still a damp patch even after remedial work

[brass]

We are an end of terrace with a damp patch on the inside of the exterior wall. It is a 4 inch wide band approx 2.5 foot long in the middle of the wall. We have checked the outside, it is clear and pointing is in good shape. Inside we have chipped out the old render, applied waterproof render, replastered and used stainblock etc . However the paint has dried but I can see the patch is still there!

What could be causing it? What are we doing wrong?

How long ago was all this done? Was the plaster 100% dry when you painted it? We had some replastering done this year... Where it was a thin skim, it took a clear week to dry, but where it was an inch or more deep, we had to leave it easily three weeks, and that was in much warmer weather than this. Might just be the moisture from the plaster..?

only a few days, but we've had the heating on. Though other research also indicates it might take a few weeks to dry out properly so I guess I'll reassess in the New Year.

Heating or no heating, it'll take longer than a few days for the plaster to dry out. Give it a chance, don't panic, and I'm sure it'll look better in the new year.

You may find that the surface finish of your paint is disimproved by the escaping moisture... If so, you can sand it back a bit, then (as long as it's fully dry) overcoat.

that sounds positive, thanks brownsauce

If that doesn't work, it's worth monitoring to see whether or not it could be penetrating damp (something that's driving me batty right now!). Does it get noticeably worse when it rains or is it damp all the time? If the former, it's possible you've got a leak or a crack in render somewhere other than right outside - we think that ours is actually caused by a leak near the roof. Water is trickling down inside the wall and just coming in where the bricks have settled

hmmm it doesn't change size or shape. It looks the same regardless of the weather.

I also suspect that there is a mix of original victorian render and newer patches from repairs/redecoration over the last 100 years!

oh and being victorian its not a cavity wall so nowhere for the water to trickle down.

brass Mine is Victorian and when we had the foundations dug out they found that it was indeed a cavity wall (as you say, repairs had been done etc). If it doesn't change then I suspect you're right and it might just be the plaster drying. Good luck! Damp is a bugger to diagnose!

that's interesting HomeIs, do your bricks follow the full brick/half brick rule in layout as well?