The hive mat (inner cover, crown board, or ceiling) is an essential part of the Langstroth hive system. Without the hive mat, the lid would be propolised down and impossible to get off. It also facilitates improved ventilation in hot weather. The hive mat mounted on the top of the hive is placed on top of four match sticks (or something of similar diameter) leaving a 2 mm gap above the hive and under the lid.
The hive mat is also the primary piece used to manufacture a bee escape board. The bee escape part (various styles and sizes available for sale from Ceracell) is mounted in the hive mat with the deep side down. These can be purchased from Ceracell for the cost of the bee escape part(s) and an assembly charge if you prefer us to do the job.
The hive mat board is also often made with a slot cut out of one end or one side, on one side of the board. This allows the board to be used as a "splitter" board. When splitting a strong hive into two hives, often a splitter board is used, with the new split being mounted on top of the old colony with the slot forming the new hive's entrance. This is oriented on the opposite side of the old colony's entrance. Later at the beekeeper's convenience, the new hive can be moved and mounted on a conventional floor.
The 22 mm wide rim hive mat is the standard size surrounding a piece of weathertex, 9.5 mm thick. You can place this with either side facing down to the bees, although most people prefer placing the white painted side down.