Laying flagstones indoors

[DustyLungs]

Hello.
I'm new to the forum, but have been lurking a while and reading lots - very informative stuff, thanks.

I'm renovating an 1840s stone and brick cottage which has had every one of the modern treatments you're not meant to do with a breathable building. Slow going, but very rewarding and my builders' cough is coming along nicely.

In one downstairs room, we have a solid concrete floor which is 1-2 inches thick, poured on a plastic sheet which is laid on 1-inch of sharp sand and then rather wet earth.
The foundations of the house are only 8-inches deep, so I've decided that the best course of action is to take up the concrete gently and lay flagstones with a nice 1-inch gap and mortar them with lime. (I'm afraid that the digging and laying of a proper insulated limecrete floor is structurally worrying and cost-prohibitive right now.)

But, laying internal stones isn't something I've done before. External, yes, but not internal.

I'm going to take the concrete up and rake out some of the old sand. I'm thinking that a dry bed for the flags might be best - sand and lime (? will it carbonate under stones?). Then a few weeks to let things settle and bed properly under foot, before mortaring with an NHL5 mix. Does that sound to be the right approach?

 

[worms]

Just to say "Hi" and Welcome! I have no clue what the answer to your query is but someone else will have been there before and know. First posts often come in a bit late and well down the list and over the weekend most folks are too busy doing stuff to be on the Forum, so either way it should help if we bump is back up to the top.

As for me, I've reluctantly reached the conclusion that working in the rain with power tools might have seen me nominated for a PPUK Darwin Award, so inside for more kit-kats.

 

[MatthewC]

I'm going to take the concrete up and rake out some of the old sand. I'm thinking that a dry bed for the flags might be best - sand and lime (? will it carbonate under stones?). Then a few weeks to let things settle and bed properly under foot, before mortaring with an NHL5 mix. Does that sound to be the right approach?

Hi Dusty

I'm not an expert on this approach- I laid a limecrete floor with flags on top and it's fine. I would think in this case that plain lime mortar under your flagstones would not set well as you say it's damp there. However, NHL would set OK, and I think 3.5 would be OK. However, I'm sure an expert will be along soon, or else Penners with a Kit Kat!

Matthew

 

[Moo]

I'm not surprised it's wet under a plastic membrane topped off with an inch or so of concrete!
It might be an idea to wait for a bit, after you've removed the impervious layers, before making any decisions about the next step. You may well be surprised to find that natural evaporation solves a lot of the problem.

 

[Zebra]

I have laid a slate hearth using NHL3.5, although it was on top of existing concrete. I have also used both NHL 3.5 and putty-based mortar for pointing brickwork which is continually damp, and if I were doing it again I would go straight for the NHL. Putty-based mortar doesn't set very well if it's continually damp, and doesn't really set hard enough in any case to do a floor in it.
What are your walls like, in terms of damp?

 

[DustyLungs]

Thanks all.
The walls are damp, as you'd expect. We've been removing various generations of gypsum, bitumen, sealant and concrete from the original brick and stone, and will be re-liming them later in the year when they're drier.

Noted re the Fat Lime vs NHL and setting, thanks for that. I'll continue to read up and report back on what the final efforts are.