Look at a chimney stack from the garden and the brickwork is only half the story. Where the stack leaves the slope there is a collar of metal, usually lead, cut into different shapes on each face. That collar is the flashing, and it has the job of keeping rain out of the one place where a brick tower, a timber roof and a covering of slates or tiles all meet.
The pieces have names. The apron sits at the bottom, step flashings climb the sides, a back gutter sits behind the stack, and small soakers are tucked in beneath the covering. A leak can come from any of them, and the repair differs for each. This guide explains the parts, the usual faults and what separates a patch from a renewal. For the wider list of reasons a stack lets water in, see our guide to why chimneys leak.
What the four parts of a stack flashing do
A chimney stack interrupts the slope on all four sides, and each side needs its own treatment because water meets it differently. On the downhill face, rain runs straight at the brickwork. On the uphill face, it arrives from behind and has to be steered round. On the two sides, it creeps along the brick and tries to find the joint where the stack meets the covering.
That is why a sound flashing is built from separate pieces rather than a single wrap:
- The apron is one sheet on the lower face. Its top edge is turned into a brick joint and its lower edge lies over the slates or tiles, so water running down the stack lands on the lead and carries on down the slope.
- Step flashings climb each side, cut in a staircase pattern so that every step tucks into a brick course. Each step laps over the covering beneath.
- The back gutter is a small lined trough on the uphill side. It collects water running down the slope and sends it round both sides of the chimney instead of letting it bank up.
- Soakers are small rectangles of lead, bent to an L shape and laid between courses of slates or plain tiles. One leg lies under the covering, the other turns up the stack, and the step flashing laps over the top.
Each piece depends on the others. A neat apron with missing soakers still leaks down the sides, and a perfect step flashing cannot help if the back gutter behind it is packed with moss. On interlocking concrete tiles the sides are dressed differently, with lead formed over the tile profile or a preformed tray.
Why lead gives way round a stack
Lead is the traditional metal for this work because it can be bossed into awkward corners and does not rust. The lead supplier Leadworx calls Code 4 its standard for general flashing, with Code 3 for soakers and heavier codes where the sheet has more to do. MyJobQuote's guide of 22 September 2026 puts a well-fitted lead flashing at more than 80 years, and longer again in the higher codes. Lead also tolerates awkward corners, since it can be bossed into shape around brick that is never quite square.
Yet stacks leak long before that age, and the reasons are usually about how the lead was fitted rather than the metal itself. Lead swells and shrinks with every change in temperature. A long piece fixed rigidly at both ends has nowhere to move, so it work-hardens and splits along a fold.
The pointing along the top edge is the next weak point. Flashings sit in a raked-out joint, are wedged with lead and then pointed. Weather eats that pointing, the wedges loosen and the edge drops away from the brick. Rain then runs down behind the lead instead of over it. A flashing can look intact from the garden and still leak, because the failure is on the back of the metal.
Why do some stacks have mortar fillets instead of lead?
A fillet is a sloping band of sand and cement run along the joint. It shrinks, cracks and pulls away from the brick, and unlike lead it cannot flex. Where one is doing the work of a flashing we usually recommend lead instead.
Faults you can spot before anyone climbs a ladder
Look through binoculars from the garden for lead that has lifted away from the brickwork along the top of the apron, a gap between the metal and the stack, or pale mortar where a fillet has cracked. Check the sides for steps that have slumped, and for slipped slates or tiles near the stack.
Indoors, a flashing fault tends to leave a pattern. Stains appear on the ceiling or chimney breast near the stack and follow heavy rain. A stain that shows only in driven rain from one side points to the flashing on that face. One that shows in every wet spell, whatever the wind, more likely comes from the top of the stack or the flue.
Can I tell the difference between a flashing leak and a brickwork leak?
Not reliably from the ground, because both stain the same place after rain. On site, a roofer can test each piece, then check the pointing, the flaunching and the pots in turn. Our survey photographs the whole stack, from the pots to the covering, so the written quote names the cause rather than guessing from a stain.
Slipped slates beside the stack matter as well, because soakers rely on the covering lying flat over them. If the courses next to the chimney have shifted, the lead can be perfect and still leak, so we check the slope for a metre or so around the stack and refix anything loose in the same visit.
