Self levelling compound over plywood base

1 Sep 2008
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United Kingdom
I am building a bathroom in a part of the house that has never had a bathroom before. As part of this I have pulled up the original floorboards and replaced with WBP ply. I have reinforced with noggins and affixed the ply to the joists with 70mm screws and it is solid.

Due to the position of joists, I had to leave around 6"-12" of the original floorboards as you come into the room. These original floorboards are 3-4 mm thicker than the ply, so there is a lip.

Can I use self-levelling compound on top of the ply to bring it up to the same level as the old floor?

(I did wonder about glueing/screwing 3mm ply over the top but I'm worried that if the glue failed the topsheet might lift and the tiles would crack)
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There's no reason why the 3mm ply should fail if it's been done properly (screwed every 150mm) but if you're tiling the floor afterwards, there's more danger of the old floorboard sections failing. You'll need a flexible SLC, but I think that I'd go for the play, and then put 6mm ply over the whole floor afterwards.

The alternative, is to lift the WBP, and apply 4mm strips to the top of the joists.
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