Damp 1880s end terrace - help!

[Dazed&Confused]

That sounds like a good plan. I have had trouble with interior walls in the past. Water had risen more than 1m in places. Removing the waterproof paint inside (fortunately painted on wallpaper) was enough to fix them without needing to remove the gypsum plaster. For me this turned out to be just a little water with absolutely nowhere to go apart from continuing up the wall.

I still have an issue with an internal wall but that's down to ground levels. They seem to build floors at different levels in these old houses and put steps everywhere. I'm planning to 'tank' that one with lime plaster (rather than the existing cement) as the trench and removal of waterproof paint is tricky on the internal wall on the other side. Hopefully it'll work enough.

Theory behind that is a waterproof wall will be completely wet through, but a breathable wall will have a fairly linear water content gradient as it dries toward the breathable side so the wall immediately behind the (breathable) interior finish will actually be quite dry and won't wet the surface. Another option if you get stuck but I like to send the water outside and be rid of it.

Found the photo. You can see the salts above the skirting boards on the stairs after I removed the wallpaper. My ventilation did not help.

View attachment 8987

And for completeness some interior wall damp where the ground levels are 2 steps higher on the other side:

View attachment 8988

Thank you Malcolm, that's really helpful. I'm learning so much!

The damp has pulled the salts out of all the walls, so I think we'll need to remove at least some plaster.

 

[malcolm]

The other thing I have learned is plan to do as little as you can. Salts on plaster will just brush away and will stop coming through once the damp has been sorted.

Also there are lots of ways to achieve the same goal, some work better and some are cheaper but might be less effective so more risk the problem isn't sorted. It's irritating that the house as originally built would probably have done every one of those things.

When fixing mistakes that are causing you problems it's not necessary to fix all of those things. Do one at a time then wait a year to see if it works. This stuff takes longer than you imagine and a year isn't too much bother.

 

[Dazed&Confused]

Thank you Malcolm. It is tempting to get impatient, but I keep reminding myself that the house has stood for this long, it's not falling down any time soon!