Repairing and Lime Rendering Old Rubble Stone Walls

[AndyPaul]

Hello all, this is my second post. I am slowly slowly renovating a house dated around 1765. I have experimented with lime rendering a old rubble stone wall based on the excellent advice received in this forum, and really happy with the result. I am moving onto another room, and here after removing the modern plaster and sandblasting, the underlying metre thick wall is still random rubble stone and lime mortar, but its very crumbly. Normally I would rake out loose material, before making patch repairs, repointing where needed and then applying the lime render by hurling. My dilemma here, is after testing the raking out process, I am finding that the more I try to rake out, the more stones and mortar become loose. My thought is to really just try and remove very obviously loose material, raking out in more of an archeological dig style, patch repair holes and gaps, then render anyway. My thinking is that if the stones are a bit loose now, applying a lime render is not going to make them any looser. The only alternative, is to be more bold and rake out hoping we get to a more solid area, I am not keen on that since its a bit of a gamble. But I am not sure? Does anyone have any experience or advice. Pics below. Many thanks!

 

[Cubist]

Hello There,

My apologies in advance but I am going to sound a few words of alarm and a strong recommendation that you proceed with extreme care.

What little I have learned about the construction of such massive rubble walls tells me that they can become extremely unstable when the outer skins are disturbed and there is an ensuing propensity for a catastrophic collapse that can threaten life and limb. There is also considerable risk of severe damage to the physical integrity of parts/all of any building that may sit on it.

Also, given that your stated intent is to apply a render to these walls then I do not understand why you are going to such extreme lengths to prepare the surface. Yes, its wise to remove loose material or in some instances rake out some of the original mortar but the object of this is merely to ensure a good bond for the new render and provide a firm seating for any larger and loose stones. Bluntly put, a sweep down with a stiff bristled brush would be all that's needed for most of these surfaces and your 'forensic' approach is contributing very little in the way of benefit.

I would urge you now to simply stabilise any loose larger stones and then get a thick render coating on the walls as soon as possible.

Frankly put I think your desire to thoroughly prepare for the target job has diverted your attention from the goals that would be delivered by its doing.

Cheers and good luck,
Cubist

BTW - I live near Ludlow Castle. About twelve years ago a lengthy part of one of the its perimeter walls collapsed and the debate about who will pay for its reconstruction and when the work will be done recently appeared in a national news article. I mention this as the wall concerned was/is constructed using outer skins of larger stones forming a cavity which was filled with stone rubble. May be an idea to take a look at similar projects to see how others have tackled the job.

 

[a twig]

If you’re worried about the dusty crumbly bits you could slather some limewash on to help bind it all together before you render, but broadly I’d agree with Cubist and stop where you have got to and crack on with the lime rendering

 

[stuart45]

Reminds me of the Bungaroosh walls I worked on in Brighton. Once the plaster or render was hacked off the walls would start to fall apart. People inexperienced with them would start taking the loose bits off and end up back in the footings.

 

[Kearn]

I expect the sandblasting process has removed far more loose material than required. I can’t work out why you would sandblast?! But that’s a different matter…

Rubble walls are largely infill within a set of barriers - stronger outer walls, quoins at corners etc. as others have said, you’re going the wrong direction which could become very serious. Ours have largely earth mortar binding them, it’s very dusty and fragile when disturbed and needs to be protected.
Give it a mist / limewash if there’s still a lot of dust, and start to solidify with plaster ASAP. Hemp lime from Ty mwar can go on extra thick in single coats which is great for daubing out bigger holes AS the base coat rather than separately, it’s very durable due to the fibres and goes off a touch quicker due to pozi additions.

 

[AndyPaul]

Thank you for your responses, and words of caution, they are duly noted. I always intended to put limewash or lime render on most of the walls, my question was really what is the best way to approach it. My concern at how the walls should be treated was exactly why a asked for advice. I removed the modern cement based plaster because I wanted the restore the house, and I was advised that modern cement based plaster should not have been put on walls like this, they do not allow the walls to breathe and will trap moisture in the walls. Hence I wanted to restore with more appropriate breatheable materials. Just to abate your fears, I have had structural engineer and architects examine the property and they were satisfied with the state of the house, it was they who advised on emoving the cement plaster. I am also following work that has been done to other houses in the area.

The best and most solid of the walls I will leave as natural stone, and mostI will apply lime render or lime wash. The sandblasting did not remove more than was expected. The picture is of a section that is the worst case, and there are a few square metres of wall like this. I assume this crumbly state could have been the result of previous owners apply modern cement based plaster, not allowing moisture to escape. I was advised that once the modern plaster was removed to leave it like this for 6 months, watch for anything falling off the wall, and then render with lime mortar. Nothing fell off the walls no powder, dust or stones. My question was if I should try to repair and go back to solid wall or not, I was favoring not, but wanted to check in. I am an experienced builder, but I know enough that I can forget everything I thought I knew with properties of this construction and age.

Once again thank you for the advice, and for taking the time to respond, it is very much appreciated.

 

[stuart45]

The walls look to be in quite good condition compared to some of the Bungaroosh walls I've worked on. Once the plaster, or render was hacked off, you often came in to work the next day to find parts of the wall had fallen off.

