A plywood board measuring 400 x 400mm. Please note that these measurements are from outside edge to outside edge. The frame is 8mm wide, which means the internal dimensions of the board (the space available for your mosaic) will be slightly smaller than the listed size. The frame rises 5mm from the board surface and the depth of the entire board, including frame, is 18mm. This 5mm depth means that these boards form an ideal base for most of our glass and ceramic tiles which are usually ±3.5mm deep (but do check). These frames should be used outdoors only with caution; apply at least five coats of varnish (but don’t varnish the surface on which you’ll be sticking your tiles). You will need to add a further coat of varnish each year. You should take boards indoors in cold weather. If you want a board that is 100% waterproof, try our new tile-backer boards.