Damp Brown Patch in Bedroom

[christian777]

Since moving in we have plastered and painted the bedroom on the first floor. Shortly after this brown damp patch appeared. I have no idea what it could be.

The damp isn't much wider than the patch itself and it goes as far as the ceiling.

On the outside there is render on the walls in good order and no pipes. So penetranting damp is unlikely

This does form part of a chimney so assumed it could be salts but still unsure. The chimney looks watertight

We do have humidy higher than we would like in the house so I wonder of its mould from condensation

Does this type of patch look familiar to anyone. Also the section on the right with slighter deeper colour is where a painting on the wall was hung and ended (didn't cover the patch)

Your help would be appreciated

 

[foxhole]

Chimney should be ventilated or moisture cannot escape except thru your walls .
Render prevents evaporation to the exterior so it escapes the only route it can .
Exterior pics of chimney condition would help , have you inspected the loft ?

 

[JohnD]

The brown is from tarry soot deposits being pushed through the chimney by damp.

Each flue needs to be ventilated top and bottom, and rain penetration at the top repaired.

If a disused chimney passes though the loft, you can demolish the top and reinstate the roof over it, this is a good method and removes the need for future repairs.