Leaking joints and sagging gutters: clean, repair or replace?

What you see, and what it usually means

Plastic gutters: seals, brackets and movement

Most leaks on plastic gutters come from the joints. A guide from The Plastic Centre, a guttering supplier, explains that "a common cause of gutter leaks is a failed rubber seal inside the gutter union bracket", and that over time seals "can dry out, shrink or become dislodged".

Sagging is usually the brackets. The same guide warns that sagging sections "allow water to collect in one area, which can strain joints and lead to leaks during heavy rainfall", and that brackets need to be "spaced correctly to provide adequate support".

Plastic moves, too. "All guttering materials expand and contract with temperature changes," the guide says, and overtightened brackets can "restrict natural expansion and lead to cracking". Cadw, the Welsh Government's historic environment service, notes that sagging is a common problem with uPVC, which it says "tends to be the shortest lived of all gutter materials".

Getting the fall right

A gutter needs a slight, steady slope down to its outlet, with "the highest point of the gutter run furthest from the outlet", as The Plastic Centre puts it. The Department for Communities in Northern Ireland recommends a fall of at least 1 in 360 in its technical note on roof drainage. Its note also warns about gutters set with the inner edge lower than the outer, which sends water down behind the gutter. A run that has lost its fall needs its brackets reset, which is a repair, not a clean.

Cast iron gutters

Cast iron lasts a very long time if it's looked after. BuildingConservation.com describes cast iron rainwater goods as having "a superb life span", but warns that "if the paint layer which protects it is neglected, the underlying metal will rust and eventually fail."

Joints between cast iron gutter sections are bolted together and sealed. Traditionally that was red lead putty; today it's usually a sealant. When the seal fails, the joint drips and the iron around it starts to rust. Cadw's advice is to remove rust and repaint on a three to four yearly cycle. It says silicone sealant can be a temporary repair, but badly rusted or split sections should be replaced like for like.

On a listed building or in a conservation area, replacements should normally match the original. BuildingConservation.com says "replacements should usually be made of the same material as the original."

Clean first, then judge

A gutter full of wet sludge is heavy, and that weight is what pulls brackets loose and opens joints. One Mumsnet user's cousin had gutters that "collapsed the other week with all the weight" of what was in them. So the honest order is: clear the gutter, run water through it, and then see what's still leaking or sagging. Sometimes a gutter that looked broken just needed emptying.

Repair or replace?

• Repair when the problem is one or two joints, a couple of brackets, or one cracked length. It's quick, and the rest of the run carries on.
• Replace when joints are failing all along the run, brackets keep breaking, the gutter is cracked in several places, or it's the wrong size for the roof.
• On cast iron, repair and repaint where you can. It's often possible to keep most of a period system.
• If the fascia board behind is rotten, fix that first, or new brackets won't hold.

How we deal with leaking and sagging gutters

We clear the gutter by hand first, then run water through it so we can see exactly where it leaks or holds water. You get photos of each problem, and a quote to fix it: resealing joints, refixing or replacing brackets, resetting the fall, or replacing a length or a run where that's the better answer.

Written by Hawkes Roofing, roofers based in St Albans who clear and repair gutters across Hertfordshire, Buckinghamshire and north-west London. Last reviewed 26 October 2026.