The short answer
A gutter sags when its support fails, and the commonest causes are loose or widely spaced brackets, a rotten fascia board holding them, or the weight of water, debris and ice dragging an overloaded run down. A dip also leaves water standing in the channel, which adds still more weight and hastens the damage. Re-fixing the brackets cures some, while a rotten board has to be replaced first.
Start with what holds the gutter up
A gutter is a long, light channel that depends entirely on its fixings. Most are carried by brackets screwed into the fascia, which is the horizontal board running under the lowest edge of the roof slope. Others hang from spikes driven into the rafter ends, or sit on rise-and-fall brackets built into the wall. Whatever the arrangement, the channel itself has little stiffness of its own.
That makes the supports the first suspect when a run drops. Brackets work loose as the screws lose grip, and a gutter that has been cleaned by someone leaning a ladder against it may have been bent out of true. One manufacturer of small gutter ranges asks for brackets at no more than 500 mm apart, and a gap wider than that lets the channel bow between supports even when every fixing is tight.
The next job is to look closely at the way the bracket meets the board. If the screw turns freely or the timber around it is dark and soft, the fixing is not the real problem.
Rotten fascia is the usual hidden cause
Where screws pull out of a bracket, the cause is often a decayed board. Water that overflows or leaks at a joint runs down behind the gutter and soaks the fascia. Timber that stays wet rots from the back, so the face can look decent under a coat of paint while the part holding the screw has turned to crumbs.
You can test this without taking anything apart. Press a screwdriver into the board near a bracket and along the lower edge. Sound timber resists, while rotten timber lets the tip sink in and may flake away in dry strands. If it does, no amount of tightening will hold the gutter, because the fixing has nothing to bite on.
This is why a sagging gutter and a rotten fascia are so often found together. Our page on what causes rotten fascia boards covers the board side, including how far the decay tends to spread along the run and into the rafter ends behind.
Weight, debris and ice
A gutter that is full is far heavier than one that is empty. Wet leaf mould, silt and standing water add a load the brackets were never sized for, and a long span between supports will start to drop in the middle. Once it has dipped, water collects at the low point instead of draining, and the load grows. A gutter in this state can fail quickly in a wet autumn.
Frost does the same thing in a more violent way. Water trapped in the channel freezes and pushes outward, and the weight of an ice-filled gutter can drag brackets from a board. If the dip appeared after a cold spell, look closely at the joints and the end stops for cracks as well. We explain that cause on our page about why gutters freeze and crack.
It also helps to know whether guards or other additions have been put in. A mesh that clogs adds load of its own, and our answer on whether gutter guards work sets out where they help and where they do not.
Wrong falls and worn materials
A gutter does not run level. It needs a slight slope toward the outlet so that water leaves under its own motion. One manufacturer of small gutters recommends a fall of about 1 in 350. That is a drop of roughly 3 mm over each metre, which is easy to lose when brackets settle or are fitted by eye. If a run falls the wrong way or has a flat spot, a puddle stays in the channel permanently.
Age matters too. Plastic gutters soften and distort in sun over the years, and a long run of old uPVC will often bow even where the brackets are sound. Typical UK lifespans for uPVC guttering are put at 10 to 20 years by one cleaning firm's guide, so a sag in a plastic run twenty years old may simply be the end of the material. Cast iron is far heavier, and a bracket that has rusted through will let a length drop all at once.
Downpipes play a part too. A blocked or undersized outlet backs water up the run, so the weight builds at the wrong end. See our notes on downpipes for the outlet side of the system.
Repair, re-hang or replace
The right fix follows from the cause. If the brackets are loose but the board is sound, the gutter can be unclipped, the fall reset and new brackets added at closer spacing. If a short length of board has rotted, that section is cut out and replaced, then the gutter is refixed to new timber. If the whole fascia is gone, the whole run of gutter has to come off to allow the board to be renewed, and it is often wise to replace the gutter at the same visit.
The costs depend on length, height and access. UK guides report guttering at about £30 to £70 per metre and fascia at £80 to £150 per metre, according to MyBuilder's 2026 guide, and scaffolding adds to both. Where a simple re-hang will do the job, that is what we recommend.
At survey we check the fall, the brackets, the board and the outlet together, and a written fixed price follows. Before then, a photograph taken during heavy rain, showing where the water spills over, tells us more than any description of the sag. Start with the gutters, soffits and fascias service page if you want to see the whole range of work on the roofline.
