damp walls in basement rooms.

[james1979]

Hi, I have a victorian property with 2 basement rooms partially underground but the one room opens up to the garden. About 10 years ago the stone and brick walls were cement rendered and gypsum plastered. There is now damp on the walls and paint pealing off. Am i right in saying if I remove all the plaster and render back to stone and brick and lime plaster the walls upto 3 coats thick this will improve? I am a capable plasterer but not tried lime plaster yet!

 

[Stevers]

I also have a Victorian property with a semi-basement, and it was tanked (patent cement slurry) then hard cement rendered and gypsum plastered. Most of our walls are OK, but only after we called the damp proof wallys back to try again with an SBR cement slurry. However, we have a chimney breast that wasn't 'fixed' under the warranty. This chimney breast was badly salted to the point where anything ferrous was aggressively corroded, and it was damp even above the builders opening. This summer I stripped off all the cement render and most of the tanking slurry from the front of the chimney breast, made good the unsound underlying brickwork and replastered in NHL render (3:1). This has completely tamed the beast, and it is remarkable how dry the chimney breast now seems. My assumption is that the NHL render allows the masonry to dry to the point where the salts (which must still be there) are no longer a problem, and of course NHL is sulphate resisting. I originally intended to use NHL render, then SBR slurry over it, but will only do so now on the junction with the gypsum plaster on the sides to keep that away from the lime finishing plaster that I will be applying shortly to the front.

There are photos and more detail here:
https://www.periodproperty.co.uk/forum/threads/basement-chimney-breast.17787/

In my experience NHL renders are easier to use than OPC renders as they 'stick' better. They also have an initial chemical set that makes them predictable to use with less time required between coats. I bought a specialist hydraulic finishing plaster from Conserv, but haven't tried that yet. All the NHLs I work with are claimed to be 100% cement free, and I am delighted (and pleasantly surprised) with how well it's all worked out so far.