14 Apr 2020
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United Kingdom
I am having a plated underfloor heating system installed between the joists of my suspended timber floor throughout the house, so, I was wondering what the best subfloor type is to lay over the joists when the UFH has been installed.

So the floor joists are at roughly 450mm centres which is pretty uncommon, I think this is due to the age of the property and the joist dimensions are 100mm by 50mm.

I was planning to use a structural, exterior grade softwood ply with a thickness of either 18mm or 21mm throughout the house. The existing floorboards downstairs have a thickness of 22mm and upstairs have a thickness of 27mm.

On top of the sub-floor, I plan to lay a 6mm hardiebacker board to create a solid surface to tile onto to give my finished floor.

I want the floors to be solid for the tile finish to be laid onto, therefore, I am more inclined to go for the 21mm thickness plywood to prevent any movement, but I am also worried that that thickness along with the rest of the floor build up may be too much for the underfloor heating to work efficiently and actually warm up the floor and the rest of the rooms.

The insulation installed below the UFH will be SuperQuilt

Am I along the right lines with my current plans, if not, what would those of you who have some experience with this type of thing do in my situation?
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I would look to beef up those joist if only 4x2 with 450mm centres. There will be a good bit of bounce I'd imagine for tiles to be laid??? Also are you tiling the full lot? The extra weight alone will imo maker the floor sag somewhat wont it?

There a few ways of doing so but I am not an expert so will wait for someone with experience to chip in.

But if i was doing it to my home doubling up with a 6x2 along side each 4x2, pocketed into the walls like the originals, and coach bolted together. That would be the way I'd go. It will raise the floor level up obviously though by 2" but I'd live with that if it meant less bounce. Is the below ceiling in place? You could maybe get away with just another 4x2 though, or even sandwich and glue the existing 4x2 with osb strips? How long are the joists? Like i said, I'm no expert.

I'd say 18mm chipboard flooring will be well up to the job! That stuff is pretty tough! Plenty of screws to hold things nice and tight! Same with your hardy backer board if you're overlaying with that. Lots and lots of screw (moisure resistant as you dont want them rusting and expanding and staining any tiles).

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