A gutter looks like a minor strip of plastic or metal along the edge of the roof, so it is fair to ask why gutters are important at all. The answer is volume. A pitched roof collects rain across its whole slope and delivers it to one line at the eaves. The gutter catches that line of water and the downpipes carry it to a drain or a soakaway. Take that system away, or let it fail, and the water simply takes the nearest route. That route usually runs down the wall, into the brickwork and onto the ground beside the foundations.
The harm builds slowly, which is why it gets missed. A green stain under a joint, a patch of damp that returns every winter, paint lifting from a window frame: each looks like a separate nuisance. Traced back, several of them can share one cause above head height. This guide covers what gutters do, where the water goes when they fail, the signs you can check from the garden and from inside, and when a repair stops being enough. The subject is the same as our older post on guttering, with the advice rewritten for British houses.
What a gutter actually does
The job sounds simple, yet it has three parts. First, the gutter catches the water coming off the roof covering, so it does not fall in a sheet past the wall. Second, it carries that water along a slight slope to an outlet. Third, the downpipe takes it down to a gully, drain or soakaway, away from the foundations. Each part has to work for the whole system to work.
Why does the slope matter?
Gutters are laid with a gentle fall towards the outlet. One manufacturer, Brett Martin, gives 1mm for every 600mm of run in its installation guidance. Too little and water sits in the channel, collects silt and overflows at the lowest point. Too much and it can race past the outlet in a storm. A gutter that has sagged between brackets has lost its fall in that stretch, which is why you often see a puddle of old water in a section after the rain has stopped.
What are the parts you can see?
Along the top of the wall you will find the gutter itself, held on brackets fixed to the fascia (the long timber or uPVC board at the eaves, nailed across the rafter ends) or to the rafter feet. Joints join the lengths together, with seals or gaskets that wear. Stop ends close the channel at each end. An outlet drops the water into the downpipe, and the downpipe is clipped to the wall down to the ground. Knowing the names helps when you describe a fault to a roofer, and it helps you spot which part is letting go.
The gutter also protects the roof edge. Where the tiles overhang the channel, the water drips into it rather than onto the timber below, so a working gutter keeps the fascia and soffit (the board underneath the overhang) drier as well.
Where the water goes when gutters fail
Water behaves the same way every time. If the channel is full, the water spills over the front edge. If a joint has opened, it drips from the joint. If the downpipe is blocked, it backs up and finds the weakest point. In all three cases the water ends up on the wall or on the ground where it should never sit.
What does it do to the walls?
Brick and render are not waterproof. They soak up water and dry out again, and they cope well with ordinary rain falling on their face. A steady trickle from a leaking joint is a different matter. The same patch of wall stays wet for weeks, which leads to dark staining, green algae, crumbling mortar and, in winter, frost damage as the trapped water freezes. A render coat that stays wet can crack and blow. Water running down a wall can also reach window frames and the brickwork around them.
What about foundations and the ground?
Rain that lands in the same spot beside the house every time a gutter overflows soaks the soil there. Saturated ground next to a foundation can soften, and in clay soils it can move as it wets and dries. We are roofers, not structural engineers, and we do not diagnose subsidence. Our view is that a gutter overflowing at one corner for several winters is worth fixing before anyone has to ask whether it has contributed to a crack. If a new crack opens at a window corner, get a chartered surveyor or structural engineer to inspect it, and have the gutters put right in the same week.
Where the water gets inside the roof edge, it can also reach the felt, the rafter ends and the ceiling plate, so the damage is not limited to the outside.
Warning signs outside the house
A wet day shows plenty if you view the house from the garden. Pick a time when it is raining steadily and stand back far enough to see the whole eaves line. Then walk round with a pair of binoculars on a dry day and look again. You are looking for water in the wrong place and for parts that have moved.
What should you look for on a wet day?
Watch for water pouring over the front edge of the gutter instead of flowing along it. Watch each joint for drips and each downpipe for a dribble rather than a steady run. A downpipe that is dry in a downpour has a blockage above it. A gutter with water curtaining over a stop end or a corner is not carrying the load.
What does the wall tell you on a dry day?
Look for vertical streaks of green or black below a joint or an outlet. Look for a pale or darkened stripe of brickwork where water has run for a long time, and for plants or moss growing in the channel itself. Check that the gutter runs in a straight line. A visible sag between brackets, a gap at the back where the gutter has pulled away from the fascia or a joint that has opened are all signs the system has moved.
