The gutter gets the attention, but the downpipe decides whether the system works. Roof water runs along the gutter to a single outlet, drops down the pipe and has to leave the base of it quickly enough for the next surge to follow. If the pipe is too narrow, kinked, cracked or silted at the bottom, the gutter backs up and the overflow lands on the fascia, the soffit or the wall below, however clean the gutter itself is.
Most downpipe trouble is quiet for years and then loud on the first heavy night. A joint that was never quite sealed starts to weep, an old bend fills with grit, or the drain beneath the pipe slowly silts up. Then the fault shows as a stain down the brickwork or a gutter spilling at one end, and the roof gets the blame.
Here we cover how pipes are sized and matched, how an offset works, what they connect to at the bottom, how blockages form, and when a replacement is the sensible answer. It sits within our wider gutters, soffits and fascias work, where we follow the water from roof edge to drain before suggesting anything.
Matching the pipe to the gutter and the outlet
Downpipes come in round, square and rectangular profiles, and the one you need is set by the gutter above it. Each guttering system is made with its own outlet, and the pipe has to match that outlet in shape and in width. A round pipe will not seat in a square outlet, and two systems that look alike from the pavement can differ by a few millimetres, which is enough to leave a leaking joint. That is why we take the make and profile from the existing system, or from a data sheet, rather than guessing from the size of the pipe.
Size matters for a simple reason. The outlet and the pipe together set how much water can leave the roof in a given moment, so a long run of gutter feeding one small pipe is more likely to overflow in a downpour than the same length with a pipe at each end. If a run keeps spilling at the far end while the gutter is clear and level, the answer may be a second outlet and pipe rather than a bigger gutter.
Where the pipe sits
The best position is at the end of a run, so water drains towards it. Pipes at the middle of a long run are common on older houses and work, but they ask the gutter to fall in two directions. On a hipped bungalow it may have to sit mid-wall to miss a window or a porch.
Whatever the layout, the pipe should stand clear of the wall on brackets, not hard against the render or brick, so it can be painted or cleaned behind and so a small leak does not soak the wall. We note the profile and position on the survey and photograph the outlet from above, so the replacement part is the right one first time.
How an offset works at the eaves
Many houses have eaves that project past the face of the wall, carrying the soffit and the fascia on a timber overhang. The gutter hangs from the fascia at the front edge, but the pipe has to run straight down the wall, which is set back. The bend that carries the water across that gap is called an offset, and it is made from two angled fittings joined by a short length of pipe, often with a swan-neck look to it.
An offset is a likely trouble spot because it is the only place where the flow changes direction twice in a short run. Leaves, moss and the grit that washes off concrete tiles collect on the lower bend and sit there until a storm pushes them into a plug. A fitting that has slipped, or a joint that was left dry, also leaks at an offset, and the water lands on the soffit above where nobody looks. Our page on mouldy soffits shows how often a weeping bend turns out to be the cause.
Getting the bends right
Bends should be gentle, and the angle should match the projection of the eaves exactly, so the connecting length slopes continuously downhill. A pipe laid almost flat holds water and fills with silt. We also keep the number of bends to the minimum the house needs, because every fitting is a place for a blockage to start.
On a deep overhang, the offset section is clipped to the wall so it does not sag. Salt deposits gather on exposed fittings near the sea, so it pays to look at these joints when you check the roofline. We cover that in our guide to salt air and rooflines.
Where the water goes at the bottom
A downpipe needs somewhere to discharge. On most houses that means one of three things: a gully at ground level connected to a drain, a soakaway in the garden, or a water butt that overflows to one of those. Which one you have depends on how the property was drained when it was built, and older houses often differ from newer extensions on the same plot.
A gully is a small trapped opening, usually with a grating, set in the ground at the foot of the pipe. The pipe should end just above the grating, or enter the gully through a shoe, a short angled fitting that turns the flow outward. Where the water runs on to a drive or path instead, it splashes the wall, ices up in cold weather and can run towards the house.
Drains and soakaways
Rainwater drains may run to a surface-water sewer, a combined sewer that also takes foul water, or a soakaway. A soakaway is a buried pit, filled with rubble or fitted with a plastic crate, that lets water seep into the surrounding ground. It works well in free-draining ground and badly in heavy soil, and it can fill with silt over the years until it overflows into a lawn. If you are unsure what your pipe connects to, a dye test or a camera survey by a drainage specialist will trace it. We are roofers, so we find where the pipe stops and the drainage starts, and we tell you if the problem sits on the drain side.
