Plywood floor; How to seal and what with? Help urgently need

12 Jun 2010
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United Kingdom
Urgently need expert advice on what to seal a Plywood floor boarding that i'm putting in our bathroom. I've looked (at numerous posts on here) and spoken to a number of people (in the trade) but nobody wants to recommend anything to help protect the ply; in the event that water seeps unnoticed under the bathroom suite or under the skirting.

We're using an exterior grade waterproof ply, and I had thought of sealing both surfaces and the cut edges.
The ply has replaced the old floorboards that were rotten and uneven.

We hope at some stage to cover the floor with ? vinyl sheet flooring or possibly something else, but not ceramic tiles. This will be another thing I may need advice with, but thats for a not so distant debate.

So I'm looking for advice that works in practice, or from somebody elses experience..

I chose to go for Ply on advice from others after having had a leak from our washing machine which transformed our Kitchen Waterproof Chipboard flooring into wave like expanded surface, despite having sealed with yacht varnish, and then having vinyl flooring fitted.

Your advice would be greatly appreciated, its the last thing thats holding me back.
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well if you really want to?seal the edges with varnish/or exterior quality wood glue.
You can use an SBR primer sealer; or similar.

This is normal practice on the underside & edges (but not top face) when tiling over WBP floors but if you use it on the top surface it’s but it not really suitable as a wearing surface so you will need to cover the floor with something. If you do decide to tile, make sure you research it thoroughly first & read the Tiling Forum sticky & archive posts to avoid making potentially expensive mistakes.

Chipboard flooring is crap
I don't know if you've alrready brought the ply but with good grades of plywood it is not necessary to seal it. Anything marine grade should be fine. I have left sheets of untreated birch plywood outside for months with no delamination or warping, just discolouration.
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WBP & marine grade plies are manufactured with waterproof adhesive to bond the ply veneers & although they won’t fall apart when exposed to water, it’s still timber which will expand/contract with moisture content. Minor movement shouldn’t be a problem with laminate or Vinyl flooring but sealing the underside & edges is recognised practice when tiling, even seasonal changes in ambient moisture content can cause movement & risk tile failure; sealing is also a warranty condition of all the major tile adhesive manufacturers.
Cuprinol Bourne Seal is suitable for sealing wood and gives a hard wearing finish, but whatever you use, it will not protect the boards from the amount of water you would get from a washing machine leak/flood.
If you seal the top surface of the plywood nothing will stick to it. If the vinyl is laid properly and sealed around the edges, only a leak beneath the floorboards will get to the plywood. Ask your floorlayer to cove form the vinyl all round and seal the toilet and pedestal with high quality silicone.

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