Which wood for this window cill?

[aingram]

Hi guys
I built this outdoor eating area and I need to fit a window board/cill. The building is outside but it's open to the elements. Although there are two blockwork walls, the other two sides to the structure let rain in.
The window is made from Acoya. Is Acoya my only option for the window board? The dimensions will be 40mn thick, 150mm deep, 1400mm long.
Oak would be nice, but I can only get kiln dried in 40mm thick. I'd worry about it moving. The area is failing well protected, but it does get rain occasionally and is very damp in the winter and very hot and sunny in the summer.

Cheers Andrew

 

[Cabinetman]

Looks like somebody’s been and will be having fun, I’m sure you will be filling the gaps around the window so it’s not going to get particularly wet just an outdoor but dry environment. Oak would be fine but wasteful? Any timber really, varnished or painted Pine? Or even lipped plywood?
Ian

 

[Katomi woodsmith]

Sapli

 

[niall Y]

Keruing, Sapele, Oak, Plastic, or even Concrete.

 

[aingram]

Many thanks for your replies. I have air dried oak posts in there so oak would look great. Excuse my ignorance but I always thought that oak boards outside would end up cupping or crowning.
Can someone please explain why that won't happen here?
Many thanks Andrew

 

[Doug71]

Go with the Oak if you want it, it will make a nice job.

If you are worried about the Oak being too dry you could leave it in there for a couple of weeks or so to acclimatise before you fit it.

It's not properly outside and if the Oak is only 150mm deep I wouldn't worry too much about movement.

European rather than American if you have the choice.