Hi, I am just in the final stages of fitting a quadrant shower tray. I had to raise it as I have a concrete floor. I built the platform level and have topped this with 18mm ply which supports the tray right up to the edges. When the tray is placed on top all is rock solid and level. So with the tray level and well supported, is it necessary to have a sand cement mix below the tray, or is it acceptable to secure the tray to the platform with some sealant. The platform is secured to the two walls. Thanks.