Most plywood manufacturers recommend spacing sheets 1/8″ to prevent buckling. The tounge and groove sheets are self-spacing and slide over each other, but full flat sheets need to be spaced. I usually use a 10 penny nail to get the recommended space between sheets. The roof sheets are most suseptible to some swelling. Failure to leave a small gap can lead to a wavy appearance, but is probably not structurally important.

If you search the words – plywood expansion gap – on the web you will find numerous references to this requirement. Most roof and floor sheeting is sized slightly smaller than 4’x8′ and has the gap requirement stamped on the back.