Don't bother, done it and it did not work. My mother was an amputee with only one leg so we thought good idea to have under floor heating so the floor dried quickly and she was less likely to slip, and also sculptured tiles again for extra grip, best under floor heating is raychem, however the cable is not covered with an earth screen so used simple resistive, and the pocket for sensor set around a meter from edge, could not find a IP68 thermostat so thermostat set just outside the room, not ideal but no real option, it was laid twice as first time the builder used out of date cement and damaged the cable, but all laid and when switched on it reached a nice toasty 27°C the maximum allowed for under floor heating.
With the extractor fan running which was insisted by building inspector the room never got warm, lucky we also fitted a large towel rail, as without it the wet room would have been rather cool, and floor could not be set any higher it was already heating floor to max 27°C.
So tests it took around ½ hour to feel the heat, an hour to get warm so would switch on an hour before care people who would shower my mum would arrive. Some times I would leave on all night if I did not want to get up early.
However as soon as shower was used, it quickly cooled the floor, so after her shower it took another hour to reheat floor, and the sculptured floor held the water on the floor so all had to be mopped when finished, with floor on the floor dried about 15 minutes faster than with floor off, so it was rarely used.
The builders were sacked due to errors, and my son and I completed the job, thinking the builders had put in for planning permission I went to tell LABC we were taking over, seems builders had not bothered and we got in trouble for not telling the LABC before starting, they were very insistence it is the home owner who must do it, although often builders do it on behalf of their client, but it's up to owner to insure its done. Wish I had never told them, we had to complete the installation certificate and submit it to inspectors for the electrics. They never visited to check it through.
The first job done by builders was to dig out floor and lay drains then fit large slabs of polystyrene which was covered with marina plywood before the heating wires were laid so massive amount of insulation, as a result even without the floor heating on the floor felt warm.
From the plywood top to finished floor level I would think around 1" (25 mm in new money) so it could have not possibly been laid on existing floor the plywood was as said 25 mm below the hall floor outside wet room, clearly up stairs this would be a problem, I did like the wet room, but when the boiler was changed the power shower had to be removed as can't pump from mains, so once the water tanks went the shower pressure was reduced, but still good, but could not wash down ceiling as easy as when first fitted. Having a loo which bolted onto the wall mean we could wash the floor under the loo.
It was still working 12 years latter when we sold house last year, but the under floor sensor had failed and had stuck in the pocket, but even left on 24/7 the floor only hit 32°C so did not worry about it. However had to be right to start with as the LABC would not have passed it.
Before leaving I did notice one or two cracks in the grouting think it needed redoing, but under the shower area there was a special shower tray which the tiles fitted into, so the grout and tanking is only taking the over spray, and the tanking was under the heating wires not on top.
Would I do it again, no way, to me it was a failure. Specially when dad was alive always going on about electric costs to heat it.
