I hope it lasts for at least 5 years!
A bath/shower room built or refurbished with quality materials of the correct specification should last at least 10 years, possibly 15 & even up to 20 years.
I do not think it is exterior grade....? But does mention "If used externally then all faces and edges should be treated with an appropriate preserver to prolong lifespan".........
Done some checking for you. The specification for ply (BS EN 636) has 3 categories covering the quality of adhesive used to bond the plies;
EN 636-1 for use in dry conditions
EN 636-2 for humid conditions
EN 636-3 for exterior conditions
WBP is EN 636-3, “exterior grade” & uses waterproof adhesive to bond the timber plies together. This is important because the adhesive & grout used in tiling contains water which could weaken the adhesive used to bond the plies. In this case the exterior grading is not to protect against flooding or wet conditions (although it will help); ply is not a suitable tile base in this case unless it’s tanked, say, in a wet room. The Wick’s ply is EN 636-2 for humid conditions, better than standard but not waterproof; it may be OK & you will find out soon enough. I would never risk using anything other than WBP, tile failure is far more expensive.
There is no benefit in you using preserver, you are not trying to protect the timber against rot; in fact you should never prime the tile surface unless your chosen adhesive manufacturer recommends doing so; most don’t as it can affect the tile bond.
The size of the floor is L: 2.65m x W: 1.60m, 45cm joist centres. I used 5x2 C16 timber as noggins and fixed 4 on each row. The ply is screwed down every 20cm along the joists and a couple through each noggins.
So is the joist span 2.65m or 1.6m? What size are the joists? The size of the noggins is fine; I usually screw fix boards every 15cm but 20cm should be fine.
The joint is across the middle, running width wise! I did have acrylic sealer, but did not use that. I gave a couple of coats of timber preserver on the underside and edges.
Joints are a potential weak point & best avoided in the middle of the room. If unavoidable, lay the boards so the joint is along the center line of a joist not across the joist if possible; any cross joist board joints must be properly supported with noggins.
The purpose of the acrylic primer on the underside & edges of the boards is to guard against moisture ingress which could affect stability of the ply causing it to warp & there go your tiles. As above, there is no benefit in using preserver; you are not trying to protect the timber against rot but moisture ingress.
If I was to tile can I not just lay 4/6mm WBP or maybe some type of tile backer?
Don’t use thin ply over board it will only make things worse. Again, the BS spec, adhesive manufacturer & Tile Association recommendations for over boarding is 15mm WBP; however it’s generally recognised that 12mm is sufficient in most cases but this is the minimum you should use. If you need to overboard the 18mm ply you have, a better bet would be to use a quality 6mm tile backer board (Wedi, Marmox, Aquapanel, Hardibacker etc) fixed in accordance with the manufacturers instructions.
If you tile directly onto the ply, apply a couple of overlapping lengths of reinforcing tape over the join & force tile adhesive well into the mesh before laying the tiles.
What size & type of tiles are you proposing to lay?
