Damp Stains on a Chimney Breast and Adjacent Ceiling - It may be Salts

publication date: Oct 7, 2011
author/source: Guest article by Bryan Hindle, Preservation Expert

Damp Stains on a Chimney Breast and Adjacent Ceiling - It may be Salts

Damp in House

The number of enquiries relating to damp stains on chimneys is on the up. The common complaint is that the roofer has either fixed the stack or the flashing, but the damp still comes in.  Usually the roofer has been back a couple of times or three and still the problems persist.

Stains on front bedroom chimney breast        Stains on rear bedroom chimney breast and ceiling
Stains on the face of the front bedroom
chimney breast
Stains on the rear bedroom chimney breast
and ceiling

This is a short example of the problem I've recently investigated. Salt contamination from concentrated flue gas salts which persist has caused this issue.

Typical symptoms are damp patches, which may or may not be salty looking.  They often come and go with damp weather, though it doesn't necessarily have to rain.  A muggy day will be sufficient for the stains to become more clearly defined. Often the damp stain will feel greasy to the touch (like your skin does when you've been swimming in the sea).

Check for Leaks

Of course, the first thing to do is check for leaks and fix them.  Leaky flashings, pointing and open chimney pots are the most common causes.

But when these are clearly in good condition, or have been repaired, hygroscopic salts are the most common cause.

On the featured survey I was able to get in the loft and check for leaks myself. In the image below there are signs of water ingress, with salts on the timber. However, the trimming joist is securely fixed in intimate contact with the chimney stack, shouldn't it be wet?

It is air dry at 11% WMC. Some say that salts make a conductivity moisture meter over-read .... that's true.  But it's also true that salts which are dry do not do this.  So clearly, with visible salts and a low reading from my conductivity meter, I know that this wood is dry and the salts are merely a sign of drying out - they are efflorescent salts.

Preservation Expert

Contrast this with the reading seen below, taken from the back of the ceiling plaster. This reading is 45% WME - substantially higher yet visibly free of salts.


This is because the salts are hygroscopic and are causing the meter to over-read, because they are absorbing water vapour.

The salts accumulated on the roof timbers are there due to years of tiny water ingress and condensation on the underside of the roof slates. This runs down and wets the timber, leaving minerals they dissolved along the way.  Repeated wetting and evaporation has concentrated those seen here. These are from the slates, back-pointing, external chimney pointing and such - they are relatively free of flue gas derived ammonium nitrates and chlorides.

Further down the stack, the chimney is encased in a porous plaster and this, combined with high humidity in the room below means that once wet, drying is slow, and there's plenty of time for salts to migrate through to the plaster and stay there.

Plaster lath ceiling goes right up against the chimney brickwork

In the above image, note that the plaster lath
ceiling goes right up against the chimney brickwork


Our houses are quite humid these days, what with double glazing and all manner of steam producing gadgets in the kitchen.  Not to mention showers, baths and drying clothes. Combine this with a cold old chimney stack and any salt contamination can develop into a serious and disfiguring problem.

The end result is that the plaster is now hygroscopic and will always be so. The only solution is re-plastering and disposing of the contaminated material to waste.

Remedial work is straightforward but some diligence is needed. Don't forget, diligence is the rarest commodity in the construction industry, so you must find a contractor who will take sufficient care to follow the rules.

For more information and to read the full article, visit the Preservation Expert website

Bryan Hindle
Preservation Expert
Building Preservation, waterproofing and structural repairs explained

All photographs copyrighted and supplied by Preservation Expert


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