 

[Cubist]

Thank you for your responses, and words of caution, they are duly noted. I always intended to put limewash or lime render on most of the walls, my question was really what is the best way to approach it. My concern at how the walls should be treated was exactly why a asked for advice. I removed the modern cement based plaster because I wanted the restore the house, and I was advised that modern cement based plaster should not have been put on walls like this, they do not allow the walls to breathe and will trap moisture in the walls. Hence I wanted to restore with more appropriate breatheable materials. Just to abate your fears, I have had structural engineer and architects examine the property and they were satisfied with the state of the house, it was they who advised on emoving the cement plaster. I am also following work that has been done to other houses in the area.

The best and most solid of the walls I will leave as natural stone, and mostI will apply lime render or lime wash. The sandblasting did not remove more than was expected. The picture is of a section that is the worst case, and there are a few square metres of wall like this. I assume this crumbly state could have been the result of previous owners apply modern cement based plaster, not allowing moisture to escape. I was advised that once the modern plaster was removed to leave it like this for 6 months, watch for anything falling off the wall, and then render with lime mortar. Nothing fell off the walls no powder, dust or stones. My question was if I should try to repair and go back to solid wall or not, I was favoring not, but wanted to check in. I am an experienced builder, but I know enough that I can forget everything I thought I knew with properties of this construction and age.

Once again thank you for the advice, and for taking the time to respond, it is very much appreciated.

That's a relief, and glad to hear that you're not playing Jenga with a pile of stones.

That aside; yes, gypsum based plasters are nowhere near as breathable as lime plasters/mortars but, despite what some may say, they are not impermeable. They can and will absorb/expel moisture from other materials and the atmosphere just not as rapidly. Therefore they do breathe - just much, much more slowly.

I'm not sure why you were told to leave walls exposed for six months - presumably to allow some of the stored moisture to escape and in doing so dislodge loose material? That strikes me as counter-intuitive if the plan was/is to apply a lime render as moisture trapped in the stones would aid bonding of the new material. I can see why someone may think the surface of the stones and extant mortar may need to be dry if the intent was to lime-wash but, so long as its not literally running water, then I can't see that it would matter that much. That said, I can't say I've ever tried it - anyone got some first hand experience?

 

[a twig]

Thank you for your responses, and words of caution, they are duly noted. I always intended to put limewash or lime render on most of the walls, my question was really what is the best way to approach it. My concern at how the walls should be treated was exactly why a asked for advice. I removed the modern cement based plaster because I wanted the restore the house, and I was advised that modern cement based plaster should not have been put on walls like this, they do not allow the walls to breathe and will trap moisture in the walls. Hence I wanted to restore with more appropriate breatheable materials. Just to abate your fears, I have had structural engineer and architects examine the property and they were satisfied with the state of the house, it was they who advised on emoving the cement plaster. I am also following work that has been done to other houses in the area.

The best and most solid of the walls I will leave as natural stone, and mostI will apply lime render or lime wash. The sandblasting did not remove more than was expected. The picture is of a section that is the worst case, and there are a few square metres of wall like this. I assume this crumbly state could have been the result of previous owners apply modern cement based plaster, not allowing moisture to escape. I was advised that once the modern plaster was removed to leave it like this for 6 months, watch for anything falling off the wall, and then render with lime mortar. Nothing fell off the walls no powder, dust or stones. My question was if I should try to repair and go back to solid wall or not, I was favoring not, but wanted to check in. I am an experienced builder, but I know enough that I can forget everything I thought I knew with properties of this construction and age.

Once again thank you for the advice, and for taking the time to respond, it is very much appreciated.

The suggestion from me was to put limewash on now before the plaster / render to help bind together any loose stuff.

Agree you are doing the right thing, was just offering an alternative to digging out more. If there’s no cement left and you’ve got a reasonable key for the lime plaster then crack on

 

[paulc]

That's a relief, and glad to hear that you're not playing Jenga with a pile of stones.

That aside; yes, gypsum based plasters are nowhere near as breathable as lime plasters/mortars but, despite what some may say, they are not impermeable. They can and will absorb/expel moisture from other materials and the atmosphere just not as rapidly. Therefore they do breathe - just much, much more slowly.

The other issue with modern plaster is the gypsum is water soluble. So if it remains damp for any length of time, it turns in to a mush. Also doesn't like temperatures over 50°C, so no good around a fireplace or stove.

 

[Stevers]

The other issue with modern plaster is the gypsum is water soluble.

There's a bed of gypsum under the Mosul Dam in Iraq, if some diligent engineers ever fail to keep up with grouting the holes where it's being dissolved away, the dam will fail and Baghdad will get washed into the sea.

Gypsum plasters don't tolerate salts very well, and can then start to absorb moisture from the atmosphere - things go rapidly downhill from there!

Also doesn't like temperatures over 50°C, so no good around a fireplace or stove.

Over twenty years ago when I fitted the new tiles in the new cast iron insert, the fireplace shop quite specifically told me to gypsum plaster in the back of the tiles. This I did with Thistle One Coat which is the only gypsum plaster I use. At the end of an long evening of an open wood fire the tiles are too hot to touch so defintely >50°C.

 

[AndyPaul]

Super interesting, and thanks for all the information. There is alot of contradictory information out there. Once again thank you, I appreciate you all taking the time to give advice. I feel more confident of the right approach now, and that is worth alot!

 

[stuart45]

That wall looks like it could have been built using formwork. Walls in Sussex were built like that in the 1700's. They normally had better quality brickwork at the quoins and jambs, and the rest was thrown in between the formwork. Early versions of Laing easy form and Wimpey no fines.