The short answer
A parapet gutter usually leaks because the lead has split at a drip or a fold, because the outlet or downpipe is blocked and the gutter overflows, or because the upstand against the parapet wall has failed. Less often the fall is wrong or the wall behind the lead is letting water in. Finding which one it is means looking at the gutter in rain and at the stains below.
How a parapet gutter works, and why it is unforgiving
A parapet gutter, lined in lead, sits at the base of a roof slope, tucked behind a low wall called a parapet. You find them on many Georgian terraces in Bath, where the front wall rises above the roof line and the rain has to go somewhere other than over the edge. The slope behind drains into the gutter, the gutter falls gently to an outlet at one end or the middle, and the outlet feeds a downpipe through a hopper or directly into the drain.
Everything about the arrangement is concentrated. Water from a large area of roof arrives in one narrow channel, so a small fault lets in a lot of water. The channel also sits against the wall of the house, over rooms that are often decorated and plastered. A leak that begins as a single failed seam can come through as a stained ceiling, a damp chimney breast or a wet cornice within a few storms.
That is also why the cause is rarely where the stain is. Water travels along rafters and laths before it drops, so a stain at the front bedroom window head may come from an outlet three metres along the parapet. A proper survey traces the water from the inside out and checks each part of the gutter in turn. Our page on parapet gutters describes the construction in more detail.
The common causes, in the order we look
Most leaks come down to a short list of faults. We look at them in this order because the cheapest to rule out come first.
- Blocked outlet or downpipe. Leaves, moss, a tennis ball or a nest can stop water leaving the gutter. It rises until it finds the lowest point, usually the lead upstand under the slates or the top of the wall, and goes in there. Many leaks that look like failed lead are in fact overflow.
- Split lead. Cracks at drips, at the ends of bays and along fold lines let water straight through to the timber deck. Our page on repairing split lead explains when a repair holds.
- Failed drips. A drip is a step down in the gutter bed that sheds water from one length of lead onto the next and allows the sheet to move. If the step is too shallow, the water backs up and creeps under the upper sheet.
- Upstands that have slipped or split. The upstand is the vertical edge of lead that turns up against the parapet wall or the roof slope. It is covered by a flashing, the strip of lead tucked into the wall joint. If the flashing has dropped out or the wall pointing has failed, rain gets behind it.
- Not enough fall. Manufacturers quoting BS 6229 recommend designing a gutter bed with a fall of 1:40 so that after construction tolerances it achieves 1:80. A gutter that has settled or was built flat holds a permanent puddle against the seams.
- Defects in the parapet itself. Cracked stone, open joints, a loose coping or old cement pointing can let water into the wall and out onto the plaster, even when the lead is sound.
How to tell what is happening without going up
You should not climb onto a Georgian parapet. The ladders and the drop are a real risk, and walking in a lead gutter damages it. From the ground and from inside the house, though, there are clues.
Look at when the leak appears. If it comes in heavy rain or during a long spell of wet weather, the gutter is probably overflowing or the outlet is struggling. If it comes after every shower, it is more likely to be a split, a failed upstand or an open joint, because those do not depend on the volume of water.
Look at where the water shows. Marks at ceiling level near the front wall point to the gutter. Marks lower down the wall, or tide lines on the chimney breast, may point to a parapet or flashing problem. A bubbling patch of paint directly below the downpipe position suggests the outlet.
Look at the pipe too. A downpipe that is dry in heavy rain while the roof is wet is blocked above. One that is running over at a joint is blocked below.
Photographs are useful. Take them from the street or an upstairs window, noting the date and weather. They help us make a plan and help you if an insurer asks about the damage. Do not rely on them to see a fault, however: a hairline split in lead does not show in a photograph taken from the street.
What to do first, and what a repair involves
If water is coming in, protect the room: move furniture, set a bucket beneath the leak and keep an eye on any electrics nearby. Leaks and storm damage come first for us, and our first task is stopping the roof taking in water, which can mean clearing an outlet or covering an open seam on a temporary basis before the permanent work.
Once the roof is safe, the repair follows the cause. A blocked outlet is cleared and the downpipe checked. Where the lead has split at a drip, the damaged bay is cut out and a new section dressed in, often with a new drip formed to the right height. Where the upstand has failed, the flashing is lifted, the lead renewed and the joint repointed using a mortar matched to the stone. On a listed house the mortar matters: in one Walcot case the Bath Preservation Trust wanted lime pointing. Before any work that would change how the building looks, put the question to B&NES conservation officers.
Sealant, felt strip or a coat of paint over a split is rarely a lasting fix. It may stop the drip for a week or two, but it hides the fault from the next person who looks. A lead repair that works is made in lead.
When to renew the whole gutter
A short run with one fault can often be repaired. A longer run showing splits at several drips, creep along the bed and thin patches is a different job. At that point we would rather re-lay it with the right code of lead, the right number of drips and a proper fall than keep chasing leaks. The sourced figures for old lead suggest a long life for correctly laid work, as the answer on how long lead lasts explains, so a gutter that has failed early usually has a construction fault that a repair will not cure.
Renewing a gutter can reveal rot in the boards or the timbers below, which we only see once the old lead is up. If that happens we show you the photographs and agree what needs doing before going on.
You pay nothing for the survey, and the pictures we take of your roof let you follow our reasoning. Then comes a written, fixed price. For the hub page that covers all lead work, including flashings and valleys, go to lead roofing.
