Huge thanks this is really helpful. It’s probably been there ages as they blocked but didn’t vent the chimney back in 2002 ish. We have just repainted ( over egg shell) so I wonder if that was hiding it.
Any ideas what is causing this wet/damp patches in our lounge and dining room please? It’s a 3 bed 1930s semi. This is all downstairs.
3 patches are either side of chimney breasts but one is on the floor near where a door has been blocked.
They fade completely in hot weather but come back...