Thanks. What's your definition of "not that expensive"? I think it's my fault for only looking at plywood prices in Wickes, B&Q etc. I guess I need to phone some wood or builders' merchants.
The garage has a flat roof made of 18mm chipboard that is rotten in places. Could I use 18mm ply to replace it? I think that might last a bit longer than chipboard. I could order it all at the same time.