Damp patch in middle of wall soggy beneath plaster

[blessedfig]

Horrible shock today when we started redecorating the kitchen-diner. When washing down one of the walls I noticed a small patch near the top where the lining paper had badly bubbled. As soon as the cloth went over it the paper came away revealing a section underneath where the plaster had disintegrated and there is crumbly, damp stuff underneath. The surrounding area looks and feels absolutely fine and there's no sign of a leak from the room above and nothing untoward outside. Does anyone know how we can make it good and what could have caused it?

Thanks for any help!

Could @PigletJohn help I wonder? He’s very knowledgeable about house issues.

photograph please, with something to show scale.

It might be a patch of filler that is not water-resistant, or it might be a pipe leak.

Look for a radiator, shower, or other watery thing in the room above.

Here we are - it's slightly longer than a pen and the bits right next to it all around feel completely dry and solid. I've looked upstairs and can't feel any damp immediately above. The showers, bath etc are all on the other side of the house and the radiator in the room above is on the wall at a right angle to this one with no sign of a leak anyway. We had quite a lot of work done on the central heating a couple of weeks ago and all radiators were checked and had new bits (valves?) fitted. It was a big job and the system had to emptied and refilled a couple of times and there was a block. There was a leak from the cylinder at one point, but again, that's in a different part of the house. Don't know if all that might have disturbed things?

I've just touched the stuff again and it's actually not damp but dry, soft and spongy.

what is on the other side of the wall?

how old is the house?

what is the wall made of?

scrape out all the soft or loose material

you might find something behind it

Thank you Piglet John - I have scraped and found that there is a pipe there with a leak so that answers that. I suppose that's going to be quite a job .good job I didn't go with my first instinct to fill it in and paper over...