Hardy backer board straight on top of joists



I’m doing a second wetroom but this time it’s from new in a extension, I’m trying to save some height and messing around and wondering if instead of floor boards then 6mm hardie backer on top, I can just lay 12mm Hardie backer then 12mm underfloor heating boards which will be the depth of my shower tray.
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You want to put HB board on top of joists, then a tray and omit the subfloor?
You're putting UFH under the tray?
Hardiebacker is not structural, is it?

In Australia, they use a similar board, IIRC 25mm thick, for floors especially in kitchens and bathrooms because it is impervious to water and insects. I believe it is available in UK but not widely used.
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They use it here, too, John - for plenum flooring in shops such as John Lewis. It is 40 to 50mm thick in that environment
You want to put HB board on top of joists, then a tray and omit the subfloor?
You're putting UFH under the tray?

god damn this forum winds me up.
I’m asking if I can. That’s the point of this forum isn’t it?
of course I’m not putting underfloor heating under the tray what would be the point of that. anyway for everyone else who answered cheers. I guess its floorboards then 6mm backer like the one I did upstairs.
I actually think you had your answer. AFAIK cement board isn't normally classed as structural so it isn't designed (or at least rated) to carry the loads associated with flooring - hence the common approach of installing a subfloor and then overboarding with a thin cement board. The obvious thing in the absence of a consensus on this would surely be to contact the manufacturers such as Hardy and Marley (who also make cement board) and ask their technical staff. As manufacturing concerns they will probably still have staff on site or at least available via email

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