This article explains how to fix structural plywood onto joists when replacing existing floorboards.
Please read my other articles entitled Tiling on wooden floors (Part 1 – Preparation) for why you may wish to replace your existing floorboards (or chipboard) with plywood.
I would recommend a minimum of 22mm thick external WBP or marine ply.If your joists are widely spaced with few noggins (over 400mm apart) then I would recommend you
use 25mm ply (if you can source it) as this will help to limit deflection.
- Clean up the top of the joists – ensure there are no old screw heads or bend over nails that will stop the plywood from lying perfectly flat on the top surface of the joists.
- Ensure the joists are level with a spirit level & pack any joists with slate that have dropped.
- Lay a few large sheets, rather than lots of small sheets, for maximum rigidity.
- Join plywood sheets along joists i.e. leave no edges unsupported.
- Add noggins where plywood edges would otherwise be unsupported to reduce flexing (see below).
Top Tip – mark the position of all cables and pipes within 50mm of the top of the joist on the plywood face when laying it down prior to screwing. This way you’ll know where to avoid screwing it down (see picture above)
- Ensure all screw heads are level with the surface of the ply to ensure tiles / tile backer boards do not catch on exposed screw heads – use the torque setting on your drill driver to accomplish this.
(This also helps to prevent you screwing the screw through the plywood if you have a powerful drill, and also helps to preserve the life of your drill motor.
- Stagger the joints for extra strength.
- Prime the underside & edges of the ply to reduce the effect of humidity in the air causing the ply to swell. This is outlined in British Standard requirements BS 5385: Part 1: 188.8.131.52.
I use this latex based primer but I’m sure there are many more types available.
Top Tip – don’t get this stuff on your clothes – it never comes off.
- Don’t prime the surface of the ply to be tiled as the adhesive will bond to the ply better than the sealer – make sure the plywood is clean if tiling onto it. Wetting the plywood before fixing the tiles will also aid adhesion.
- Some people recommend leaving a slight (2-3mm) gap between adjacent boards to allow for swelling of the ply with changes in humidity but I do this more to prevent lipping – where one board is higher that the other creating an uneven surface to tile onto. This could cause your tiles to crack so make sure board edges are perfectly level.
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