Where a chimney comes up through a pitched roof, the covering stops and masonry begins. That junction runs all the way round the stack, on slopes that fall towards it, away from it and past it, and water has to be kept out on every side. The job is done in lead. The flashing is lead laid across the joint, a soaker is a small sheet of lead worked into the roof covering, and a back gutter is the lead channel behind the stack that turns the water round it. Together they are often called the leadwork. Taking the parts one at a time, how each is built and which faults let water in, makes it far easier to trace a stain on the ceiling to the right part and to know whether repairing that part will do or whether it needs full renewal.
The parts of the leadwork round a stack
Stand on the roof next to a chimney and you can pick out four pieces, one for each side of the stack. They are made differently because each side faces the slope differently.
On the lower side, facing down the slope, sits the apron. This is a single piece of lead that lies over the roof covering and turns up the face of the chimney. Water running down the roof meets it first, so it is dressed to sit over the covering below, and it is the part most exposed to flowing water.
Up each side run the step flashings, also called stepped flashings. The brick or stone courses are level while the roof slopes, so the lead is cut in steps that follow the courses. Each step tucks into the masonry. Under the lead on each side, the soakers do the work between the roof covering and the stack. A soaker is a small, flat sheet of lead, bent to an L, with one leg under the slate or tile and one turned up the side of the chimney. There is one soaker for each course of slate or tile, so they overlap like the covering itself.
On the upper side, facing up the slope, is the back gutter. It is a shallow channel of lead that sits behind the stack, takes the water running down from the ridge, and carries it to either side so it can run round the chimney and onto the roof below. Without a back gutter, the water would pile up against the stack and find its way in.
Each part overlaps the next, and the whole is only as sound as its weakest lap.
How the lead is fixed into the stack
The lead does not stick to the chimney. It is held in position by the way it is fixed, and that is where good work differs from poor.
The top edge of each flashing is tucked into a groove in the masonry. This is a raglet: a slot cut into the mortar joint, or into the stone, about the depth of a joint. The lead is turned into the raglet, wedged with small lead wedges, and then pointed over with mortar. The wedges hold it in place and the pointing keeps rain out of the slot. According to the Lead Sheet Association, cover flashings are wedged into walls at a maximum spacing of about 500 mm. Closer spacing holds the lead tighter against wind.
Over the lead on Bath stone, the pointing is lime, for the reasons given in our guide to lime repointing, and the raglet follows a bed joint instead of cutting into a stone face, so no block is damaged.
The lower edge of the flashing sits free over the roof covering, held by the weight of the lead and by clips. It can move a little as it heats and cools, and this movement is why pieces are kept to a sensible length. One supplier's guidance gives a maximum piece length of 1500 mm for flashings in the common lead codes. A very long unbroken run is likely to ripple and tear as it warms and cools, which is why long runs are made in lengths with lapped joints.
Lead thickness is graded by code. Codes 3 and 4 are the lighter sheet used for soakers, while flashings are typically laid in code 4 or 5, according to Royston Lead's guidance. Choosing a code means weighing the part, the exposure and the length of run.
How flashings fail and what it looks like inside
A leak at the stack rarely announces itself at the stack. It tends to show as brown staining on the breast inside, a damp patch on the ceiling next to it, or a drip in the loft that follows the timber. Tracing it back is a matter of looking at each part in turn.
Lifted or loose lead
If the pointing over a raglet has crumbled, the top of the flashing can drop out of the slot and flap in the wind. Water runs behind it. This is one of the commonest faults and also one of the easiest to put right, as long as the lead itself is intact.
Cracks, splits and holes
Lead can crack where it has been bent too tightly, where it has been stretched across an over-long piece, or where it has been walked on. Hairline cracks let water in slowly, and from the pavement they are easy to overlook.
Cement and mastic fillets
A fillet of cement or mastic smeared along the joint is a quick fix that sometimes passes for a repair. It cracks as the lead moves, and it hides the real fault under a layer that looks sound. Where a stack has a fat grey fillet at its foot, the lead behind it is usually the problem.
