Where a chimney rises through a roof, two very different structures meet: a heavy masonry stack that stays still, and a sloping covering that expands, contracts and sheds water towards it. The junction between them has to be sealed with metal, and the stack itself has to resist rain from every side. When either fails, water gets in at the stack, and it usually shows up as a stain somewhere else.
This guide follows the water on the roof. It names the five leak paths, explains how a roofer tells them apart and sets out the usual repair for each. If the stain is the problem you can see and you want to know what it means from the inside, our answer on damp patches on a chimney breast starts from the room. The full range of work is on our chimney repairs page.
Finding the leak before choosing the repair
Water travels. It enters at one point, runs down a timber or along a membrane and drips from another, which is why a stain on a ceiling seldom sits beneath the hole. Guessing leads to repairs in the wrong place, so we trace the leak before we quote.
Start with the evidence indoors
First questions: where is the stain, when does it show, and was it blowing or still when it appeared? A stain that arrives in heavy rain from one direction points to one side of the stack. One that appears after a long wet spell and dries slowly suggests water soaked into the masonry. In a loft, the underside of the roof around the stack shows tide marks, damp timber and sometimes daylight.
Then the roof
On the roof we look at each part in order: the lead at the foot and sides of the stack, the gutter behind it, the cap at the top, the joints and the masonry between, and the pots. We photograph each part in turn, so the pictures show you the same things.
A water test, where needed
When the visual check is not conclusive, a controlled water test settles it. Water is run on one part of the stack at a time, starting low and working upwards, while someone watches the loft. The point at which water appears gives the leak path. It is slower than guessing and avoids paying for a repair that does not work.
A word on timing
Waiting makes a leak worse. Where rain is entering, a temporary cover goes on first if one is needed, and the permanent fix follows when the house has dried out. Our winter chimney damage guide covers the faults that follow frost.
Leak path one: failed flashing
Flashing is the sheet lead that seals the joint where the stack passes through the roof slope. It is cut and folded to fit the shape of the stack, with a skirt that lies over the roof covering and an upper edge tucked into a joint in the masonry.
How flashing fails
Lead is soft and it moves. Over the years it can lift at the edges, split along a fold, tear away from its fixings or pull out of the joint it is tucked into. Where the joint has been sealed with mastic or cement, the sealant shrinks and cracks. A flashing that looks intact can still be letting water behind it, so the edges matter more than the face.
Soakers and steps
On a tiled or slate roof, the sides of the stack use stepped flashing, which follows the slope, together with soakers, small pieces of lead that interleave with the courses. Where the roof covering has been changed and the flashing has not, the two no longer fit.
The repair
A local fault can be patched, but a flashing that has failed across its length is better renewed. New lead is dressed over the roof covering, tucked into raked-out joints and wedged in place, with mortar or lead wedges and not a smear of sealant. Lead is sold in codes, each a different sheet thickness, and Leadworx calls Code 4 the standard pick for everyday flashing. MyJobQuote says correctly fitted flashing can last for over 80 years, and longer in the heavier codes. Costs are in our chimney repair cost guide, and our page on replacing chimney flashing covers the job itself.
Leak path two: the back gutter
A stack that rises through the slope, away from the ridge, blocks the water running down the roof behind it. Without a channel to carry that water round the stack, it would pool against the masonry. The channel is a back gutter, a lead-lined trough set into the roof behind the chimney, falling to either side so that the water runs round the stack and back onto the slope.
Why it fails
The back gutter is the part of the junction that carries the most water and the least attention. Leaves, moss and grit settle in it and hold water. Lead in a trough that stays wet lasts less well than lead that dries. Where the fall is too shallow, water stands in the gutter and finds its way over the upstand or through a seam. A back gutter that has been formed in sheet that is too short can leave a lap that lets water in.
How a roofer checks it
We look for debris, ponded water or staining along the gutter, and for splits where the lead has been folded. Inside the loft, a damp patch behind the stack, on the uphill side, is the usual sign. Our answer on the chimney back gutter explains it in more detail.
The repair
A clean-out solves a blocked gutter. A split or sagging one is renewed in lead, with a sound fall, a decent upstand against the stack and an upper edge that laps correctly beneath the roof covering. The new back gutter is built to carry water round the stack and clear of the masonry, and we check the fall before the covering goes back over it.
Leak path three: flaunching and the stack top
Up at the pots, the flaunching is the mortar that beds them and slopes away to throw rain off. Because it sits highest, with nothing above to take the weather, it is the first part of the masonry to suffer.