Do not move, add or remove a soakaway without checking where it can go. Its position relative to buildings and boundaries matters, and your building control body can say whether a change needs consent. A water butt on a downpipe is simpler, but it still needs an overflow route to a drain.
Why downpipes block and how to tell
Blockages begin at the points where the flow slows: the gutter outlet, the offset bends, the pipe joints and the gully or shoe at the base. The material is mostly what the gutter collected and could not hold. Leaves and seeds, moss from the roof, grit from ageing concrete tiles, nesting debris from birds and, near the sea, wind-blown sand all find their way down. They sit in a bend, bind together and trap more.
The signs are easy to read once you know them. A gutter that overflows at the outlet end in heavy rain while the rest of it is clear. Water pouring from a pipe joint partway down, because the blockage below has made the pipe fill and push out through a seam. A gurgling sound after rain. A dark, wet trail on the wall below a bracket or joint. Green staining and damp at the foot of the wall are a sign that the base is discharging poorly.
Clearing a blocked pipe
Start at the top. Take out whatever you can reach from the outlet by hand, wearing gloves, and flush the pipe with a hosepipe while watching the bottom. If water comes out freely, the pipe is clear and the problem is higher up. If it backs up, the plug is lower. A flexible rod or a plumber's snake can free a plug in a straight run, though it can scratch a plastic bend or push the blockage on to the drain, so go gently and never force it.
Where a pipe blocks again and again, there is usually a reason: a bend that holds water, a joint that has dropped, or a gully that is silting. We look at the cause instead of only clearing the symptom. Our page on a blocked downpipe goes through the same steps in more detail, and clearing the gutter above matters too, as covered in our gutter cleaning guide.
When to repair, when to replace
A single cracked section or a loose bracket is a repair. A whole pipe that is brittle, bowed, split along the seam or loose on its fixings is better replaced, because patching old material rarely holds for long. uPVC downpipes go brittle with age, particularly on a sunny elevation, and can crack with the force of a knock or a thaw. Aluminium dents but does not split. Cast iron lasts a long time when painted, but rust blisters and cracked joints show where it is failing, and a pipe that has rusted through at the back may look sound from the front.
Fittings are the weak point on any material, so check the joints before you condemn the pipe. A joint that was push-fit with no seal can be refitted, and a bracket that has pulled out of soft mortar can be re-fixed into sound brickwork.
Older materials and asbestos cement
On a building from before 2000, HSE advises assuming that asbestos could be present, and rainwater goods made of asbestos cement are one of the places it can turn up. A pipe in good condition is not a hazard if it is left alone, but breaking or sawing it releases fibres. If a grey, rigid pipe looks old, do not drill or snap it. Ask for it to be identified before any work starts. Our guide to asbestos soffit boards explains what to do next.
When we replace a downpipe we fit parts from one system, so the profile, the brackets and the outlets match. We also check where each pipe discharges, so the water goes to a gully or soakaway and not against the wall it was meant to protect.
Splash, shared pipes and roofs below
Some downpipes cause trouble not because they are faulty but because of where they are. On a two-storey house with a single-storey extension, the upper pipe often ends on the flat roof below. That works for a while, but the water scours the covering at the point where it lands, stains it and eventually wears through. The better arrangement is a pipe carried down to the ground, or at least into a gutter on the lower roof that has its own outlet.
Shared pipes cause a different kind of problem. On terraces and semis, a single gutter and pipe may serve both houses, and a fault belongs to both. If a joint on a shared pipe leaks into one neighbour's wall, it helps to know whose responsibility it is before work starts. Our page on shared gutters covers the questions to raise with the people next door.
Splash and the base of the wall
If a pipe stops well above the ground or a grating, rain bounces back on to the wall and wets the lower brickwork. That shows up as green algae, flaking paint and damp at skirting level. A short extension or a shoe at the base can solve it, but the water needs somewhere to go, and a drive that slopes towards the house is a poor answer. A neater fix is to route it into the gully with a pipe that discharges just above the grating.
When we survey the roofline we look at the pipe from top to bottom and photograph each stage, including the base. After the survey you get a written quote with a single total, shaped by the length of run, the access, the system you pick and anything the survey turns up. If the issue turns out to sit in the drain, we tell you so before anyone starts digging.
