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Chimneys & Salt Contamination

September 28th, 2018

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Salt contamination

Nice example of salt contamination on a chimney recently.

Have you ever noticed discoloured patches which often appear damp on the face of your chimney? If so, this could be caused by salt contamination.

Salt contamination within a chimney occurs due to the long term effects burning of fossil fuels (coal and wood etc) in an open fire. When fossil fuels are burnt, contaminates are released into chimney flue where they migrate into the brickwork. Long term the accumulative effects can result in extensive contamination of the masonry where these salts aggressively attack the brickwork. Often coupled with water ingress these salts can move in solution and are deposited within the surface plaster where they tend to give the appearance of a damp, discoloured greasy residue.

These salts can be hygroscopic meaning they have the ability to absorb moisture from the atmosphere, therefore their presence within the plaster causes the plaster to become damp simply by attracting airborne moisture. You can often see the effects of salt contamination throughout the year as these damp patches becomes more visible in the colder wetter months when humidity levels are high and reduce in the warm summer months as humidity levels lower. This is the effect of the contaminate salts absorbing moisture from the air. The effects may also be increased where chimney flues have been capped absent from ventilation.

Therefore misleadingly, whilst the chimney itself maybe dry and no longer be suffering with issues of water ingress, symptoms of damp will remain due to contamination of the plaster.

Over the years I’ve seen many cases of salt contamination misdiagnosed as water ingress leading to thousands of pounds being spent by worried homeowners on expensive scaffolds, replacement flashings, chimney rebuilds and roof repairs. I’ve even met one client who wrapped the chimney stack externally in plastic for over a year to prevent the possibility of water ingress continuing after many failed attempts at repair. In the end we diagnosed salt contamination as the problem.

Salt contamination can be difficult to diagnose however, to the experienced professional should prove no problem. If you take readings from the contaminated plaster with an electronic moisture meter you will inevitably receive an erroneous high readings. This is the effect of salts which are conductive. Similarly, if you take electronic readings beyond the surface plaster into the brick behind (you can use deep wall probes for this) you may receive similar results again, due to salts. A calcium carbide meter could be used although this method of onsite analysis will not differentiate between salts from the flue and free moisture, therefore any elevated readings obtained could lead to misdiagnosis.

The only accurate way to establish whether the chimney is wet/dry is to take samples of the masonry and put them through the gravimetric process. This process will separate the moisture content of the material sample highlighting how much moisture is attributed to free moisture / water ingress and how much moisture is present due salt. At Dryfix our surveyors have the skills, knowledge and equipment to do this for you, if you are worried about potential salt contamination, just give our office a call and book in a survey.

With regards to repair. Well, unfortunately if the contaminated plaster remains in situ so will the discolouration caused by the hygroscopic effects of the salts. There is no magical surface paint or treatment that can be used to disguise or hide the problem.

As such in almost every case it is essential that the contaminated plaster is removed. The brickwork beneath will require preparing and some contractors may choose to treat the masonry prior to reinstatement of a new plaster. This is simply to ensure that long term the salts are retained within the structure and there is no possibility of them migrating back into any newly reinstated plaster. It is essential however that if you are re-plastering directly back to the masonry that the backing plaster contains and essential salt inhibiting additive. Gypusm plasters and dry lining should be avoided.

What type of treatment, if any really does depend upon the circumstances and the contractors preference.  Sulphate washes, physical slurry barriers, water proof renders and above ground membranes either adhered or stapled are considered by many. The only thing I would advise is, if using physical membranes which are stapled to the walls, be careful. I have seen cases of these used to prevent salt contamination in chimney flues where the staples / plugs have perforated and damaged the bricks leaving holes directly into the chimney flues. If you don’t have an aluminium flue liner inside your flue there is a possibility that holes could lead to the escape of dangerous gases.

If you suspect a problem with damp or salt contamination, please don’t hesitate to get in touch. At Dryfix our damp reports are only undertaken by experienced and fully qualified surveyors who hold the National Qualification for remedial surveying C.S.R.T. awarded by the BWPDA / PCA, our industry body. This proves that our surveyors have been independently examined and tested, proving their extensive knowledge and expertise.

All of our surveys are carried out in accordance with The British Standards Code of Practice.

Our surveyors carry all the necessary diagnostic and inspection equipment onsite, including thermal imaging and fibre-optic cameras to examine hidden cavities, highlighting blockages and bridged damp proof courses. Salt analysis testing and equipment for in-depth moisture analysis of masonry walls and plaster is also available. Photographic evidence of our findings is recorded and included within our report, so you can be sure of correct diagnosis.

Our reports contain comprehensive advice of any problems identified and include our recommendations, specifications and costs for repairs, if necessary. Edited pictures along with category damage ratings are also included to assist your understanding about the urgency of any repairs needed.

Our reports are available in both printed media and/or converted into secure electronic Portable Documents (PDF) for email.

Suspect a Problem?

If you suspect a problem and require a survey, or need advice please Contact us.

Dryfix Preservation Ltd, York















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