The short answer
A gutter sags when it is not supported properly or its supports have failed. The causes are brackets too far apart or pulled loose, a fascia that has rotted so the fixings have nothing to grip, the weight of leaves, water and ice, and plastic that has softened with age. The cure is to find which, fix the support and re-hang the run to a fall.
Brackets and spacing
A gutter is held up by brackets fixed to the fascia, or to the rafters in a rafter-fixed system. They must be strong enough, close enough together and firmly fixed. If any of these fails, the gutter drops between them.
Spacing is the first thing to check. Manufacturers give a maximum distance between brackets for their products, and the spacing is shorter for large gutters or where heavy loads are expected. Too few brackets leave long spans that bow under the weight of water, and a bracket missing at a joint leaves it unsupported.
Quality and fixing matter. Thin plastic brackets can crack or snap with age and cold. Metal brackets can corrode. Screws pulled out of soft timber, or nails fixed into an unsuitable board, no longer hold. A bracket that has bent or rotated tilts the gutter out of line.
Brackets should also set the fall. Brett Martin's guide is 1 mm for every 600 mm of gutter, so the brackets must be at the right heights along the run. Where one has dropped, the fall reverses locally and water ponds.
On the ground, look along the line of the gutter against the fascia. A visible curve, a section that has dropped or a gap between gutter and board shows where support is failing. Our page on leaking gutter joints explains how a sag opens joints.
Rotten fascias and fixings that will not hold
The commonest hidden cause is the board. If the fascia has rotted, brackets screwed into it have nothing solid to grip. They pull out under load, one after another, and the gutter drops. Replacing the brackets alone is no use, because the new fixings will soon loosen in the same soft wood.
Signs of this include screws that spin without biting, brackets that wobble, a fascia that crumbles when probed with a screwdriver, and paint that is flaking or blistered in the same place. A gutter that sagged soon after being repaired points the same way.
Where rot is the cause, the fascia must be cut out, repaired or replaced before the gutter is rehung. Our page on rotten fascia boards explains how that is judged.
Rafter ends and the wall plate can also be at fault. If the roof structure has moved, or the rafter feet are rotten, the whole eaves line can drop, and the gutter follows. A visible wave in the fascia itself, and not only the gutter, suggests this.
Fixings need to suit the material: corrosion-resistant screws long enough to bite into sound timber, and in masonry-fixed systems proper plugs. A gutter fixed with the wrong fixings is likely to sag again, whatever the brackets look like.
Weight, ice and tired plastic
Water is heavy. A full gutter run carries a great deal of weight, and debris adds more. Leaves, moss and silt that hold water turn a light gutter into a heavy one, and the brackets bear the load. A gutter that sags only after storms may be overloaded by blockages, so clearing it relieves the strain.
Ice adds to the load. Water that freezes in the gutter expands and pushes the sections apart, and a frozen mass can pull a gutter away from its fixings. Freeze-thaw tests a gutter's support regularly, with South Farnborough averaging about 53 air-frost days a year (Met Office, 1991 to 2020).
Plastic ages. In sunlight and heat, uPVC softens and loses stiffness, and an old gutter can deform under its own weight and bow even with good brackets. Colour fades and the surface chalks. Brittle plastic cracks instead of bending.
Metal gutters can sag if the brackets corrode or the sections are too thin, and cast iron is very heavy, so its fixings must be strong. A gutter overloaded with snow or fallen branches may pull away from the wall.
The sensible response to a sag after a storm is to clear the gutter, look at what failed and fix it before the next heavy rain. Left alone, the sag holds water, accelerates the fascia rot and spills over the wall.
Re-hanging the run properly
The repair starts with the cause. If the board is sound and only brackets have failed, new brackets are fitted at the right spacing and heights, with strong fixings, and the gutter is re-hung to a fall. If the board is rotten, that is dealt with first.
Re-hanging means checking the line with a string or a level and setting each bracket so the gutter falls evenly towards the outlet. Joints are checked and resealed as needed, and any section that has deformed beyond recovery is replaced. Outlets and downpipes are checked for blockages and fit.
If the gutter is old, brittle or deformed throughout, it is better to replace the run than to re-hang a tired one. Our page on how often guttering needs replacing sets out the signs.
Access usually drives the cost. Height may call for a tower or scaffold, and a frame on the road needs a licence, arranged through the scaffolder. Low gutters can often be done from a ladder or small tower.
Our survey covers the run, with photographs of the sag and the board behind. The price is quoted as a single written figure. The gutters, soffits and fascias hub covers the whole roofline.
