laying plywood and backer board

13 Mar 2011
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United Kingdom
I'm planning to rip up the floorboards, lay 18mm plywood and backer board in an upstairs room I'm turning into a bathroom, there is no ceiling underneath so lots of access

The room is stripped bare including plaster

A few questions
do I need a small expansion gap between plywood sheets or the backer board?
Should I lay the backer board now and all cover the entire floor, or should I fit the furniture first then the board cut around it?
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You don't need an expansion gap in between the ply, it's screwed down so does not move as one mass like a laminate floor does. Personally I wouldn't bother with the hardibacker, the ply is expensive enough at 18mm without needing the hardibacker. I know tiling shops love to say it's essential but in reality you can just seal the plasterboard with something like BAL primer/sealer http://www.theunderfloorheatingstor...r-7267-p.asp?gclid=CODqhKbD8cMCFUrpwgodehgAkg I used this on my bathroom walls which are plywood and then fitted the tiled to it with adhesive. It's applied with a normal brush. What you are doing is sealing the timber so that it cannot draw the moisture from the adhesive preventing it from setting hard. The hardibacker is an expensive way to do the same thing. As for basins/toilets etc, have your first fix pipes fitted and ready to connect to, in otherwords have your hot and cold sticking up through the floor and your WC waste ready to connect to. It doesn't matter which way round you fit the furniture, I do what is easiest so I fit the floor covering and then fit the pedestal/toilet/etc afterwards placed on top, no awkward cuts

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