Chimneys take more weathering punishment than almost any other part of a house. Projecting above the roofline and exposed on all four faces, they are subject to frost cycles, driving rain, UV degradation, and wind loading that the rest of the roof structure is largely shielded from. On Victorian and Edwardian properties in particular — which make up the majority of housing stock across Brighton and Hove — chimney stacks are a persistent maintenance concern and one of the most common sources of water ingress we are called out to investigate.
Why Chimney Mortar Fails
The mortar used to bed and point traditional brick chimneys was typically a lime-based mix. This was appropriate for the era — lime mortar is slightly flexible and breathable, which suits older masonry. Where problems arise is when that original lime mortar has been repointed at some stage using a harder Portland cement mix. Cement mortar does not flex, does not breathe, and bonds more strongly than the brick itself. As the stack moves thermally and the brickwork expands and contracts, it is the brick rather than the mortar joint that eventually gives way. The result is spalling brickwork, cracked beds, and open joints that let water in directly.
Common Types of Chimney Damage
- ✓Open or eroded pointing — mortar joints that have recessed, cracked, or washed out, leaving gaps between the bricks.
- ✓Failed chimney cap or crown — the mortar cap over the top of the stack deteriorates faster than the sides and is a very frequent source of water ingress.
- ✓Spalling brickwork — face of individual bricks breaking away, usually caused by frost acting on saturated masonry or by incompatible hard mortar trapping moisture.
- ✓Failed lead flashing — the junction between the stack and the roof slope is sealed with lead; when it lifts, cracks, or loses its pointing into the chase, water tracks down into the roof structure.
- ✓Loose or displaced chimney pots — pots that have shifted or cracked allow water in at the top of the flue.
- ✓Structural leaning — a stack that is noticeably leaning requires a structural assessment, as it may need partial or full rebuilding.
Repointing vs Rebuilding
Repointing — raking out the deteriorated mortar joints and repacking with a fresh mix of the correct specification — is the appropriate repair where the brickwork itself is still sound. A properly raked-out and repointed stack in a suitable lime-based mortar will restore full weather resistance and, depending on the original condition, may not need attention again for 20 to 30 years. Where the brickwork has spalled significantly, where courses have shifted, or where the stack is leaning, repointing alone is not sufficient and a partial or full rebuild is the correct approach. We always assess the structural condition of the stack before recommending a scope of work — there is no value in repointing a stack that needs to come down.
Using the wrong mortar mix on an old chimney stack is one of the most common and most damaging mistakes in chimney repair. If you are getting quotes for repointing, make sure any contractor specifies the mortar mix they intend to use. A hydraulic lime mortar (NHL 2 or NHL 3.5) is appropriate for most pre-1920s stacks. Hard OPC mixes should be avoided on traditional masonry.
The Lead Flashing at the Base of the Stack
The junction between a chimney stack and the roof slope is sealed by a combination of step flashings (running up the sides), cover flashings (lapping over the top of the step flashings), and an apron flashing at the front face. This is often where water ingress originates — not because the brickwork is failing, but because the lead has lifted from the chase, cracked through thermal movement, or was installed too short to begin with. A thorough chimney survey always includes a close inspection of the flashings as well as the masonry, since one can look perfectly sound while the other is the actual source of the problem.
Can You Leave It?
Open chimney mortar joints and a failing cap are not something that improve over time. Once water is getting into the masonry in volume, frost cycles through the winter accelerate the breakdown — what is a repointing job in summer can become a partial rebuild by the following spring if left through a wet winter. The cost difference between the two is significant. Summer is an ideal time to have chimney work carried out: access is straightforward, conditions are dry, and fresh mortar cures properly in warm weather rather than risking frost damage before it has set.
Our chimney repair service covers everything from repointing and cap replacement through to full stack rebuilds and lead flashing renewal — we survey from the roof, not from the ground, so the assessment reflects the actual condition of the stack.