Marmox board is an economic lightweight substrate. The boards are made of waterproof extruded polystyrene, faced with a fibreglass mesh embedded into a cement polymer on each side, which adds extra strength and rigidity. It is extremely light, tough, and can be cut with a sharp knife into any shape (though it will tend to blunt the blade on your craft knife). No power tools are needed. It is waterproof and suitable for outdoor use if the exposed foam edges are protected, either by a frame or a coat of cement-based adhesive. For extra protection, you can apply alkali resistant mesh tape to the edges before applying cement-based adhesive. In the building industry it is used as an underlay for tiling applications in wet areas such as bathrooms.