Do not seal a suspect joint with mastic. It hides the evidence and traps moisture, and every trace has to be cleaned out before the lead repair.
The back gutter, the part nobody sees
On a stack that sits in the middle of the slope, water running down the roof reaches the uphill face of the chimney and stops. If nothing guides it round, it builds up against the brickwork and finds any gap it can. The back gutter solves this. It is a lined channel set into the slope behind the stack, shaped like a shallow tray with a ridge-like peak that divides the flow and sends it left and right.
It is also the part most often missing. Older roofs, quick re-roofs and many repairs leave the uphill side with a thin strip of flashing or a lump of mortar. Where a proper gutter exists, it fills with grit from concrete tiles, moss and leaves. Once blocked, it spills over the lip into the roof structure behind the stack.
Does a stack always need a back gutter?
A stack across the slope, with a wide uphill face, needs some way to divert the flow. A stack on the ridge sheds water both ways, so the issue does not arise. Our chimney back gutter page covers how the channel is built and when we add one where none existed.
A failed back gutter shows as dark staining on the rafters behind the stack when seen from the loft, and it may not reach the ceiling below until the damage is well advanced. A torch and a quick look after a wet spell is a useful check.
Repair or renew: how the decision is made
Our rule is to do no more than the fault needs, so the choice rests on what the survey turns up. A sound flashing with one lifted corner can be dressed back and repointed. A stack with a good apron, a split step and a perished joint can often have just the failed length replaced.
Renewal becomes the better choice when the problems run through the whole collar. The signs are lead gone hard and brittle, splits at several folds, old sealant and missing soakers. Mending one section then just moves the leak along. A full renewal strips the old lead, checks the brickwork and timbers behind it, and replaces apron, steps, soakers and back gutter in turn.
What weight of lead is used?
Lead sheet is specified by code, which is a thickness. Code 3 is the thin end, kept for soakers, while Code 4 suits most flashings and Code 5 is common for aprons and larger pieces. The surveyor picks the code to suit the exposure and the size of each piece.
A fillet or bitumen strip that has failed once is a sign to replace, not to patch again.
What chimney flashing costs and what moves the figure
Published guides give a broad picture. MyBuilder's chimney repair guide, updated 21 August 2026, puts a flashing repair at £250 to £1,200. MyJobQuote, in a guide dated 22 September 2026, gives £150 to £250 for a lead patch and £350 to £460 for a full replacement without scaffold, or £700 to £920 with it. Checkatrade's October 2026 guide gives stepped flashing at £80 to £120 per metre, excluding VAT. These are national third-party ranges, not our prices.
Four things push a job towards one end of the range or the other:
- Access. A low rear stack is within reach of a roof ladder, while a tall one usually needs scaffold. MyBuilder's scaffolding guide gives £400 to £900 for chimney-height scaffold.
- Size. A single-flue stack takes far less lead than a wide stack with six pots.
- Brickwork. Joints that crumble when raked out need repointing before the lead goes in.
- The covering. Slate and plain tile need soakers laid course by course, which takes longer than lead over a profiled tile.
Whichever quote you hold, ask whether it includes VAT and scaffolding. Ours is a written total that follows the survey, with your own stack photographed, and the work carries our 10-year workmanship guarantee in writing. The chimney repair cost guide goes further into the total.
Shared stacks, repointing and the winter question
Flashings on a terraced or semi-detached house often sit on a stack that serves two homes. A failing joint on the neighbour's face can still send water into your roof space, because the brickwork ties the two halves together. Our guide to the shared chimney stack covers raising it with a neighbour.
Flashing and repointing often belong together, because the lead sits in raked-out joints. Doing both in one visit shares the scaffold and the call-out. Our page on chimney repointing shows how the two jobs fit.
Winter exposes weak flashings, since frost opens small gaps and wind lifts loose edges. Our guide to chimney winter damage covers what freezing does to a stack.
A stack letting water in today gets a temporary make-safe from us first, and the lasting repair waits for settled weather. Send a photograph of the stack and of the indoor stain through the form or WhatsApp, or call 01793 626096, and a survey follows. When the work ends, no rubbish stays on your drive. For the other repairs a stack may need, start from our chimney repairs page.