Blocked or failed back gutter
Moss, leaves and debris pile up in the back gutter. A blocked channel overflows into the stack and the roof void. The gutter sits behind the chimney, out of sight from the street and from most windows. Check it first when a ceiling stain lines up with the rear of a stack.
Missing or slipped soakers
When the covering is repaired or replaced and the soakers are not renewed, water gets in at the sides. This often happens after a patch repair that did not include the leadwork.
Repair or renew: what the work involves
Repairs come in three sizes, and the survey decides which applies.
The smallest is repointing the raglets and refixing lifted lead. The pointing is raked out, the flashing is wedged back and the joint is filled with lime mortar. If the lead is sound, nothing else is needed. A patch repair covers a split or hole, by cutting out the damaged area and welding or dressing a new piece in. Both are small jobs, but both need access to the stack, and so scaffold or a platform usually comes into the price.
The middle size is renewing one part, such as the back gutter or the apron, while keeping the rest. This makes sense when one part has failed and the others are sound. It takes more care than it looks, because the new lead has to lap correctly under and over the old.
The largest is full renewal of the leadwork. Covering around the stack is lifted, the old flashings and soakers are removed, and new lead is dressed all round. The covering is relaid over the new soakers. We do this when the old lead has failed in several places, when earlier repairs have been bridged with cement, or when the covering has to be lifted anyway.
Cost guides give a range for flashing work: MyBuilder (2026) shows repairs between £250 and £1,200 nationally, while MyJobQuote (September 2026) lists patches between £150 and £250 and an average of roughly £570 for full replacement. Only the survey produces a fixed price. MyJobQuote adds that lead fitted correctly may serve for more than 80 years, which is why we repair sound lead and renew only what has failed. For the cost of the wider stack, see our guide to chimney repairs.
Chimneys on slate, tile and stone roofs
The principles are the same on every roof, but the details change with the covering, and a good repair respects the covering it sits in.
On slate, soakers are cut to the size of the slate and sit between the courses, so each slate overlaps the soaker beneath. On plain tiles, which are small, the soakers are also small and numerous. On profiled tiles, such as pantiles, a soaker does not work, and the junction is usually sealed with a lead flashing dressed to the profile, or with a manufactured flashing. On stone tiles, which are heavy and uneven, the lead must be formed to follow the uneven surface, and it needs to be lapped carefully so the water is shed.
The stack has its own say. A stack set low on the slope has water running down onto it from above, and the back gutter does the heavy work, while a stack on the ridge has less water arriving from behind it. A stack at the party wall between two houses is flashed against the wall and sometimes has the covering of both houses meeting at its foot.
On older Bath houses the stacks are often built in stone, and the lead is dressed into stone raglets. The combination of lead, stone and slate needs matched materials throughout, and the repair should look like the original work. If the stack has to come down at some point, the lead is part of that process, and our guide to chimney stack removal follows that sequence, and the roof is made good with covering to match.
Planning a flashing repair
A flashing repair is a small job in a high place, and most of the planning is about access and the weather.
Access first. The stack is often at the ridge, and the lead runs all round it, so working there needs a safe platform: scaffold, a tower or edge protection with a proper working platform. The cost of access is part of the price, and if other roof work is planned, it is cheaper to combine the two than to put access up twice.
Weather second. Lead can be laid in most conditions, but the pointing over it needs dry, mild weather to set properly, particularly if it is lime. A forecast of heavy rain or hard frost is a reason to wait, even for a leak. If water is coming in meanwhile, the roof can be made safe with a temporary cover until the repair is done. Leaks come first in our diary, and the first visit makes the roof safe.
Consent third. Like-for-like repair of leadwork on an unlisted house does not normally need consent. On a listed house, ask B&NES Council before the work if it goes beyond like-for-like, and read the council's advice on listed buildings. A chimney that is going to be reduced or taken down is a bigger question, and our page on chimney removal planning covers it.
Once the work is agreed, the written price should name each part being renewed, the lead code, the pointing mortar and the access. Our workmanship sits under a 10-year written guarantee, and we clear the site and remove the waste at the end.