Re- rendering an undressed stone wall

[Frazer G]

Some forum users may have seen my questions before but unfortunately I have had various suggestions which I am trying to distil into a positive action plan. At times recently I have felt like someone in a strange city who askes for directions from a local trader, only to be told "well I wouldn't start from here!". So, my current "here" is the wall of the old house I am renovating which is random undressed stone built in about 1890.  I am renovating it principally to reduce damp issues. It has been stripped of all old plaster, patched where major cavities were found using a 3:1  cement mortar, and "painted" with a waterproofing slurry (Sika) as per their guidance, approximately 2 - 3 mm thick. So far so good. Now at this point all the "guidance is to fit a mesh membrane and plaster on top of that. There are endless "how to do it" guides but none cover walls as described below. It is virtually impossible to use mesh on such uneven walls due the difficulty of fixing it to random stone.

 

I need to prepare for the final plastering (which I will not be doing as I am not a plasterer). The wall is, as stated, random stone and now, after application of the waterproofing slurry has a surface which is sound but has bumps and hollows, in some cases 20mm deep, peak to trough. The final surface render / plaster, after completion will be approximately 15mm - 35mm thick due to the undulations between stones.

 

It seems to me that given the primary objective is to reduce / eliminate damp in the wall it could either be a) rendered with thick bonding plaster / render with a waterproofing additive prior to a final skim or b) a moisture barrier plasterboard be stuck to the wall, again to be ready for a final skim.

 

If option a) what exact type of render should I use with what additive, how many coats for these thicknesses etc?

 

If option b) how would the plasterboard be fixed. Dot & Dab can't be used with foil backed PB (as I have read) on its own. I have also read the it can be used with D&D if the foil is painted with SBR and allowed to partly dry. I have also read that a foam PB adhesive will work on foil backed PB but will that be OK with the bumps & hollows described. Finally will the PB need mechanical fixing for safety? Drilling into random stone is extremely difficult as many will know because the drill tends to slide away from stone surfaces which are angled and old lime based "mud" cements will not hold modern screws and plugs (drilling through the waterproof slurry would also defeat its purpose). 

 

Why use option b) with all the questions? IF the problems can be overcome it would be much quicker, need virtually no drying time and provide a perfectly flat surface for the plasterer to finish the job.

 

Is a combination of a) & b) feasible. A thinner coating of plaster / render to flatten the surface (all questions about option a) are still valid),  which would then be suitable to use foam adhesive without mechanical assistance??? Would this generate other questions about using foam adhesive on foil-backed PB which I have not thought about???

 

I can't be the first to face such a problem, there are thousands of similarly built houses, so how would forum experienced members address my problem without saying "well, I wouldn't start from here!.

 

 

Edited December 8 by Frazer G

[Gone West]

I've got a similar problem, but only a small area fortunately. I am going to use Lime Green Ultra insulating lime render base coat which can be laid up to around 30mm IIRC. Then when more or less level I'm going to skim with a ready mixed lime finishing plaster. I'm not a plasterer and have never done well plastering but I'll give it a go.

 

https://www.lime-green.co.uk/products/lime-plaster/ultra