What goes wrong
Cracks open between the mortar and the pots, or where the mortar meets the brickwork. Frost breaks the surface up, and the cap becomes hollow underneath. Water getting in at the top tracks down beside the clay liner inside the stack, soaks the brickwork and emerges wherever it can. The stain it makes in a bedroom can be a floor away from the cap.
The repair
Weak flaunching is cut off and rebuilt, with the pots re-bedded, not skimmed over. Our chimney flaunching guide runs through the method and the mortar choice step by step. A patch on a crumbling cap fails quickly because it has nothing sound to grip.
Why the cap and the pots are linked
The cap exists to hold the pots. Where a pot has cracked or the bed has loosened, the water enters round it, so we check both. Replacement of a pot is a small job on the scaffold, and cheaper than coming back later to do it. Where the flaunching is poor and the top courses of brick are loose as well, the repair reaches further down the stack, and rebuilding the top of it may be the more lasting answer. Our stack rebuild guide explains when that applies.
Leak path four: porous brick, stone and open joints
The stack itself can leak even when the lead and the cap are sound. Masonry takes up water, and a stack that stands in the full weather has more surface exposed than a wall.
Open joints
Mortar that has worn back lets rain into the middle of the stack. Water can pass through a joint and across a flue lining, and in time the damp reaches the plaster inside. Repointing the worn courses closes this route. Our chimney repointing guide covers raking out and the choice between lime and cement.
Soft or damaged faces
Some bricks and stones are naturally porous, and old ones may have lost the hard outer skin they were fired with. Frost then flakes the face off, which is called spalling, and exposes softer material behind. Limestone stacks on older Stroud-area houses need a lime mortar and a matching replacement stone, because a hard cement repair forces moisture out through the stone instead. Our guide to repairing stone chimneys with lime explains the reasoning.
Render and coatings
Rendered stacks leak when the render cracks or loses its bond, since water gets behind it and cannot get out. Sealing the surface with a coating can trap water already inside the wall, so we do not apply one without finding out where the moisture is coming from.
The repair
Open joints are repointed, damaged bricks or stones are cut out and replaced, and failed render is hacked back and redone. If the structure has moved or cracked right through, the answer is a rebuild.
Leak path five: open pots, cowls and unused flues
The fifth route runs straight down the flue. Rain falls into an open pot just as it falls into a bucket. In a working flue the heat and the draught carry much of it away, but in an unused one it can reach the damp, cold masonry and the fireplace below.
Where the water ends up
Water entering the top of an unused flue runs down the liner and collects in the throat of the fireplace, where it soaks the soot and the lining. The first signs are a damp smell, wet soot dropping into the grate, or a stain on the breast. Rain can also be driven across the opening by wind and run down the flue wall.
What stops it
The fix is a cowl or cap that keeps the rain out and still lets the flue breathe. A solid cap on an unused chimney, with no ventilation, can make damp worse by stopping air moving through the flue, so a vented cap or cowl is preferred. Our guide to chimney pots, cowls and caps compares them, and our answer on capping an unused chimney deals with the decision to cap.
Cracked and missing pots
A pot with a split in it lets water pass into the flue and into the stack below. A missing pot leaves the flue unprotected. Either is a simple replacement, normally done with the flaunching.
Taking the stack down instead
Sometimes the best fix for a chimney nobody uses is to remove the stack to below the roof and make good. That job has its own page, removing a chimney stack, along with the structural and consent points. Listed status, or a conservation area, should be checked with Stroud District Council's planning team first.
Putting the repair in order and what you receive
Because a stack usually has more than one weakness, the order of the repair matters. Starting at the top means the water from the work above does not undo the work below.
A sensible sequence
The scaffold goes up first, because every repair depends on a safe platform. The pots and flaunching are dealt with next, then the joints and any masonry repair, and finally the flashing and the back gutter, which seal the stack to the roof. The covering round the stack is made good last. Each stage is photographed.
What is in the survey and the quote
At the survey we trace the leak, take photographs of your own roof and stack and list the repairs needed. The written quote covers scaffold, repairs, materials, labour, clearing waste and making good, with VAT shown on the quote. National ranges for chimney work are in our cost guide, and none of them stands in for a price on your own stack.
Guarantee and site
The 10-year workmanship guarantee comes on paper, and we remove our rubbish. To book the survey, phone 01453 933613 or send a WhatsApp message with a photograph of the stack and the stain, or fill in the enquiry form.