Had an flat roof single storey extension built just under two years ago – there are some 18mm plywood fascias just below roof height, which we dutifully primed and painted.

Had a pre-winter inspection today and found the ply had been merrily disintegrating underneath the paintwork. In a couple of places my trusty screwdriver now goes the full distance through it with ease.

Looks like the whole panel will need ripping out and replacing. Given that it’s so new, obviously I’m not a happy bunny.

Anyone have any experience of this kind of nonsense? Can 18mm ply ever be expected to stand up to this, or should we have treated it differently?

Marine ply should have lasted longer than that.

Yep, marine / exterior ply should have lasted longer with no protection at all on it. You’ve got an interior ply there, and not a good one I’m afraid.

As above, but my local timber yard said there is a lotof cheap imported ply out there , that delaminates when wet, and the builders are not very happy as if they install it, in good faith the customer expects them to fix it.

Proper marine ply should be stamped WBP.in the corner of a sheet

I’ve seen WBP veneers separating due to exposure prior to covering up, but also due to the rip of the saw.

I wouldn’t have specified plywood for a fascia(Marine of WBP) due to cut edge / veneers being exposed. If you’re going to protect it by overcladding then I wouldn’t have a concern.

Cheers all. I would agree that the cut edge is the weak spot, as this is exposed underneath in this particular design. I’ll be raising it with the builder.

What materials would people recommend for a fascia board in these circumstances, and if ply is to be used, can I treat it with anything to reduce the chances of a repeat?

Use timber Par,and treat with clear preservative before instalation,

What really makes fascias last better is to get a couple of coats of paint on before they go up; touch up any fixing holes afterwards.
Painted softwood with or without preservative really doesn’t last well in this climate, though pre-painting with a microporous paint will improve life expectantcy.

If wood , then a durable timber, generally softwood but could be a hardwood, PAR, treat or apply a external paint system (generally painted hence softwood)

If ply, then cover with UPVC or aluminium fascia system.

Now you’re underside why UPVC is so popular. Hate the stuff myself and my extension has PCC aluminium fascia and soffits.

wbp ply refers to the glue – not the quality of the plywood.

proper marine plywood is a lot more expensive (and better ) than external quality wbp plywood.