Damp Chimney Treatment

Water can enter via the chimney stack

Water can enter via the chimney stack

A damp chimney 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.

Lets deal with the possible causes of the damp chimney breast one at a time:

- Water entering uncapped chimney pots

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.

- Water entering through defective lead flashings around the chimney stack

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.

- Leaking adjacent rainwater goods

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 repaired or replaced.

Salt Contamination of the Wall Plaster

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 effective 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.

Permanent Treatment With Damp Proof Membranes

Newlath offers a cost effective long term solution for damp problems.

Newlath offers a cost effective long term solution for damp problems.

A very effective solution to this problem is to apply a damp proofing membrane to the chimney breast. Damp proofing membranes are essentially plastic membranes which are moulded in a stud pattern much like an egg box is. The membrane is applied to the chimney breast either over the existing plaster or after plaster has been removed. The membrane is fixed into position using special ‘mushroom’ fixings and the surface of the membrane has a mesh on it in order that a new plaster finish or ‘dab fixed’ plasterboard can be applied. What it achieves is an effective and permanent solution to your damp walls and any salt contamination because the membrane does not allow moisture or any of the harmful salts to reach the wall finishes. These membranes will also deal with the problem of residual moisture after an external problem has been rectified.

Often damp walls will take up to a month per inch of their thickness to dry once a moisture source has been removed.

As they dry the residual moisture can show itself as damp staining, salting, blown plaster, peeling surfaces etc. so if you apply a damp proofing membrane to the problem areas you can forget about any of these detrimental affects because they are completely impervious to dampness and the associated salts.

Buy Newton Damp Proofing Packs

Newton damp proofing packs for penetrating damp

Buy Newton Damp Proofing Packs

Newton damp proof membranes can be installed by a builder or DIY. Our damp proofing packs and other damp proofing products like our condensation paint and external water repellent can be purchase here on next day delivery: Damp Proofing Shop

Alternatively we could send you a list of Newton Specialist Basement and Damp Proofing contractors in your area.


  1. Jamie Westwell says

    Hi. Can i put the damp proofing membrane over the contaminated plaster does the plaster have to come off and then apply it to the brick work

    • Becky Relf says

      Hi Jamie, thank you for your question. It is recommended that the substrate is cleaned to remove all loose debris and organic matter, plaster can soften overtime weakening the fix of the membrane to the wall therefore we recommend it is removed.

      Wall render in a good condition does not have to be removed and the wall membrane can be fitted directly over however If the render is damaged, local repairs may be required or if it is in a bad state of disrepair it may need to be removed.

      Obtain as flat a surface as possible. If the wall is undulating, better results are produced if the wall surface is dubbed-out flat with sand & cement prior to installation of the membrane rather than dubbing out over the top of the 803 Newtonite. A flat uniform surface to the substrate means that the plaster or render above the 803 Newtonite will be uniform also and less likely to have cracking caused by differential thicknesses. Hope this helps but For further information please contact us on 01732 360 095 or to purchase a damp proof pack visit our store http://newtonwaterproofing.co.uk/store/

      Kind regards
      Aimee Goodes
      Technical Dept

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