Suspended timber floor + ufh + limestone travertine

[electricianid]

I have just ordered wet ufh for my upstairs bathroom which I intend to finish with travertine tiles but I am unsure how to install it. Bearing in mind that I don't want to raise the floor too much I thought about screwing 15-22mm exterior grade ply to the underside of the joists every 400mm or so, filling the void with celotex or equivalent up to the top of the joists, laying the pipe onto the insulation and covering with a dry sand/lime mix to a depth of perhaps 50mm before laying the limestone tiles. The room is 4.3 X 3.3M, the joists are 7 X 3" spaced 6" apart and they run across the 3.3M length. Does this sound reasonable? Any comments/advice would be gratefully received.

 

[Pford75]

I'd consider using thermal board instead of just ply.

Are you sure that the joists will be okay with the weight of the tiles? How thick are they?

One of the reasons for ply on the tile side of the joists is to help stop flexing. Depends on the size of the tiles (smaller are more prone to this), but you may get some movement between tiles resulting in cracks.

But probably the key question is one of weight (especially if you also have a cast iron bath)

 

[electricianid]

Not only a cast iron bath but also a toilet, boiler, hot water tank and a washing machine plus obvious live loads.
I asked a charted (structural) engineers firm to calculate the above dead load plus tiles when the joists were 12" apart and he laughed and said that the existing joists would easily cope with that but if I was anxious then I could nail some timber between the joists down the middle of the room, adding that people are always timid about existing joists!
I have since put more joists in between the existing joists making them under 6" apart now, and have still nailed timber pieces between. This has all been well documented by the engineer making it an easy sue should they snap.

I think that ply or better still thermal board on top of the joists/sand/lime/pipes would be a good idea though, thanks very much.