Damp problem around internal wall chimney breast

Postby godfrey_52 » Tue Jun 22, 2010 1:55 pm

I have recently moved to a renovated (approximately 100 year old) ground floor flat. The chimney breast has been dry lines and is non-operative. Recently (even though it is summer) large damp patches have occurred. The landlord told me the damp was not coming from the chimney stack, could it be a leaking pipe or appliance from the flat above. The flat is usually left with the windows open a bit so do not think it is condensation
Any suggestions or comments would be greatly appreciated
Many Thanks
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Simply Build It

Postby NewtonM » Wed Jun 23, 2010 1:04 pm

Ok this is quite a common problem associated with chimney breasts and can be a real pain.

It usually happens for one or more of the following reasons:

• Water entering uncapped chimney pots.

• Water entering through defective lead flashings around the chimney stack.

• Leaking adjacent rainwater goods

• Salt contamination of the wall plaster

The first thing to do is to check outside. Try to inspect the chimney and the associated lead flashings. You may well be able to see any defects clearly from the ground but if not you may need access equipment. If you are not confident about doing this yourself then contact your local building surveyor and for a fee they will undertake a survey for you.

If you suspect water is entering through an uncapped chimney pot then you may need to buy a suitable chimney pot cowl (like a hat for the pot) and install it or commission a builder to do it. However, even if this is a remedy for the cause you may still need to deal with the issue of residual moisture which I will cover further on.

Lead flashings are installed at the junctions between your chimney and the roof and are dressed into the brickwork and over the first tile to prevent water getting in. If following your inspection/survey you find that your lead flashings require some attention you will need to employ a building/roofing contractor to undertake the necessary repair works. Again residual moisture resulting from the problem needs to be considered.

An external inspection carried out by yourself or a surveyor should easily pick up any defects with your rainwater goods. If you find that the defect on the guttering, downpipes etc is directly adjacent to the damp patch on your chimney breast then you may well have found the problem. Now it’s a case of getting the defective rainwater goods repared or replaced.

Salt contamination especially in older chimney breasts is a common problem. The salt contamination is usually a result of fossil fuels like coal being burnt in the fire place. Because coal comes from the ground it can contain salt minerals like nitrates, chlorides, sulphates etc. When the coal is burnt these salts manifest themselves in the brickwork of the fireplace. Over a period of time and especially if the fireplace is no longer used as such the salts can migrate from the brickwork and settle in the wall plaster. These salts then become what is technically known as hygroscopic, which means that they can attract and hold atmospheric moisture from the surrounding environment. This is why damp patches associated with salt contamination are often worse when the weather is. If it is salt contamination there are a few ways it can be dealt with. If a damp patch is small and isolated I have known a very simple and effective solution to work which is certainly worth trying. That is to apply a couple of coats of an aluminium wood primer over the patch. In some cases this can act as a cheap but affective sealant.

If this does not work then more drastic measures may be necessary. You can remove the areas of defective plaster and then re-plaster the wall incorporating a salt retardant additive and sulphate resistant cement in the plaster mix. Having said that patching plaster like this can be a difficult thing to blend and hide in that it can shrink and crack and may require fairly regular attention. A permanent damp proof membrane might be the best solution

Hope that helps
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