Damp Chimney Breasts and Hygroscopic Salt Contamination.


  • How to fix tricky damp patches on chimney breasts.
  • Why roofing works do not always solve this problem.

Do you have a problem damp patch on a chimney breast which will not go away?  Perhaps you have already had roofing or chimney repair works undertaken? We explain why damp on chimney breasts can persist, even after the source of water has been addressed. Problem chimneys can provide a route for water to access the interior of a property.  Water can run down within a chimney flue and then soak into masonry over/around a fireplace, resulting in damp patches internally. The first course of action should be to address the ingress, typically requiring chimney/roofing repair work (or possibly ventilation of redundant flues).   If the damp patches do not go away, please call us and we can provide a solution to this issue.

The problem is hygroscopic salt contamination:

  1. Where fires have burned, chimney flues will be lined with soot.
  2. Soot contains salts which are dissolved by water running down the flue.
  3. Salts held in solution within water, move wherever that water moves to.
  4. Water moving into plaster/finishes, carries salts with it.
  5. When water evaporates out of the plaster, the salts do not evaporate, they reform as a solid within the plaster.

Hygroscopic - means capable of absorbing moisture from the air.

Where hygroscopic salts are present in plaster, they will draw moisture from the air, resulting in visible damp patches, which will persist even when water ingress problems have been fixed. Trace commonly remedy, using systems which isolate new plaster from salt contaminated masonry, so that a dry finish is provided - please call if you require assistance.

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    Kealey Clarke

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