Chimney repointing is the process of raking out old, decayed mortar from between the bricks of a chimney stack and replacing it with fresh mortar to restore weather resistance and structural stability. Stack repairs cover the wider job: renewing the mortar cap (the flaunching), refitting or replacing pots, and addressing the flashing where the stack meets the roof. Together these are the most common reasons a chimney leaks or becomes unstable.
What chimney repointing involves and when it becomes necessary
Repointing means removing the exposed face of the mortar joints to a sound depth, then filling and finishing them with new mortar. The mortar used to bed and joint the bricks is the weakest part of a chimney, and it is exposed to driving rain, frost and temperature swings on all four faces with little protection. Over time it crumbles, washes out and loses its grip on the brickwork.
A stack typically needs repointing when the mortar can be scraped out easily with a screwdriver, when joints are visibly recessed or hollow, or when pieces of mortar are found on the roof or in gutters. Other signs include damp patches on chimney breasts inside the house, white salty staining (efflorescence) on the brick, and bricks that have begun to crack or spall — where the face flakes away after water has soaked in and frozen.
The choice of mortar matters. Modern dense cement mortars are stronger than many older bricks, and on a period chimney they can trap moisture and accelerate decay. For older or solid-wall properties a lime-based repointing mortar is often more appropriate, as it is softer and allows the structure to breathe. The correct mix depends on the age of the building and the type of brick, so it is reasonable to ask any contractor what mortar they intend to use and why.
Why chimney stacks are such common leak sources
Stack repairs cover the wider job: renewing the mortar cap (the flaunching), refitting or replacing pots, and addressing the flashing where the stack meets the roof.
A chimney is one of the most exposed parts of any roof. It stands proud of the slope, catches wind-driven rain from every direction, and has multiple joints and junctions where water can find a way in. Each of those junctions is a potential weak point, and a fault in any one of them can show up as damp inside the house — often some distance from the actual source.
The usual culprits are predictable:
- Decayed pointing — open joints let water soak into the brickwork and track down inside.
- Cracked or missing flaunching — the mortar cap around the pots fails, allowing water straight into the top of the stack.
- Failed flashing — the metal weatherproofing at the roof junction lifts, corrodes or pulls away from the brickwork.
- Spalled or porous bricks — once the brick face breaks down, the whole stack absorbs more water.
- A redundant flue with no cap — an unused chimney left open lets rain fall directly down the void.
Because water can enter at the top and travel down through the masonry before appearing on a ceiling or wall, diagnosing the true cause often needs the stack to be inspected close up rather than guessed at from the ground. Several faults frequently occur together, since they share the same cause: years of weather exposure with no maintenance.
Flaunching, pots and flashing explained
These three terms cover the components most often involved in stack repairs, and they are worth understanding before getting any work assessed.
Flaunching is the sloping mortar bed on top of the stack that holds the chimney pots in place and sheds rainwater away from the flue openings. It is formed as a smooth, weathered fillet so water runs off rather than pooling. Because it sits flat on the most exposed surface of the chimney, it cracks and breaks down faster than almost any other part, and failed flaunching is a frequent cause of leaks straight into the top of the structure.
Pots are the clay or terracotta cylinders that sit on the stack and extend the flue upward. They improve the draw of a working fire and protect the flue opening. Pots can crack, become loose as the flaunching fails, or simply be the wrong type for a flue that is no longer in use. A disused flue can instead be fitted with a capped ventilated cowl, which keeps rain out while allowing a little air movement to prevent damp building up inside.
Flashing is the weatherproof junction between the chimney and the surrounding roof covering. It is usually formed in lead, dressed into the brickwork joints and over the tiles or slates so water is guided away from the gap. Front aprons, side step flashings and a back gutter all combine to seal the stack to the roof. When flashing corrodes, lifts, or was poorly fitted, water enters at the base of the chimney — one of the most common and most misdiagnosed sources of internal damp.
How a stack is repaired or rebuilt
The approach depends on the condition of the masonry. Where the bricks are sound and only the joints have failed, repointing alone may restore the stack. The old mortar is raked out to a consistent depth, the joints are brushed clean, and new mortar is worked in and finished to match the surrounding brickwork. Cracked flaunching is broken out and recast, and any loose pots are reset at the same time.
Where decay runs deeper — bricks spalling, the stack leaning, or large sections of mortar gone — a partial or full rebuild may be needed. This involves carefully taking down the stack, often salvaging the existing bricks and pots where they remain serviceable, and rebuilding to the original profile with appropriate mortar. Flashing is normally renewed during a rebuild, since the surrounding tiles are disturbed in the process anyway.
Access is a significant part of the job. A stack cannot be reached safely from a ladder for sustained work, so scaffolding or a suitable platform is generally required, and this is often the largest single element of the cost. Anyone commissioning work should expect a close inspection first, a clear statement of what is being repaired versus rebuilt, and confirmation of the mortar and flashing materials proposed. It is also worth checking whether the property sits in a conservation area or is listed, as that can affect what materials and methods are permitted.
Reviewed: June 2026