Flagstone floor: UFH, cracking and how to bed.

[Roger H]

Hi all.

Barn conversion. At the stage of laying a stone flag floor on insulated concrete subfloor with UFH under 65mm screed. Looking at new 30mm sandstone flags, 600x600mm or similar. (I know, limecrete was an option for the floor).

Was planning to lay them on a lime mortar but was blinded by science at tile stockists. He suggests planing surface of screed to dead-flush, turning on UFH for a few weeks to open up any cracks; filling them with a filler compound, sealing the screed, laying a 'flexible adhesive'; then stone; then seal. This sounds quite process-heavy.

Anyone any experience of this and any pointers? Would prefer a simpler route if possible. (Have also posted this on Green Building Forum but its a bit quiet there today)

Thanks for any pointers,
Roger.

 

[chuckey]

What is that man on? Go ahead and use ordinary lime mortar. If you use a flexible filler under the flags, when will it cease moving?, at least with mortar if it cracks, just open the cracks up a bit and whack in some more. I used patio sealer on my flags, made 'em a bit darker and ONE came out a bit glossy. Too early to say how it wears.
Frank

 

[Nemesis]

Remember non-hydraulic lime may not cure without exposure to air.

You could use sand.

 

[Jon Maine]

We have laid slabs on a bed of tile adhesive over underfloor heating within a screed , you have to have a solid bedding to help transfer the heat as any pockets will act like double glazing units. I am not suggesting you have to use tile adhesive hydraulic lime would be good.

As long as you do not have immense areas cracking should not be a problem. Did you install an expansion gap around the edge of the room? We were supplied with a roll of firm foam rubber about 1/2" thick for the job and put it around the perimeter of the room.

Whatever happens the heat should not be put through the screed until say 30 days after laying, should not crack though.

 

[Roger H]

Thanks all.

As I suspected. I will go for bedding it in lime mortar. Oversite and ufh/screed all vertically insulated from perimeter walls by continuous insulation off-cuts. I thought the ribbon of insulation the plumber came with far too thin. Nemesis, I suppose if it doesn't fully cure, the worst that it is is a passive bedding material, like sand.

Best wishes, Roger.

 

[Nemesis]

Well ours are mostly on earth, but where we had to lay new (on earth) we used sand and hydrate and wet it a bit. Seems fine.

However - I would recommend lime putty and aggregate pointing, done with care, and masking tape. Rub back when green with a stainless scouring pad.

 

[Lime]

I'd avoid modern materials, lime works well for flooring.

Another tip.
Invest in a laser level.
I used one to relay a victorian tiled floor recently and I don't know what I would have done without it.
It enabled me to keep a perfect level no matter where I was. (as long as in line of sight)
Much superior to just a spirit level.