How to Fix Overflowing Gutters (6 Common Causes)
Your gutter is overflowing because of one of six common problems: a blockage in the gutter or downpipe, incorrect fall causing water to pool, sagging or misaligned brackets, an undersized gutter for the roof area, a failed joint leaking water over the edge, or back-pitch where the gutter tilts outward. The most frequent culprit by far is a simple blockage — leaves, moss, and debris collect in the gutter channel and clog the downpipe outlet, causing water to back up and spill over. Before you call a roofer or replace anything, work through each of these six causes systematically.
Overflowing gutters are not just an annoyance. Water cascading over the front edge saturates fascia boards, stains brickwork, promotes damp, and sends water straight to the foundations where it can cause serious structural problems over time. Fixing the root cause quickly saves you far more than the cost of a new gutter system.
Cause 1: Blockage in the Gutter or Downpipe
The Problem
The single most common cause of gutter overflow. Leaves, twigs, moss, bird nests, tennis balls, and general wind-blown debris accumulate in the gutter channel. Over time, this debris migrates to the downpipe outlet and blocks it, either partially or completely. Water has nowhere to go, so it backs up and pours over the front edge.
How to Identify It
- Water overflows during rain but the gutter is not visibly damaged
- You can see debris piled up in the gutter when you look from a ladder
- The downpipe runs dry even during heavy rain — water is not getting through
- After rain stops, water sits in the gutter for hours
How to Fix It
- Clear the gutter by hand. Wear gloves, use a gutter scoop or old kitchen spatula, and work from a secure ladder or scaffold tower. Bag the debris — it makes excellent compost.
- Flush the downpipe. Push a hose into the top of the downpipe and run water through it. If it is blocked, the water will back up. Use a plumber’s drain rod or pressure washer lance to break the blockage.
- Check the shoe (bottom outlet). Debris often collects at the bottom of the downpipe where it connects to the drain or gully. Clear this too.
- Install a gutter guard or leaf mesh. This is the preventative fix. Mesh guards sit over the gutter and let water through while catching leaves. They do not eliminate cleaning entirely but reduce it from twice a year to once a year.
Prevention
- Clean gutters at least twice a year — spring (after blossom) and autumn (after leaf fall)
- Trim overhanging branches where possible
- Fit gutter guards if the property is near mature trees
Cause 2: Incorrect Fall (Pooling Water)
The Problem
PVC gutters should be installed with a slight fall — typically 3 mm per metre — towards the nearest downpipe outlet. If the gutter is level, or worse, falls the wrong way, water pools in low spots. During heavy rain, these pools overflow before the water can reach the outlet.
How to Identify It
- After rain, water sits in certain sections of the gutter but is empty near the downpipe
- You can see a visible dip or bow in the gutter line when you sight along it
- Overflow happens at the same spot every time — usually the low point furthest from the outlet
How to Fix It
- Use a spirit level or laser level to check the fall along the entire gutter run.
- Reposition brackets to create a consistent fall of 3 mm per metre towards the outlet. Start at the highest point (usually the end furthest from the downpipe) and work down.
- Use a string line between the highest and lowest brackets to ensure intermediate brackets are set at the correct gradient.
- Re-hang the gutter on the repositioned brackets.
The Right Fall
| Gutter Run Length | Total Fall Needed (at 3 mm/m) |
|---|---|
| 4 m | 12 mm |
| 6 m | 18 mm |
| 8 m | 24 mm |
| 10 m | 30 mm |
| 12 m | 36 mm |
If a gutter run is longer than 10–12 m, consider using two downpipe outlets — one at each end — with a central high point so the gutter falls in both directions. This also halves the flow rate each outlet needs to handle.
Cause 3: Sagging or Misaligned Brackets
The Problem
Over time, gutter brackets can bend, pull away from the fascia, or work loose — particularly on older timber fascia boards where screws lose their grip. When brackets sag, the gutter droops and creates low spots where water pools and overflows.
How to Identify It
- Visible sagging in the gutter line when viewed from the ground
- One or more brackets have pulled away from the fascia
- The fascia board behind the bracket is soft, split, or rotten
- Water overflows at the sagging section
How to Fix It
- Replace damaged brackets. PVC gutter brackets are cheap and widely available — keep a few spares.
- Replace rotten fascia sections. If the fascia board has deteriorated, new brackets will pull out too. Replace the damaged section with new PVC-U or treated timber fascia before re-fitting brackets. Kalsi’s fascia board range covers standard sizes.
- Increase bracket spacing if needed. The recommended spacing is 800–1,000 mm. If previous brackets were too far apart, add intermediate brackets to prevent future sagging.
- Use longer screws. When re-fixing to timber fascia, use screws long enough to penetrate through the fascia into the rafter feet behind — typically 40–50 mm stainless steel screws.
Cause 4: Undersized Gutter System
The Problem
The gutter is simply too small for the roof it is trying to drain. This is common on:
- Properties where an extension has increased the roof area draining to an existing gutter
- Houses where the original builder specified the minimum size (or undersized to save money)
- Properties in high-rainfall areas where a system sized for average conditions is overwhelmed by heavy storms
How to Identify It
- The gutter overflows during heavy rain but is fine in light showers
- The gutter is clean, properly aligned, and the downpipe is clear — nothing is wrong except capacity
- The property has a steep roof pitch (45°+) or a large roof area relative to the gutter size
- A mini gutter (76 mm) or standard half-round (112 mm) is fitted on a property that needs more
How to Fix It
-
Calculate the effective roof area draining to the gutter using the formula: (W + H/2) × L
-
Compare to the gutter’s capacity — see the gutter sizing guide for a full capacity table
-
Upgrade to a larger profile. If the numbers show your gutter is undersized, the fix is replacement. Consider:
- Deep-flow guttering for domestic properties with large roofs
- Hi-cap guttering for maximum domestic capacity
- Square-line as a step up from half-round
-
Add a second downpipe outlet. Sometimes the gutter profile is adequate but the single outlet cannot drain fast enough. Adding a second outlet halfway along the run effectively doubles the system’s throughput.
