint is too small — or was not left at all during installation — this movement pushes the gutter ends past the seal, breaking the watertight contact.
Over many heating and cooling cycles, even correctly installed joints can see seals shift out of position.
2. Debris Trapped in the Joint
Leaf fragments, grit, moss, and silt can work their way between the gutter end and the seal. This prevents the seal from making full contact with the PVC surface, creating a channel for water to escape.
3. Seal Has Hardened or Degraded
Rubber and EPDM seals lose flexibility over time — especially on south-facing elevations where UV radiation accelerates ageing. A hard seal cannot compress against the gutter surface and water seeps through.
4. Gutter Sections Have Shifted
If a bracket near the joint has pulled away from the fascia, or the fascia board itself has bowed, the gutter sections may have moved apart inside the union. Even 2–3 mm of displacement is enough to break the seal.
5. Incorrect Joint Type
Occasionally, a joint leaks because the wrong fitting was used — for example, a union designed for a different gutter profile, or a running outlet where a union should be. If the fitting does not match the gutter, no amount of sealant will make it watertight long-term.
How to Fix a Leaking Gutter Joint
Tools and Materials
| Item | Purpose |
|---|---|
| Ladder or scaffold tower | Safe access |
| Screwdriver (flat-head) | Releasing union clips |
| Old cloth or sponge | Cleaning |
| Warm soapy water | Cleaning gutter ends and fitting |
| Replacement rubber seal | If existing seal is hard or damaged |
| Clear silicone sealant (exterior grade) | Belt-and-braces sealing |
| Sealant gun | Applying silicone |
Step-by-Step Repair
Step 1: Identify the Leaking Joint
If you can see the drip from the ground, you already know which joint it is. If not, run a hose into the gutter and walk along underneath, watching each joint for drips. Mark the leaking joint(s).
Step 2: Clear the Gutter
Remove any debris from the gutter on both sides of the joint. You need a clean, dry working area.
Step 3: Release the Union Clip
Most PVC union joints have a front clip or locking mechanism. Release it by pressing or levering with a flat-head screwdriver. Once released, the gutter sections can be lifted out of the union on both sides.
Step 4: Remove the Union
Lift the union off the gutter ends. It should come away freely once the clips are released.
Step 5: Inspect and Clean
- Clean the gutter ends — wipe with warm soapy water to remove grime, algae, and old sealant residue. The PVC surface must be clean for the new seal to grip.
- Clean the inside of the union — same process. Check for cracks in the fitting itself.
- Inspect the seal — remove it from the union and check its condition:
| Seal Condition | Action |
|---|---|
| Soft, flexible, no cracks | Re-use — it is fine |
| Hardened, inflexible | Replace |
| Cracked or split | Replace |
| Flattened (lost its cross-section) | Replace |
| Displaced (out of its groove) | Reposition — may be reusable |
Replacement seals are available from builders’ merchants, or you can buy a universal gutter seal strip and cut to length.
Step 6: Apply Silicone Sealant
Run a thin, continuous bead of clear exterior-grade silicone sealant along the inside of the union where the seal sits. This is the belt-and-braces layer — it catches any water that gets past the primary seal.
Do not use silicone as a substitute for the seal — it does not have the flexibility to accommodate thermal movement on its own. Use it in addition to the seal.
Step 7: Refit the Union
- Place the seal back into its groove (or fit the new seal)
- Slide the union over the first gutter end — push until you reach the expansion mark inside the fitting (usually a moulded line)
- Slide the second gutter end into the other side — again, up to the expansion mark
- The gap between the two gutter ends inside the union should be 6–10 mm — this is the expansion gap
- Lock the union clip back into position
Step 8: Check for Expansion Gap
Look inside the union (if you can see in) and confirm there is a visible gap between the gutter ends. If they are butted tight together, the next hot day will push them apart and the seal will fail again.
Step 9: Test
Run the hose through the gutter and watch the repaired joint from underneath. No drips? Job done.
When Silicone Alone Is Not Enough
A common bodge is to smear silicone over the outside of a leaking joint without dismantling it. This might work for a few weeks but fails because:
- Silicone does not bond well to dirty, wet PVC
- It cures rigid and cracks when the joint moves with temperature
- It traps water behind it, potentially making the problem worse
- It looks terrible
Always dismantle the joint, clean it, fix the root cause (seal or gap), and then use silicone as an additional internal seal — not as a surface plaster.
When to Replace the Fitting
Replace the union clip entirely if:
- The fitting itself is cracked or warped
- The seal groove is damaged and cannot hold a new seal
- The clip mechanism is broken and will not lock
- The fitting is from a different system and does not match the gutter profile
- Repeated repairs at the same joint keep failing
Replacement union clips are cheap — typically £2–5 each. It is not worth nursing a damaged fitting when a new one costs less than a tube of sealant.
Browse Kalsi’s rainwater system fittings for replacement union joints, running outlets, and accessories.
Preventing Future Leaks
Correct Expansion Gaps
The number-one prevention is ensuring every joint has the correct expansion gap. Check during installation and re-check if you are doing any gutter maintenance.
| Temperature at Time of Fitting | Expansion Gap to Leave |
|---|---|
| Cold (0–10°C) | 8–10 mm |
| Mild (10–20°C) | 6–8 mm |
| Hot (20°C+) | 4–6 mm |
In cold weather, the gutter is contracted and needs more room to expand when summer arrives. In hot weather, it is already expanded, so less gap is needed.
Regular Joint Inspection
During your twice-yearly gutter clean, visually inspect every joint for:
- Visible displacement (gutter ends no longer centred in the union)
- Staining on the fascia below a joint (sign of a slow drip)
- Gap between the gutter and the union body
Use Quality Fittings
Cheap gutter fittings with thin seals and weak clips fail faster. Quality PVC-U systems from manufacturers like Kalsi Plastics use thicker seals, more robust clip mechanisms, and deeper seal grooves that maintain a watertight connection for decades.
Bracket Support Near Joints
Fit a bracket within 150 mm of each side of every union joint. Unsupported joints sag, move, and leak. This is especially important on long runs where the weight of water in the gutter creates lateral forces.
Frequently Asked Questions
How to fix a leaking gutter joint?
Remove the union clip, clean both gutter ends and the fitting, check and replace the seal if it is hardened or damaged, apply a bead of silicone sealant inside the joint, refit the union with a 6–10 mm expansion gap, and lock the clip back in place. Test with a hose to confirm the leak is fixed.
Why do gutter joints keep leaking?
Repeated joint failure is usually caused by inadequate expansion gaps. PVC expands in heat — if the gutter ends are butted tight inside the union, thermal movement pushes them past the seal every summer. Ensure a 6–10 mm gap exists between gutter ends at every joint.
Can I fix a gutter leak without dismantling the joint?
Applying sealant over the outside of a joint is a temporary patch at best. For a proper, lasting repair, you need to dismantle the joint, clean the surfaces, fix or replace the seal, and reassemble with the correct expansion gap. It takes 30 minutes but lasts years.
What sealant should I use on gutter joints?
Clear exterior-grade silicone sealant is the best choice. Apply it inside the union as a secondary seal behind the rubber gasket. Do not use silicone as a substitute for the gasket — it is not flexible enough to handle thermal movement on its own. Avoid acrylic sealant, which degrades faster in outdoor conditions.
How often should I check gutter joints?
Inspect all gutter joints at least once a year — ideally during your autumn gutter clean. Look for drips, staining below joints, and any visible displacement. Catching a failing joint early prevents water damage to the fascia and wall behind.
