Did you know that plywood is always laid up in uneven numbers of layers, such as 5, 7, 9 and 13? The reason this is so is due to the fact that each layer of plywood has the grain oriented at 90° to the previous layer. To get the grain oriented as it is with the starting face veneer – the grain running the length of a 4′ x 8′ sheet – the grain of the second veneer would be oriented from side to side.
It’s the third layer that again matches the grain of the first veneer. As the plywood grows in thickness, it has to grow by two pieces of veneer. This process also balances the plywood’s movement due to contraction and expansion to keep the panels flat. Or so they say!