Is Damp Coming from My Chimney?

Last Updated 23 August 2018

Damp from chimney

Leaky chimneys are a common source of penetrating damp in the roof of a property. Penetrating damp is especially dangerous in the roof due to the abundance of timber, which is at high risk of developing mould, wood-boring insects and wet and dry rot.

While damp anywhere in a property is unpleasant, damp in a roof can lead to severe structural issues and costly repairs, such as replacing rotten timber.

Here are some of the common causes of chimney damp and the signs to look out for.

Damp from leaking chimney flashing

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.

Damp from damaged chimney stack

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.

Damp from cracked chimney flaunching

Chimney flaunching is the mortar or concrete mix that holds the chimney pots in place. If your chimney flaunching cracks, these cracks can fill with water which widens the crack further through erosion and freeze-thaw during cold weather. If the cracks get wide enough, water can trickle down into the chimney stack or down the flues.

Damp from damaged chimney pots or cowls

Chimney pots and cowls are designed to stop droughts and rainwater from entering the flues if the chimney is in use. If the chimney’s unused, the pots may be capped.

If these pots or cowls are damaged or missing, rainwater can trickle down the flues, which can lead to damp and increased moisture levels inside the property, increasing risk of condensation.

You should have your chimney regularly inspected

Chimneys are a part of a building that’s usually out of sight and out of mind, and in most properties it isn’t used at all. This can lead to chimneys becoming seriously decayed before anyone notices, by which point you may also have further property damage from penetrating damp.

This can be an especially nasty surprise for someone who’s about to sell a property, only to find its value significantly reduced by the need for expensive chimney and/or damp repairs.

It’s recommended that you have regular chimney inspections to catch any erosion, damage or leaks early on before they build up into more serious and costly problems.

If you have any concerns about the health of your chimney, feel free to get in touch with us by calling now or requesting your call back today.

How a leaking chimney causes damp issues

  • Flashing (the metal sheets which cover the intersection between the chimney and the roof) can become damaged and allow water into roof.
  • Brick chimney stacks can decay like any other wall, where damp can penetrate into worn pointing or shattered bricks.
  • Chimney flaunching is the mortar or concrete that holds your chimney pots in place. This highly exposed material can wear down and become leaky.
  • Missing or damaged chimney pots or cowls allow rainwater to trickle into the flues, which often goes unnoticed if the chimney is not in use.
  • Damp from a leaky chimney can cause severe damp and rot in your roof timbers, leading to dangerous structural issues and costly repairs.

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