I'm building a 2.4m x 3m (8 x 10) shed/office in my garden. For the subfloor, I am going to use two layers of 18mm (3/4 inch) plywood. The second layer is there because I plan to tile the floor and want to minimise flex. On a building this small, is it necessary to allow expansion gaps between the plywood sheets (I'm using 8' x 4' sheets so will use approx 2.5 sheets per layer) or can I just lay them tight? Ideally, I'd like to glue the first layer to the joists as well as screwing, and then glue (and screw) the second layer on top of the first.