Bob LaCivita replies: The effect is caused by gravity. Plywood should be stored horizontal flat. Most of us do not have the space to do this. I store plywood up either standing on end or edge at as close to 90 degrees as possible. Veneer core plywood will warp to some degree because the core veneers are not the same thickness due to cutting, pressing and in some cases, sanding. The face veneers are also sanded and are not equal. the slightest difference will cause the sheet to warp.