Crumbly damp patches on my chimney breast

[joanneO]

We bought a 1850 end terrace last year and have been doing it up. The living room faces north (I think) so its quite dark and can be cold. About a month ago I noticed a patch on the chimney breast which appeared all of a sudden. It's now about the size of my hand and the plaster is crumbling. Its towards the bottom of the wall.
The bay window was leaking and with all the nasty rain recently alot of water had got in. But we've had it all fixed now.
Before I get it replastered I want to know what it is and how we can fix it (cheaply!). Does anyone know?
Thanks!

 

[JohnD]

a chimney needs to be ventilated top and bottom or it will get wet with condensation. If yours has been capped and/or bricked up it needs to be ventilated. An airbrick will do. If the fireplace is permanently disused, you can sometimes allow air into the chimney from under the floor. This prevents draughts and has the added advantage of improving subfloor ventilation.

If the room was humid following a leak, the condensation will be worse.

If the stain is brown or black or has crystals in it, the affected plaster needs to be hacked off and the brickwork sealed, this will be acids and tar coming out of the chimney.