The building
A Portland stone chimney stack on a period building in Central London, with a moulded corbel detail at the base — a common feature on stacks of this age, and one of the first things to lose definition when soiling and growth take hold at height.
The problem
The stack showed extensive biological growth — algae, lichen, and moss colonisation — combined with atmospheric soiling. More seriously, the stack had been repointed with hard cement at some point in the mid-twentieth century. The cement had cracked, and water was getting into the stack itself rather than running off it.
The method
DOFF at 150°C removed the biological growth and soiling first. Once the stack had dried, the defective cement pointing was cut out with an oscillating multi-tool and replaced with a feebly hydraulic lime mortar (NHL 2), matched to the original mix. The corbel detail at the base was cleaned with particular care to preserve its profile. Works were carried out from a mobile elevated work platform — no scaffold required.
Cement pointing is harder than the stone around it. When it cracks, water gets in and has nowhere to escape. Lime is softer than the stone, deliberately — it moves and sheds water instead of trapping it.
The result
The stack is watertight again, and the biological colonisation has been removed without any abrasive contact with the stone surface. The lime repointing gives the joint somewhere to move as the stack expands and contracts, rather than forcing that movement back into the stone.
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