Sandstone window cill repair

[Mr M]

I need to repair the damaged/missing sections of the sandstone window cills to the bay window shown. The cills have unfortunately not been maintained by the previous owner and have deteriorated to the extent that they have crumbled and fallen away in places

I was thinking of doing the following:

i) Wire-brush, clean-off and unibond the areas which need to be repaired
ii) Drill several holes into the face of the areas to be repaired and screw in some stainless steel screws leaving the heads protruding by about 15mm (so the filler material has something to grip onto), or use some small bluebird type fishtail ties instead of screws
iii) Knock-up some timber formwork to act as a cast and fix this to the brickwork below the cill. The brickwork will eventually be painted so any holes that remained would be filled
iv) Fill the damaged/missing areas with either sand and cement or grano and cement or something else? (any suggestions are welcome)
v) Remove the timber formwork so the cills can then be painted

Does anyone have any views on whether the above would be successful? or has anyone done any similar repairs a different way?

 

[Mr M]

Does anyone have any views, alternatives or advice on what I am proposing to do?

 

[mrrusty]

I would have a go with a mortar made up with a high concentration of SBR. Made like that it really does stick like the proverbial. I would try it without the screws. If it does drop out you are no worse off. Rather than unibond use a sbr slurry of just cement and sbr. If you use sbr you will need release oil on your formwork! https://www.pavingexpert.com/sbr_01

 

[Mr M]

I would have a go with a mortar made up with a high concentration of SBR. Made like that it really does stick like the proverbial. I would try it without the screws. If it does drop out you are no worse off. Rather than unibond use a sbr slurry of just cement and sbr. If you use sbr you will need release oil on your formwork! https://www.pavingexpert.com/sbr_01

Thankyou for your reply.

What aggregrate would you suggest I would be best to use in the mortar mix? Red sand, sharp sand, mersey grit, grano or something else and what ratio of aggregrate to cement should I gauge.

I will clean the sandstone off first and will then coat the areas that are to be filled with an sbr/cement slurry as you suggest.

I have read your link on paving expert so thanks for providing that. I have never used sbr before but I was considering whether to build-up the areas to be repaired in stages over several days, rather than fixing some formwork on day 1 and casting it all in one go. I would basically clean and then sbr/cement slurry the sandstone to start with and then apply some mortar with sbr in the mix to a maximum thickness of around 15mm. Leave it to go off for a day or two then repeat the process by coating the new mortar surface with sbr/cement slurry and building the repair up with another layer or layers of mortar.

Once I got to within around 10 to 15mm of the finished surface, I could then either fit the oiled formwork to the bottom and to the face of the sandstone either side of the repair and cast the last part of the repair or (and I don't know how feasible it would be) cast the last part of the repair by hand and then rub it up to as smooth as I can get it as its going off? Thinking about it, I would probably fix a length of timber under the cill which finished flush with the face of the sandstone on day 1and then at least the mortar which is applied has something to rest on so hopefully it wont slump too much and doing that would give a nice straight finish to the underside of the repair. That said, I would welcome any further thoughts, particularly on bulding-up the repair in layers as opposed to casting it all in one go

 

[mrrusty]

I think I would have a go at making it up with 1:3 cement to sand (I don't think it makes much difference what sand) and use 75:25 SBR to water - i.e. mostly sbr. I wouldn't bother building it up. Make your form, oil it otherwise the sbr mortar will stick too much, and put it all in one go.

I've been using SBR slurry and mortar recently to consolidate a flaky, crumbly outside toilet wall, and I can tell you as a mix it really sticks like anything.

 

[Mr M]

Ok, thanks for taking the time to reply. I'll make-up the timber formwork and give it a go doing it all at once. What's the worst that can happen

As I said, I haven't used sbr before and in addition to the large repair shown in the photographs in my original post, I also have some other areas to patch up which are only quite small. With sbr in the mix, will I still be able to leave these slightly proud of the surrounding sandstone until the repair is starting to go off and then rub it up smooth or will having sbr in the mix not allow this because its too 'sticky'?

Also, is there a particular type of oil that I should use on the timber formwork?

 

[mrrusty]

you won't sand it back once it's set - it sets rock hard. You can get special release oil, though I believe diesel is sometimes used. I reckon you'd get away with olive oil too - anything to stop the mix sticking to the wood.

 

[Mr M]

you won't sand it back once it's set - it sets rock hard. You can get special release oil, though I believe diesel is sometimes used. I reckon you'd get away with olive oil too - anything to stop the mix sticking to the wood.

Sorry, I wasn't suggesting trying to sand it back once it had actually set. I meant rubbing down the smaller repairs with say a plastic float before it's set ie when it's just starting to go off. I was trying to establish if that is possible with sbr in the mix?

 

[mrrusty]

Yes, it'll trowel down when wet no problem.

 

[Mr M]

Yes, it'll trowel down when wet no problem.

Thanks for all your replies mrrusty, you've been very helpful. I'll give the repairs a go.