Why is my chimney leaking?

Start with the flashing at the foot of the stack

The most frequent culprit is the flashing. This is lead sheet turned over the junction of chimney and roof covering, and its job is to guide water down onto the slates or tiles beneath. Most stacks have a front apron, stepped pieces up each side, and a back gutter where the slope runs into the rear face.

Properly laid lead serves for a very long time, yet it fails in familiar ways. Edges lift in a gale, it splits along a fold where it has flexed through years of sun and frost, and the old mortar used to hold the top edge into the brickwork crumbles away. With the top edge loose, rain tracks behind the lead instead of over it. In some cases the flashing was never right: a bead of sealant or a smear of bitumen tape over a gap is a patch, and it tends to open again within a season or two.

The back gutter is the part that gets overlooked. It sits behind the stack, hidden from the garden, and collects leaves and moss. A blocked one lets water back up and finds the first weakness in the lead, so lead that seems fine from the front may be failing at the rear. A damp mark on the breast within hours of heavy rain often points here.

Our page on chimney flashing explains how the lead is cut in and replaced.

Eroded pointing and soft brickwork or stone

The second cause is the stack itself. Pointing is the mortar between the bricks or stones. A chimney stands in the open on every side, with wind, rain and frost working on it, and it takes more weather than any wall in the house. Over time the mortar erodes back from the face of the masonry, leaving open joints that let water in sideways.

The water does not run straight through. It soaks into the stack, travels down inside it, and appears as a damp patch on an upstairs breast or in the loft, often some distance below the true entry point. A stack with open joints will also hold water in its bricks after the rain has stopped, so staining can appear days after a storm.

On a Bath stone chimney, the mortar type matters as much as its condition. Hard cement holds moisture and drives it out through the softer stone, which then crumbles. If the stack was repointed in cement at some point, the damage can be worse than the open joints themselves. The page on cement and Bath stone explains the mechanism, and the remedy is to rake the cement out and repoint in lime. Brick stacks usually take a mortar chosen to suit the age of the brick.

Where the masonry itself is spalling, flaking or leaning, repointing alone will not do, and a partial rebuild may be the sensible route.

Flaunching, the pot and the cowl at the top

At the very top, the flaunching is the cambered mortar collar that fixes each pot and throws rain outward. Flaunching cracks with frost and age, and once it does, water gets in around the base of the pot and runs down the flue. A pot that has cracked or tilted will also let rain straight into the flue, where it has nowhere to go except the fireplace or the breast walls.

This sort of leak shows up differently. Water in the flue tends to appear as damp or drips at the fireplace opening, a sooty stain low on the breast, or a smell of wet soot in rainy weather. If the fireplace is blocked off and unused, the water collects behind the infill instead, and the first clue may be a patch of damp wallpaper.

A cowl or cap on an unused flue keeps rain out while letting air through, which matters because a sealed, unventilated flue can breed damp inside the breast. If the pot is cracked, it is normally replaced on its own; our answer on whether a chimney pot can be replaced on its own sets out how it is done.

Ask for the top of the stack to be photographed. Cracks in flaunching are hard to judge from the ground, and a clear picture removes any doubt about whether it needs replacing.

The roof covering and gutters beside the stack

Not every chimney leak starts at the chimney. Slipped slates, cracked tiles and a failed underlay beside the stack can all let water in, and the water may then run along a rafter until it meets the brickwork and appears to come from the chimney itself. The same applies to a blocked or overflowing gutter at the eaves below.

Roof pitch and stack position both affect where water gathers. A stack on the ridge sees less of the runoff than one halfway down a slope, which has a lot of roof above it feeding water down against its upslope face. That upslope face is where a back gutter and its lead carry the heaviest load.

On older roofs, the ageing of the whole covering is a factor. If several slates beside the stack have slipped, or the battens beneath have softened, fixing only the flashing leaves the cause in place. We look at the surrounding roof as part of any stack leak so that the repair is not undone by what sits next to it.

If water is coming in now, we prioritise it and make the roof safe first. That temporary measure only holds things, and the lasting repair follows when conditions allow.

Damp with another cause

Some stains on a chimney breast are not rain at all. Condensation is common on a cold breast wall in a room that is used often, especially where a flue is capped with no ventilation and the air inside it cannot move. Salts drawn out of old soot and the lining can also stain wallpaper brown and make a wall feel damp even when the stack above is watertight. Tar staining, where tar from past fires has soaked through the plaster, is its own problem and we deal with it separately.

A simple test helps. A stain that appears or grows within hours of rain suggests water entry from outside. A stain that follows cold, still mornings and fades by afternoon suggests condensation. Staining that never changes with the weather may be salts or an old, finished leak that left its mark.

Do not rely on the guess. Painting over a stain hides it, and a damp breast can rot the joists and plaster behind it while it is hidden. Our guide on the damp chimney breast goes through the signs in more depth.

What we check and what a repair involves

The survey is free. We take photographs of the stack from several sides and from above, and share them with you. We work from the top down: the pot and flaunching, the pointing, the flashings and back gutter, then the covering and gutter beside it. Water is traced back to its entry point, not the point where it shows.

Repairs match what we find. A split flashing may be replaced in new lead; failing pointing is raked out and redone in the right mortar; flaunching is cut out and relaid. Where the stack has moved or the masonry is past repair, we lay out the options, including a rebuild or, for a stack no longer in use, removal. The decision is covered in repair or remove a chimney.

As a guide, MyJobQuote reports a lead patch at the chimney typically costs £150 to £250 in the UK (2026), and a full leadwork replacement averages about £570 depending on scaffold. These are typical UK ranges, and access and extent move them. Once surveyed, you receive a fixed written price; our chimney repair cost page covers what moves the price. If you would rather see the stack cleaned before judging it, read about cleaning a Bath stone chimney. The wider chimney repairs service covers every stack job.