Cause 5: Failed Joints
The Problem
PVC gutters are joined using union clips with rubber seals. Over time, seals can degrade, shrink, or be displaced by thermal movement. When a seal fails, water leaks through the joint — sometimes visibly dripping, sometimes running behind the fascia board where you cannot see it.
A leaking joint can also cause water to escape over the front edge of the gutter if the leak is on the outer side of the union.
How to Identify It
- Water drips from the underside of the gutter at a joint (not the full length)
- Staining or algae growth on the fascia board directly below a joint
- The gutter sections have shifted apart visibly at the union
- During rain, you can see water escaping where two lengths meet
How to Fix It
- Remove the union clip by releasing the locking mechanism.
- Clean the gutter ends and the inside of the union — remove old sealant, grime, and debris.
- Check the seal. If it is cracked, hardened, or deformed, replace it. Seal kits are available from most merchants.
- Refit the union ensuring both gutter ends sit fully into the clips with the seal correctly positioned.
- Apply silicone sealant along the inside of the joint as a belt-and-braces measure.
- Allow for thermal expansion. When re-fitting, ensure there is an expansion gap (usually 6–10 mm) between the gutter ends inside the union. PVC expands in heat — if the lengths are butted tight, they push apart when it warms up and the seal fails again.
Cause 6: Back-Pitch (Gutter Tilted Outward)
The Problem
This is a less obvious but surprisingly common issue. When a gutter bracket is fitted so the front edge of the gutter is lower than the rear edge, the gutter tilts forward. Water runs to the front lip and overflows before it reaches the outlet.
Back-pitch is often caused by:
- Brackets fitted to a fascia board that is not vertical
- Brackets bent during installation
- The fascia board itself warping over time
How to Identify It
- Water overflows from the front edge even when the gutter is not full
- The gutter appears to lean forward when viewed from below
- Placing a spirit level across the gutter (front to back) shows the front is lower
How to Fix It
- Bend the bracket arm slightly to tilt the gutter back. Most PVC brackets have a malleable arm that can be adjusted.
- Pack behind the fascia bracket with a slim shim to adjust the angle.
- Replace the bracket if it is damaged or a type that cannot be adjusted.
- Check the fascia board. If the fascia itself is angled outward, fitting a new fascia board to the correct vertical alignment will solve the problem permanently.
Diagnostic Flowchart
Use this quick troubleshooting sequence:
- Is the gutter or downpipe blocked? → Clear the blockage
- Is the gutter level or falling the wrong way? → Reposition brackets for correct fall
- Are brackets sagging or pulling away? → Replace brackets and check fascia
- Is the gutter the right size for the roof? → Calculate effective area and upgrade if needed
- Are joints leaking? → Replace seals and re-fit unions
- Is the gutter tilted outward (back-pitch)? → Adjust bracket angle
If you have worked through all six causes and the gutter still overflows, there may be a structural issue with the roofline — a warped fascia, a rafter that has dropped, or a soffit that is directing water behind the gutter rather than into it. At that point, a professional roofline survey is the next step.
When to Replace vs Repair
| Situation | Repair | Replace |
|---|---|---|
| Single blockage | ✓ | |
| One sagging bracket | ✓ | |
| One leaking joint | ✓ | |
| Multiple leaking joints (old system) | ✓ | |
| Persistent overflow from undersized gutter | ✓ | |
| Cracked or UV-damaged gutter lengths | ✓ | |
| Fascia rot behind the gutter | Fix fascia + possibly replace gutter | ✓ if gutter is also old |
If the gutter system is more than 20 years old and showing multiple problems, a full replacement is usually more cost-effective than patching individual issues. A new PVC-U system from Kalsi Plastics — including gutters, downpipes, and all fittings — can be fitted in a day and will last another 30+ years.
Frequently Asked Questions
Why is my gutter overflowing in one spot?
An overflow at a single point usually indicates a localised blockage, a sagging bracket creating a low spot, or a failed joint. Check the gutter at the overflow location first — clear any debris, inspect the bracket, and examine any nearby joints.
Can overflowing gutters cause damp?
Yes. Water running down the wall from an overflowing gutter is a leading cause of penetrating damp. Over time it saturates brickwork, soaks into the mortar, and can reach the internal wall. The fascia and soffit boards behind the gutter are also vulnerable to rot if they stay wet.
How often should gutters be cleaned?
At least twice a year — once in late spring after tree blossom and once in late autumn after leaf fall. Properties surrounded by mature trees, especially conifers, may need quarterly cleaning. A gutter guard reduces the frequency but does not eliminate the need entirely.
Should I repair or replace old gutters?
If the gutter system is fundamentally the right size and the PVC is in good condition, repairing individual issues (brackets, joints, blockages) is cost-effective. If you are dealing with multiple failures, UV-damaged plastic, or a system that is undersized, full replacement is the better long-term investment.
Will gutter guards prevent overflow?
Gutter guards significantly reduce blockage-related overflow by keeping leaves and debris out of the gutter channel. They are not a guarantee — fine sediment and moss spores can still enter, and the guards themselves need occasional cleaning. But they are one of the most effective preventative measures, especially for properties near trees.
