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Deleted member 174758
If we get a badly twisted joist that's exactly what we do - and not always a couple of millimetres, either. The way to check is to place a straigjt edge (e.g 4ft level) across 3 or more contiguous joists. If the diffetence is a couple of millimetres i wouldn't bother planing it out in many casesSome of the joists are at an angle, so even though they are level, the level is of the top edge (ie not sat flush). do you advise planing 1mm off this edge to try and give the board more of a flush base...
Plywood is generally a bit more waterproof than chipboard, but even it won't stand being constantly waterlogged without either delaminating and/or rotting. If your chip ozrd is P5 then things are six of one and half a dozen if the otheredit: just had a thought - would 18mm plywood be better for bathroom instead of 22mm chipboard? I will be putting a shower tray on a mortar bed.. cheers