A chimney stack is often the most exposed part of a house. It sits at the highest point of the roof with no shelter, and in South Wales it faces driving rain, strong wind and the occasional hard frost. Understanding how the weather attacks a stack helps you spot problems before they become expensive.
Driving rain
Rain blown sideways soaks into brick and mortar. If the pointing is already weakened, water gets deeper into the stack and can show up as damp inside the house. Bricks that stay wet for long periods are also more likely to be damaged by frost.
Frost
When water inside brick or mortar freezes, it expands and can break the surface off the brick, a process called spalling. Over several winters the face of the bricks can break away, leaving the stack weaker and more porous. This is one reason a stack that looked fine can deteriorate quickly.
Wind
Strong wind puts a steady load on a tall stack. A leaning or cracked stack can be at risk in gales, and loose pots or cowls can be dislodged. Fallen masonry is dangerous to people below and can damage the roof, so any stack that looks unstable should be looked at promptly.
The details that suffer
Flashings, flaunching and mortar joints are the weak points. Cracks in the flaunching let rain into the flue. Lifted lead lets water behind the stack. All of these are found early with a routine inspection and are cheap to put right compared with rebuilding.
What to do
Look at your chimney from the ground a couple of times a year, especially after a storm, and watch for missing mortar, leaning, cracked pots or brick fragments in the garden. Do not climb up.
See our Chimney Repairs section or call us on 07766 547309.
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