Damp and moisture meters

Is green algae on the outside wall the same thing as black mould indoors?

No. Green algae on an outside wall is growth on a shaded, often damp outer face. Black mould inside is a different surface and usually a different mechanism. One does not prove the other, and neither proves 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?

North walls, walls under trees and walls that never dry grow a green film. That film likes moisture on the outer face. It can sit over sound brick. Black mould indoors grows on cold indoor finishes where room air is humid. You can have a green outside wall and a dry inside room, or mould inside with a clean brick face. A surveyor should say which surface they are describing. Mixing the two words in one sentence makes a buyer budget for the wrong job.

A home survey looks at both faces where they can be seen. It does not sample the growth and it does not guarantee the cavity or the solid wall between them. If rain is also getting through, that is penetrating damp and it will have its own signs inside. Algae alone is not that sign. Do not scrub the outside on the viewing and call the house dry, and do not treat indoor mould by cleaning the algae.

What should you ask your surveyor?

Did you see green growth outside, black mould inside, or both?
Is there an internal stain that lines up with the green patch?
Are you describing a finish, or water getting through the wall?

What are the typical next steps?

Keep the outside growth and any indoor mould as separate notes.
Do not inject a damp-proof course because the brick has a green film.
If the inside is mouldy, ask about heat and ventilation before you ask about the outer algae.

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 algae mean the brick is saturated?

No. It means the surface stays damp enough for growth. Saturation through the wall is a further finding, not the colour.

Should algae be removed before exchange?

Not as a condition of understanding the survey. Removal is cosmetic unless the survey has tied it to a defect in the wall.