We often see and hear about homes with yellowy brown damp patches at the top of chimney breasts. The most likely cause of this kind of damp patch or stain is hygroscopic salts in the flue along with creosote and tar. These stains can come and go as moisture levels in the air change.

Hygroscopic Salts
Burning wood or coal produces a variety of gases which can mix with moisture to create acids that attack the mortar or plaster on the flue wall. This reaction causes salts to form including hygroscopic salts – salts that draw moisture from the air. If the chimney is well ventilated, moisture levels inside the flue should be low and sweeping regularly can help control the salts and salt build up. However, if the flue is not ventilated well (e.g. if it is closed off completely at the bottom so no air can flow through it) the salts will continue to suck moisture from the air, making the wall more and more damp. It is possible for the salts to penetrate so far into the flue wall they can draw moisture from within the actual room. Other chemicals created within the flue like Tar and Creosote are also soaked up by the salts adding the dirty yellow or brown colouring to the stain.

The solution to this problem is NOT to cover the stain in some sort of anti stain paint and then paint over it with emulsion. This is not going to stop the damp. You need to contact a few builders and get them to quote for repairing the wall. This might include removing the existing plaster from the wall and re-plastering and opening up sufficient ventilation in the flue to allow air flow. Repairing the wall inside the flue or adding a lining will help prevent the problem from reoccurring as will sweeping your chimney regularly to remove any residues and soot.

Horizontal Line Stains
If the damp appears as yellow, brown or black horizontal lines across the front or sides of the chimney breast, this might be caused by the lining inside the flue being fitted incorrectly in the first place.
Since 1965 building regulations specify that all properties built in the UK must be fitted with a flue lining. It sounds incredible but it was known for some builders to install the lining upside down. This allows moisture to easily escape through the lining to the brick wall and from there through to the room. As the linings comes in square or rectangular segments, the damp would appear as a straight horizontal line. The solution to this is again to have a builder in to remove and replace the lining and re-plaster.