On Feb 22, 2:04*pm, Mark Bluemel wrote:

What board should I use? OSB? WBP ply? Something else?

I wouldn't board and felt it, I'd use (for simplicity) something that
was inherently both strong enough and waterproof. Corrugated asbestos
cement is the traditional one, or these days that probably means
Onduline.

If you do want to board it, OSB is the stuff. However there are a
couple of grades about and most builders merchants only sell the less
water resistant of the two, so check first. OSB is quite resistant to
moisture on its surface, but it does need to have the edges well
protected.