I am building a built-in desk for my home office. I plan to use 3/4" plywood over a 2x4 frame and attach the frame to the studs. I understand that the table top needs to be allowed to expand/contract so I'm planning to attach the plywood to the 2x4 frame using cleats or similar hardware for this purpose. The desk will be from wall to wall (~8'). Do I need to leave a gap between the plywood top and the walls to allow for expansion and contraction? If so, how much would you recommend?