Crumbing sandstone plinth - how to save/restore...

[rowleyburkin]

Grateful for any opinions on this. Photo shows bottom of the wall on my detached house (built in 1879). Its built on these sandstone plinths all the way round.
Is it possible to prevent further deterioration ?

 

[Cubist]

Hi Rowley,
Sandstone is one of the more porous stones and consequently weathers faster than most. The best approach in my view is to help them to shed rain-water as quickly as possible by ensuring that any pointing around them is kept in good condition and only lime mortars are used - get rid of any Portland based cements asap. It's also a good idea I think to get rid of any non-breathable plaster and paints used indoors as well. Once the pointing is in good order, remove any loose surface material - rain-water will get trapped behind it - and apply a few coats of lime wash coloured to match the sandstone. Don't be tempted to apply a sealant coating of any kind, that will create more problems for both the plinth and the timber frame.

May be worth considering a coat of lime render to afford them with some further protection but you should also repair all the caulking around the infill panels as soon as you can as any fissures, however small, will allow water to accumulate between the panels, plinth stones and the wooden frame.

Hope this helps,
Cubist

 

[rowleyburkin]

Thanks Cubist - I should maybe add that the house is brick with rendered panels and timber cladding (so not timber framed). Unfortunately I suspect the render is cement as its very hard about an inch thick and seems strongly bonded to the brickwork.
Ideally I'd like to strip it all off, make good the brickwork, fix woodwool boards for insulation then lime render and finally re-apply the timber framing but it would be a big undertaking....

 

[malcolm]

I see what looks like cement pointing below the plinth. That is bad for soft stone. It's well worth replacing the pointing with lime to help reduce water coming up from below. Even though most of the water on a plinth will come from above every little bit helps.

The cracked face of the limestone will trap water and it's the water that causes the deterioration when it freezes in the winter. But is that really limestone on the surface? Is there a chance someone has skimmed it with cement? If it's cement then getting rid of it would be a good idea to let the stone dry out. If it really is limestone on the surface then getting rid of the loose bits and skimming with a lime render (plaster) would help protect the sandstone and let the water shed off without being trapped. It'll be more obvious to you what is there than us looking at photos.

 

[88v8]

Thanks Cubist - I should maybe add that the house is brick with rendered panels and timber cladding (so not timber framed). Unfortunately I suspect the render is cement as its very hard about an inch thick and seems strongly bonded to the brickwork.

And what with impermeable cement render and presumably no damp-proof course, all the rising damp is having to escape through the sandstone. More so if there is cement pointing beneath, as stone walls breath primarily through their joints and if they cannot then the stone can become very wet.

So I think you should remove any cement pointing as Malcolm has said, and if possible chisel off at least the bottom foot or so of the cement render, even though you cannot sweep all the way across because of the applied timber. That will give the wall some chance of breathing.
Then you could repair the spalled stone with NHL2 lime and as mentioned above, protect and conceal with limewash.