Chimneys and parapets
Render on a disused stack is bulging and the mortar behind it is crumbly. What has the survey not sampled if it mentions sulphate?
The mortar. If the survey mentions sulphate, it has not sampled the mortar to show it. Render on a disused stack is bulging and the mortar behind it is crumbly, which is what the eye could see. Sulphate is a word for a chemical effect people discuss on old stacks, where flue deposits can attack mortar and throw the render off. Mentioning the word is not a laboratory sample. The survey has recorded bulge and crumbly mortar. It has not taken mortar away for testing, and it has not proved sulphate. The sample is what is missing.
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?
Disused flues still hold soot and condensate in the brick, and old render can be pushed off by expanding, failing mortar whether or not a chemist would call the salt sulphate. The surveyor can tap the bulge, see the crumbly joints and use the word carefully as a possibility. A sample would be a piece of that mortar tested, which a home survey does not do on the roof.
Ask the surveyor whether 'sulphate' is a suspicion or whether any sample was taken, and correct the note if the word overreaches. Tell the solicitor the proved points are the bulge and the crumbly mortar. A bricklayer can say how the stack should be repaired from what is visible. Do not let a specification claim a chemical result the survey did not obtain. If someone later tests the mortar, that result is new information, not a reread of this visit.
What should you ask your surveyor?
- Did you see bulging render and crumbly mortar, without taking a sample?
- Is sulphate mentioned only as a possibility, not as a test result?
- Was any mortar removed for analysis?
What are the typical next steps?
- Keep bulge and crumbly mortar as what was seen.
- Do not treat a mention of sulphate as a sample.
- Specify repairs from the visible stack, not from an untested salt.
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
If the flue is capped, has sulphate been stopped?
A cap changes how rain enters a disused flue. It is not a sample of the mortar, and it does not prove what salt is there. The survey still has not sampled the crumbly mortar behind the bulging render. That is as far as the recorded facts about the bulging stack render can be taken.
Does sound render on the house walls clear the stack?
It clears those walls, which are not the stack. The bulge is on the disused stack. House render that is firm does not sample the chimney mortar and does not cancel the word if the survey used it. The report leaves the bulging stack render where it is, without a second finding.