Hardwall and waterproofing

[robinbetts]

Hi,

I'm renovating a 1900 house with stone walls. There was a problem with penetrating damp and on the affected walls I stripped away most of the old render and plaster, except in a couple of places at the bottom where the render was very hard s&c which stopped the damp.

I have had a plasterer in to render the walls, and I was expecting him to put a think render of waterproof sand and cement, but instead he has used PVA and Hardwall plaster (not particularly thick) and is suggesting to dot and dab plasterboard on to make up the thinkness, before skimming.

My question is, will this Hardwall be a good enough barrier? I'm not sure if he has mixed any waterproofer in with it, and I'm not at all confident that the PVA he put on the stone beforehand will create any kind of barrier given the quality of the mortar etc between the stone.

Any advice would be much appreciated.

Cheers,

Robin

 

[JerryM]

i am amazed that the original sand and cement "stopped" the damp.

once i've completed the damp proofing (either injection or tanking) then i have to apply a water proof render.

this i choose on cost/ability/tools to be plastering sand & cement with approved waterproofing agent. The alternative would be to use what's called renderlite. this is a lightweight plaster which contains waterproofing additives. the approval refers to the British Board of Agreement www.bbacerts.co.uk. what this means is that it's a recognised product which is suitable for it's intended purpose.

i've not come across hardwall plaster so i can't comment. it does not sound the right thing though. the pva is purely for adhesion and will give no protection against damp.

the introduction of an air gap between wall and plasterboard is good in principle. however the dabs of plaster will enable this air gap to be bridged and i would expect areas near to the dabs to be damp upto 1m if rising damp is present in the wall or all the way up potentially if penetrating damp is present.

i would ideally recommend you get a couple of quotes from damp proofing companies to at least get an idea of what the situation is. or hire/buy a damp meter to take reading yourself to assess how bad or good the wall is.

given where you are if i was doing the job i would tank the walls. if the damp is not that bad then a paint on membrane might just do it but the trick with them is to get the plaster/render to adhere. i have only experience of treating small areas ie not full walls. the link gives a typical product http://www.laydex.ie/laydexdocs/Aquaprufe.pdf

 

[Lounge About]

Hi, i'm also in the process of renovating a sand stone lodge and i'm going to tank it, theres a good video on sovereign chemicals website, makes it look easy but time will tell. If you ring them they give you free advice and full specification for your individual needs

 

[JerryM]

Lounge About, good to know you've seen the video.

i feel its spot on too. the tanking is as easy as it looks. it does go off quickly so stick to the 20min batch size, the other trick is to get well prepared ie plenty of containers, scales and all laid out to make doing the job easy. it also helps if you can get someone else to do the wetting of the walls.

 

[Lounge About]

There's going to be three of us when we do it, hope it means a very quick job (45-50m2), should get that done in a day, what do ya think

 

[JerryM]

I feel you will need the 3 people. the application rate is 2kg/m^2. i mix 4kg batches which i can put on in 20mins so i reckon would take me over 8 hrs none stop which i could not physically do.

with 1 person mixing & wetting the walls and 2 applying then 1 day seems good enough.

make sure you do the prep well. on brickwork it is crucial. i brush the walls vigorously with water & yard brush. then i repoint any poor joints and fill out any crevices/holes (using 4:1 washed sand gauged with 4:1 water:SBR bonding agent). i aim to get the surface as "flat" as needs be (difficult to explain what i mean here but trade off between time pointing and time tanking - i aim for the flatness to be enough to ensure the K11 will cover completely quickly and easily - i 'don’t want to be brushing too much).

the special block brush is also a must.

 

[Lounge About]

Preperation starts this weekend, nocking out the loose mortar and then power washing it all, anti sulpate on Monday and then repointing for a few days, should be ready for the tanking the following Monday