Check the ground beneath too. A trench of washed-out soil, a damp patch of path or a green strip at the base of a wall points to repeated overflow in that spot. On a house with a concrete or cast-iron gutter, hairline cracks and rust blooms along the underside are worth a note. Pick the clearest of your findings, write down where it is, and you have what a roofer needs to start.
Warning signs inside the house
Not every gutter fault shows from the garden. Some of the clearest signs appear inside, often a good while after the fault began, because the wall has to soak through first.
How does damp from a gutter show on the walls?
A damp patch on an outside-facing wall that gets worse in wet weather and dries in summer is a classic sign. Look at internal plaster and wallpaper for tide marks, a brown or yellow stain, bubbling paint, a musty smell or black spots of mould. Check the corner nearest a downpipe and the wall beneath a long gutter run, then compare to the outside. If the stain on the inside lines up with a streak or leaking joint on the outside, you have your link.
What about ceilings and the loft?
Where a gutter overflows at the eaves, water can wick up into the roof edge and show as a stain along the top of a bedroom wall or in the corner of a ceiling. In the loft, use a torch and look along the eaves for dark, damp timber, wet insulation and felt that is stained or torn. Stay at the hatch or on boarding, and never step between joists.
Windows give early notice too. Condensation is common in cold weather and has many causes, so do not blame the gutters for all of it. But paint that peels from the bottom corner of a window frame, or a sill that is always wet when the frame above it is dry, suggests water running onto the frame from above.
When you see damp, take dated photos in the same spot each month. Patterns that follow rain point to an external cause such as guttering, while damp that does not follow the weather needs another explanation, and a damp specialist or surveyor is the right person to ask.
Repair, replace or just clear it out?
A lot of gutter trouble is not a failed gutter at all. A channel clogged with leaves, moss and silt overflows even though the gutter is sound, and a clear-out and a check of the joints puts it right. Our guttering maintenance tips cover that routine, and the answer on why gutters overflow goes through the usual causes one by one.
When is a repair enough?
A single leaking joint, a slipped bracket or a short sagging stretch can normally be mended on their own while the rest stays as it is. Resealing a joint, refixing a bracket and adding another support are small jobs. If the gutter is old but the channel is flat, firm and uncracked, a repair makes sense.
When does replacement make more sense?
If joints leak all along the run, the plastic has gone brittle and cracked, the metal is corroding through or the fascia holding the brackets is soft, patching will keep you calling someone back. A new run on sound fascia boards is the better buy. If the boards are rotten, the gutter and the fascia are usually done together, which is where our gutters, soffits and fascias work comes in.
On price, a Checkatrade guide dated October 2026 gives a typical UK range of £650 to £1,700, with an average of £1,000, for replacing 15 to 20 metres of gutter with three downpipes. Access is the big variable. If a ladder will not reach safely, scaffolding adds to the job, and Checkatrade's September 2026 fascia and soffit table gives £300 to £600 a week for each elevation. Our written quote is one total price after a survey. With anyone else's, find out if the figure covers VAT and scaffolding.
For a quote, send photos of the eaves taken from the ground, one of any stained wall, and a line on where the water spills. The first thing we check on the survey is the fall and the brackets at that spot.
Part of our Gutters, Soffits & Fascias in Crawley
Gutter questions, answered
Can blocked gutters really cause damp inside?
Yes. Overflowing water soaks the outside wall in the same place again and again, and in time it can reach the plaster inside. The tell is a stain that grows after heavy rain and lines up with a wet streak outside. A roofer can confirm the gutter side, and a damp specialist can check other causes.
How often should I look at my gutters?
A good habit is a quick look from the ground after autumn leaf fall and again in spring, plus a glance during any heavy downpour. You are checking that water runs along the channel and out of the downpipe, not over the edge. Anything that looks wrong from there deserves a closer look from a roofer.
Is a sagging gutter always a sign it needs replacing?
No. A sag between two brackets can often be cured by adding a support or refixing a bracket and resetting the fall. Replacement makes sense when the fascia is soft, the plastic is cracked or joints leak along the run. Photos of the sag and the wall behind it let us advise before we visit.
