Waterproof Back Board.

[woodchopper]

The tiled archway above the front door was faced originally with some asbestos type fibre board, but the face of it after a few years started to boste and the paint over it would peel/flake off. So - I got a joiner to replace it. He used 12mm plywood, and for the design on the face of it was of a triangle and diamonds used soft wood which wasn't treated - it was soon rotting and is now due for replacement again.
Is there any new stuff on the market these days for a backing board that is waterproof and that can be painted ?? For the design on the front I will just paint it on.
The roof timbers are directly behind the plywood at present so it must be waterproof.
I've been told to use Marine grade plywood and paint it - would that suffice?
Or use Tile backing board.

[The Builder]

"Masterboard" is used mainly for soffits and ceilings of porches and carports etc. They are water-resistant though: not completely waterproof. It'll depend on your exposure to driving rain mostly.

Tile backer boards would probably do. There are waterproof ones, but They are designed to be tiled over, so I can't find any info as to whether they will take paint.

Marine ply would certainly do it, but preferably painted with something "breathable" to avoid the paint being blown off.

My first choice would be something inert and untroubled by driving rain, AND available in a finished colour - no painting needed.
You're not going to like this... but uPVC panels. Easily cut and fixed, with various finishing trims available.
My own dislike for uPVC, and probably most people's is due to the ubiquitous "brilliant white" which makes everything look like an ice-cream kiosk.
Today there are plenty of colour choices. They cosy more, but once they're fitted, you can forget about them.

With luck, Alavahalf will see this and reply. He'd be more up on the decorating and finishing side of things than I am.

[The Builder]

oops.... ^^^^ "cost more"