Thoughts as to new (or at least changed...) damp issue in external wall - ~1900 house

[dbrb2]

I have an end of terraced 1900ish house, and the pictured wall is the end of the terrace.
For a long time, the corner by the various aerial sockets showed a triangular damp patch soon after it rained (second photo below). It was quite constrained to just that corner though, both visually and with a damp-meter. It would sometime vanish entirely in dry weather, then be very obvious after downpours.

After some investigation between me and a builder, we found a few deep un-pointed holes in the external mortar near that hole that had been hidden behind bits of garden - and a test with a hose pipe on the external wall showed water dribbling into the corner.

These joints were all repointed, the damp area of plaster hacked off and patched with sand/cement then a skim of filler (visible as a darker triangle in the top photo below)
At the same time the space behind the old skirting was given a coat of sand/cement with a bostik water impervious additive (I don't have a photo of this), and the skirting boards replaced

This was all done ~7 days ago. Confusingly (to me and him), there is now a faint tide mark to the left hand side, running parallel to, and a couple of inches above, the new skirting board, that has never been there before.
Initially I thought is was just dirt - but it survived a clean, vanishes with a fan heater, then reappears a day later. When present, it drives a damp meter crazy (and likewise when dried out, readings return to normal)

Any thoughts as to what this could be? Trying to be methodical, the only obvious new thing to the left hand side of the image is the sand/cement below the skirting. The builder is very confident this isn't the problem, as it is quite thin and he used a "bostik integral waterproofer".

Of possible relevance, that wall is below ground to about the height of the top of the skirting board on the left hand wall. The wall to the right has ground level at floorboard level. However, this is a constant - and has been the case since well before this recent work (possibly since forever....)

 

[dbrb2]

Ok - I got curious:

Unscrewed the skirting board. The cement is still slightly damp I think - looking at the difference in colour between the bit by the skirting (light grey) and the main body of it (dark grey)

So I guess the question is:

Is my new damp line above the skirting because the cement is still damp, and so it will improve as the cement dries off, or is the cement itself causing this dampness in some more fundamental way

I measured bricks in the wall before I started using a meter, and they didn't seem to register damp, though they are very cold on an exposed wall, and we found some routes for water ingress in the far right corner.
My hope is that the new damp line is from the moisture in the sand and cement as it drys and/or being leached out of the bricks - and that since the obvious holes in the brickwork have been filled (far corner) it will eventually dry and stay dry.

If that is the case the best bet is to wait until the cement all goes light grey, and see what happens then.

 

[Cubist]

Theres a golden rule - external ground levels must never be above internal floor levels. Water collecting in the ground outside will leech through the wall to appear inside and will then migrate up the wall carried by warm air from the internal heating system. The external ground needs to be excavated to get its surface well below the level the wooden joists supporting that internal floor.

 

[dbrb2]

Ideally yes. Unfortunately the floorboards are level with the external ground all the way around the house (the air bricks have snorkels to get below boards). These walls though are all dry.

This one wall backs onto the neighbours garden, and is the one that is slightly below ground (due mainly to topography)

 

[Cubist]

That's unfortunate and sadly means that that section of wall will be a permanent weakness and source of damp. Some may think/suggest tanking the low section would help but I don't believe it would - the damp would merely rise above it. Leaving it alone may be your best option while hoping that the snorkels help to prevent rot in the floors void.

Hopefully someone will come along to offer more helpful and constructive advice - good luck.

 

[dbrb2]

Thanks. It is interesting (to me) that the new sand cement seems to have made a _new_ section of previously dry plaster damp.
I'd much rather not remove the new S+C - and am _hoping_ that it is curing slowly rather than providing a new source of damp from somewhere....
But I don't have much evidence to support this!

I suspect I could chisel it off again if needed - it's not a large area - but I would rather not if it can be avoided. Maybe I leave it to dry for a bit (perhaps with a dehumidifier) and see if it gets drier or not over time.
If _not_ I will likely have to remove it....though presumably the longer it is on, the harder it becomes to ever remove as well!

Lowering the ground on the far side of the wall would be nice, but may prove impossible, due to it not being my property

 

[Stevers]

Some of us have spent the last twenty years hacking wet cement of walls and have had success in replacing it with lime renders. Cement renders seem to hold on to the damp rather than let it go, which is what you might be seeing. Very often the brick or stone behind the render will appear dry. Similarly I've been removing a thin hard cement screed from a Blue Lias limestone floor, initially there were any number of damp spots, but those have now dried out completely.

I have used concrete with integral water proofer for the floor of an unheated shed (against a retaining wall), and that appears to be dry - just luck of the draw I suppose!

 

[dbrb2]

I'll see what it looks like after a day with a dehumidifier running.

Key will be:

Once/if it looks dry, does it stay that way

If so, great. If not, it's soaking up water from somewhere else, and from there into my previously dry plaster