Damp and moisture meters

Is black mould on a cold external wall proof that the damp-proof course has failed?

No. Black mould on a cold outside wall is often condensation: warm, moist room air meeting a cold surface. It can also grow where rainwater is getting in. It does not, by itself, prove that the damp-proof course has failed.

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?

Government damp and mould guidance tells housing providers to find the source, reduce humidity and deal with cold surfaces. A bedroom wall with no extractor, drying clothes indoors and a blocked trickle vent is a different problem from a wet outer leaf. A survey should say what it saw. It cannot measure how you will live in the house.

Wiping mould with bleach is not a repair, and it is not a survey. If the report also notes a leaking gutter, a failed tray or high readings at the floor, read those as separate points. The mould page and the condensation page in these guides are the longer versions. This answer is only the distinction.

What should you ask your surveyor?

Is the mould at the floor, in the corner of a ceiling, or across a whole cold wall?
Did you see ventilation, or was the extract and the trickle vents marked not inspected?
Is there a separate comment about rainwater goods or a damp-proof course?

What are the typical next steps?

Do not buy a damp-proof course because a corner is mouldy.
Ask how the house is ventilated, and whether the surveyor could see the extracts working. A home survey does not test the ventilation system.
If rainwater is also mentioned, fix the reading of that defect before you argue about lifestyle.

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

Does government guidance set a safe mould score?

No. The guidance is about risk and about removing the cause. It is not a buyer’s price list.

Can you redecorate the mould away before exchange?

You can hide it. The next cold spell can bring it back if the surface is still cold and the air is still humid, or if water is still getting in.