How do I fix a sagging gutter?

Why gutters sag, restoring the fall so water actually runs, and the bracket spacing that stops it recurring.

Difficulty: intermediate
Time: 2 hr
Read: 4 min
Safety: warning

Short answer

Sagging is nearly always failed or too widely spaced brackets. Restore a fall of roughly 3mm per metre towards the downpipe, add brackets at no more than 800mm spacing — closer near joints — and replace anything rusted or split rather than trying to reuse it.

A gutter that sags stops draining, holds standing water, gets heavier, and sags further. It is a problem that accelerates, and the fix is mostly about support spacing.

Safety

Working at height is the most common cause of serious DIY injury. Use a ladder at the correct angle on firm level ground, never lean it against the gutter itself, and have someone footing it. For anything above single storey, or a long run, use a scaffold tower or a contractor. Never work on a roof edge in wind or wet conditions.

What you'll need

Replacement brackets and any damaged sections
String line and a tape measure
Drill, driver and appropriate screws
Ladder or tower, and gloves

Step by step

Set the access up properly first.Ladder at about 75 degrees on firm level ground, tied or footed, never resting on the gutter. This is the part of the job that actually hurts people.
Clear it out before diagnosing.A gutter full of wet debris is extremely heavy and that weight may be the whole cause. Clear it, then look at whether it still sags.
Find the failure.Look along the run for a bracket that has pulled out, rotted fascia behind a bracket, a split in the gutter, or simply too few brackets. That inspection decides everything else.
Check the fascia board.Push a screwdriver into the timber behind the brackets. Soft or crumbly means rot, and no bracket will hold in it. That has to be repaired first or the gutter will sag again immediately.
Work out the correct fall.Roughly 3mm per metre towards the downpipe — about 25mm over a typical 8 metre run. Too little and it holds water; too much and it looks wrong and can overshoot at the outlet.
Set a string line.From the highest point to the outlet at the correct fall. Every bracket then follows the line. Doing it by eye is how gutters end up draining the wrong way.
Space brackets at 800mm maximum.And closer — within about 150mm — either side of every joint and at the outlet. Wide spacing is the most common underlying cause of sagging.
Add brackets rather than only replacing.If the original spacing was marginal, replacing like for like recreates the problem. Extra brackets cost very little.
Fix into solid timber.Into the fascia where it is sound, or into the rafter feet if the fascia is not. Screws rather than nails, and stainless or coated for longevity.
Replace split or rusted sections.Plastic that has gone brittle and grey has had its UV life. Trying to re-seat a split section wastes the effort of the whole job.
Check the joints and seals.Union brackets have rubber seals that perish. Replacing the seal or the union is cheap and prevents the drips that stain a wall.
Test it with water.A hose at the far end. Water should run steadily to the outlet with nothing standing. Any pooling means the fall is wrong and it is far easier to correct now.
Check where the downpipe discharges.Into a gully or a drain, not onto the ground next to the wall. A correctly draining gutter feeding a broken gully just moves the problem.
Clear it twice a year.Autumn especially. Debris is what adds the weight that starts the sagging, and clearing is far easier than refitting.

Tips

String line, 3mm per metre. Setting brackets by eye is why gutters hold water.
Brackets at 800mm and closer at joints. Wide spacing is the underlying cause almost every time.
Test with a hose before packing up. Correcting the fall then is minutes; later it is the whole job again.

Common mistakes

Leaning the ladder against the gutter — It is not a structural component. It distorts or breaks, and the ladder moves — which is how gutter jobs cause falls.
Refitting brackets into rotten fascia — The timber will not hold a screw, so the gutter sags again within months. The fascia has to be sound first.

If it doesn't work

Water standing in the gutter

Cause: Insufficient or reversed fall — Fix: Reset the brackets to a string line with about 3mm per metre towards the outlet.

Gutter sagged again after repair

Cause: Rotten fascia or too few brackets — Fix: Check the timber with a screwdriver and add brackets to 800mm spacing.

Dripping at a joint

Cause: Perished union seal — Fix: Replace the rubber seal or the whole union bracket.

Water overshooting the gutter in heavy rain

Cause: Blocked, or fall too steep at the outlet — Fix: Clear it, and check the fall is not excessive near the downpipe.

Green staining on the wall below

Cause: Persistent overflow or a leaking joint — Fix: Find the leak, fix it, then clean the wall. The stain will return otherwise.