I've recently removed solid wood flooring from a conservatory, with the intent of replacing it with engineered wood. As the engineered wood is quite thin (13mm), I intend to first lay a sub floor of plywood, then float the engineered wood on top. The old solid wood flooring has been nailed battens with installation between them, and has some sort of plastic covering its back, see picture below.
1 - Yes, 18mm structural ply is the stuff to use for sure 2 - It is always a good idea to do this. Although you are above dpc level, there is always the possibility of spilt drinks, radiator leaks etc…. and treating it will help prevent damage from any of the affair mentioned. This Cuprinol 5 Star Wood Treatment is generally the best to use: http://www.diydoctor.org.uk/DIY-superst ... tment.html 3 - No, you should be fine as you are above doc level
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