Render & plaster on damp internal bedroom wall below ground level?

9 May 2004
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United Kingdom
Hey ho - our flat is the basement of an 1860's building and as expected it's plagued with damp :(
We stripped the plasterboarding in the corner of our bedroom today as the wallpaper was lifting and we could tell the plasterboard underneath was pretty manky.
Pic shows the wall we've uncovered - first off we found loads of very blown very coarse render (age indeterminate, but we've been here 20 years and the conversion was done in the 80's) about half an inch thick that was just crumbling away, with a thin plaster coat; much of the render had fallen off the wall and the resulting dust was bridging between the bricks and the plasterboard. We hacked away everything that was loose and friable.
The floor level of the flat is about three feet below ground level and I think this shows in the pic where there's a line of blown render at that level, but much of the render on the rest of the wall was blown too. The stuff that's still there is pretty sound, though. Also there's a line of blown render at about 6 inches above floor level, not sure how that's come about.
ANYWAY, long story short - there's zero chance of doing anything on the outside to attempt to prevent damp ingress as there's a humungous block of flats the other side (and as I said, we're below ground level anyway). What do we do to prepare, re-render and plaster this wall? Any (sensible!) suggestions most welcome!


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Re-render with waterproofer additive..will break down through time! force the moisture to migrate elsewhere.

Tanking, possibly with ventilation added.
it looks like there's been remedial damp work previous up to 1000mm ht. or so?
knock it all off and have the whole wall(s) rendered with a sand and lime 3:1 mix, and skimmed with limelite finish.
dont use any gypsum plaster.
keep the bottom edge of the render about 40 - 50mm off the floor.
dont allow a rendered plinth.
take out suction and the render should take to that surface but you could prep with SBR or a sand an lime an SBR slurry.
use plastic beads not galvanised beads.

besides penetrating damp, condensation will be an issue - 24/7 trickle venting and trickle heating will help, so might a humidifier.
night time in your bedroom will bring on condensation.
is there a chimney breast in the flat?
If the damp cannot be cured then the wall is not suitable for lime plastering.
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OK so further hacking and so on today and it's become a bit clearer and also more confusing, par for the course I guess...
There are three distinct areas; there's a 6" high border at the bottom of the walls that is rock solid concrete, as hard as iron, and is showing no sign of deterioration or lifting.
Above that there is render to about waist height, three foot 6 or so (although on one wall it's a couple of inches higher) of a greyish render that is mostly sound, it's only the plaster over it that is rubbish in a few places but generally a tap-test doesn't sound hollow (the corner that's come off as you can see in the pic below was loose, but that's all); and the render on the narrow wall is the same greyish stuff all the way up, some of which as you can see has come away, although the rest is sound.
Above that, and the cause of the dampness on the plasterboard, the render is a coarser yellowy orange sandy stuff that had largely fallen away. I can only compare it to bricklaying mortar in its consistency, but it had loads of hair/fibre in it as well. I surmise that this is older than the lower render? Anyway, that's the worst part. Some large patches are still adhering strongly (again, a tap test doesn't sound hollow over the bits that are still up, although I reckon I'll have them down anyway).

The darker area at the bottom left corner doesn't seem any damper than anywhere else so not sure the cause of this; the skim plaster at this level all along the wall is loose and crumbly though (as I mentioned in my previous post).

There's a likely source of water ingress in the top left corner above the window which is an outside wall we can get to and may have caused some of the original problem, but the extent of the crumbling along the other wall makes me think it was just too old to stay up any longer.

The bricks are certainly a little damp but not catastrophically so - the fact that the render etc that is below ground level is still holding fast would imply it's not a really bad case of penetrating damp but just generally a case of old render...
Anyhoo, my original question still stands - what's the best stuff to use to prepare, re-render and replaster? Lime or not? What sort of waterproofing additive (I'm guessing SBR) and what mix etc?
Thanks for the input to date guys!


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I would paint on a cement slurry with sbr and waterproofer.
scratchcoat 3 sand 1 cement with waterproofer.
finishing coat of 4 sand 1 cement with waterproofer/plasticiser..then skim.
malc_p, lookat this in a logical way.
you dont want any damp or condensation build-up in your bedroom. damp walls attract condensation.
any penetrating or potentially penetrating damp areas are bad news.

hire a corded rotary hammer an knock off all the remaining render or plaster down to the floor.

dont tank the walls unless you install gutters/sump and sump pump - otherwise you simply shift the damp to somewhere else.

prep the walls as i mentioned, and then two coats of lime renders at 10mm each - skim with limelite skim.
In D&T work far from needing dry conditions lime render is ideal for remedial rising or penetrating damp conditions - by definition the masonry will be damp.
lime render allows the passage of moisture thereby taking back pressure off the render.

cement render forms a hard shell allowing back pressure to build up, and after a few years to start blowing the render.
cement render's hard surface creates conditions for condensation.

i've removed rock hard cement render dripping with condensation on the surface, and with soaking wet disintegrating brickwork behind
Fine if the penetrating damp issue is addressed otherwise the finished surface will be affected over time.
you keep repeating yourself - i've addressed the penetrating damp issue with my explanations.
the ultimate issue for a basement wall is the water table an the only way to deal with that is tanking, which i've already explained, will cause its own issues.

without tanking, both cement and lime renders "finished surface will be affected over time" - only lime render will give you more time, and far less possibility of condensation, an blowing off the render.
the tanking i'm talking about, as explained above, is tanking,gutters, sump an sump pump.

"get it tanked" you say. At what height would you stop tanking?

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