Chimneys and parapets

What is sulphate staining on a chimney breast from an old coal fire?

Old coal soot contains salts that migrate through the plaster and leave brown or yellow patches, often with a fluffy deposit. They can appear long after the fire last burned, especially if the breast is damp. They are not the same as a live roof leak, though a leak can wake them up.

Last reviewed 24 October 2026. Written for buyers in England and Wales unless this answer says otherwise. No named surveyor has signed this page. It is general guidance, not a report on your house, and not legal advice.

What is the fuller explanation?

A surveyor describes the stain and whether it is salty, soft or wet on the day. They cannot date the last fire. Hygroscopic salts pull moisture from humid room air, so the patch can look active in winter even when rain is not coming in. The Property Care Association’s approach to salts is to find out whether water is still arriving.

The usual investigation is to check the flashings, the flaunching and whether the flue is ventilated, before anyone replasters. Replastering with an ordinary gypsum skim on a salty breast often fails. Ask a plasterer who understands salt-resistant systems only after the water question is answered. There is no tariff for that specification.

What should you ask your surveyor?

Is the patch salty, or is it a soft wet stain under the ceiling line?
Could you see the flashings where the stack meets the roof?
Do you think water is still entering, or are these old deposits?

What are the typical next steps?

Check rainwater entry before you pay to skim the breast.
Do not treat upper-floor chimney salts as rising damp.
If you will open the breast for a stove, tell the installer about the salts so the lining specification is honest.

Costs vary with the building and with who you instruct. Where this page does not cite a published figure, ask the surveyor, a relevant specialist, or your solicitor for a written quote. Do not treat a forum average as the price.

Also asked about this

Can you paint a salt stain with stain-block and forget it?

You can hide it until the salts come through again. Paint is not a test of the flashings.

Does a decommissioned gas fire leave the same salts?

Coal deposits are the classic source. A later gas fire does not remove them. Any current gas fitting still needs a Gas Safe engineer if it is in use.