Is Damp Coming from My Chimney?

In a typical period house, the point where the brick chimney meets the roof tiles is covered by sheets of metal called flashing, which seals any gaps between the two and stops water from pooling in the intersection of the roof and the chimney.Over time, these metal sheets can peel away from the roof or the caulk that forms a watertight seal around the sheets can become eroded. DIY attempts to fix flashing is likely to result in recurring leaks, so it’s best to leave it to a professional company who can offer a long term guarantee on the works.Leaky flashing is the most common cause of chimney leaks and dampness, the symptoms of which are usually wet, discoloured or dripping roof timbers. If the roof has been converted, you may see the plaster sagging, bubbling or forming mould from the penetrating damp.

Like any other brick wall, the chimney stack is vulnerable to decayed pointing and cracked bricks, which allow water to seep inside the brick and damage them through freeze and thaw.

Many chimneys have been repointed with concrete mortar, which damages the brickwork. Mortar should be soft and porous to allow movement in the structure and for the bricks to release moisture, otherwise you end up with wet, cracked bricks.

You can learn more about how decayed or low quality pointing can result in damp problems here.

A compromised chimney stack can result in roof leaks or penetrating damp. In severe cases, the structure of the chimney may be compromised, though this is very rare.