What is a rainwater hopper and why does it block?

What a hopper is and where it sits

A hopper head is the wide, open top of a rainwater pipe. Water leaves the roof through an outlet in the wall, drops into the hopper, and runs down the downpipe to the drain. On older buildings the hopper is often cast iron, sometimes with a date or a decorative front, and on grander properties it may be lead. It stands proud of the wall on a bracket or sits tight against it, and its job is to take a concentrated flow and feed it into a pipe that is narrower than the outlet.

You see hoppers most often on buildings with a parapet. A parapet is the short upstand wall along the roof edge. Behind that wall runs the parapet gutter, a trough finished in lead sheet that gathers rain from the slope and carries it to an outlet cut through the wall. That outlet discharges into the hopper. On a terrace with a continuous parapet this arrangement may be the only drainage the roof has, which is why one blocked hopper can affect a whole slope of rain.

The design is simple, and the simplicity is the weakness. A hopper has an open top, a shallow bowl and a small throat. Anything that lands in it stays unless the water flowing through is strong enough to carry it on.

Why the hopper head blocks

The causes are few, and each is the ordinary litter of a roof.

• Leaves and seeds. Autumn leaves, sycamore keys and blossom wash down the gutter and gather at the outlet. A wet mat of leaf is enough to slow the flow, and everything behind it then settles.
• Moss and slate or tile grit. Moss loosens from the roof, and weathered slates and tiles shed fine grit. Both are carried into the gutter and pack into the throat of the hopper.
• Bird nests and droppings. A hopper is a sheltered ledge, and pigeons and other birds will use it. Nests wedge into the neck, and nesting birds are protected, so a nest in use must stay where it is until the young have gone.
• Fallen debris from the pipe below. A downpipe that is crushed, bent or part full of silt backs water up into the hopper from underneath.
• Ice and old paint flakes. Standing water in a cold spell can freeze in the bowl, and flakes of paint and rust from a neglected cast iron hopper add to the sludge.

Trees nearby make it worse, but a roof with no tree in sight can still block. Wind carries debris a long way, and the roof itself keeps producing grit.

What happens when it overflows

When the hopper cannot take the flow, the water has to go somewhere. The first sign is usually the parapet gutter filling and standing, because the outlet can no longer empty it. A properly formed lead gutter has a fall toward the outlet, and the usual guidance for flat surfaces is a design fall of about 1 in 40 to give a finished fall of 1 in 80 (BS 6229, as quoted by Rockwool). A gutter that holds water above its design level is working outside what it was built for.

From there, three things tend to happen. The water can spill over the front of the hopper and run down the face of the wall, staining it and soaking the masonry. It can back up the gutter until it reaches the top of the lead and gets behind the flashing or under the slates and tiles, and that is how a ceiling stain appears inside. Or it can find a joint in the lead, a drip or a seam, and enter there.

Wet stone is the quiet cost. Bath stone and other limestones take up water, and a wall that is soaked below an overflowing hopper will stay damp, grow algae and in a freeze lose surface from the stone. The same wall in a dry year would have lasted far longer. Our page on a lead roof and Building Regulations explains when gutter work needs formal approval, and that matters if the clearing job turns into a rebuild.

How a blocked hopper is cleared

Clearing is a hands-on job, done in a set order so the debris does not simply move further down.

First comes safe access. A hopper at parapet height is usually reached from a scaffold, a tower or a roof ladder with proper edge protection, not from a ladder leant against the wall. Then the gutter is cleared from the far end toward the outlet, so loose material is brought to the hopper and removed by hand rather than pushed into it. The hopper itself is emptied of silt, nests (once any birds have gone) and rust flakes.

The downpipe is then checked. Water is run through it from the top to prove it flows freely, and a rod or a flush clears any soft blockage lower down. If the pipe still backs up, the fault is below the hopper, and may be a collapsed run or a blocked drain rather than the roof.

We also look at what the blockage has done. Hand cleaning shows whether the lead in the gutter has split at a drip, whether the outlet lining has lifted, and whether the hopper is cracked or pulling away from its bracket. A cracked cast iron hopper leaks at the joint and sends water straight into the wall, and it needs repair or replacement, not another clean.

Stopping it happening again

A hopper cannot be made blockage-proof, but it can be made much easier to keep clear.

The simplest step is regular inspection, with the gutter and hopper cleared in late autumn after the leaves have come down and again after any storm that brings debris off nearby trees. For a roof with a deep parapet gutter this takes a short visit, and it costs far less than repairing a soaked wall.

A mesh guard fitted over the outlet keeps leaves out of the throat, and a properly sized overflow in the parapet gives water an exit if the main outlet does block. Both are worth considering where the parapet is tall, because a gutter with no overflow will fill until it spills somewhere less convenient. Our guide to the lead codes used for gutter linings shows why the thickness of the lead matters, since a thicker code resists the wear that standing water and debris cause.

Finally, treat a recurring blockage as information. If the same hopper fills every autumn, the outlet may be undersized for the roof it drains, or the gutter may not hold its fall. Our free survey includes pictures of the roof itself, and the fixed price that follows in writing shows whether a clear-out is enough or the drainage needs reworking. For the wider picture on this kind of work, see our lead roofing service.