Most polystyrene panels for XPS products are slotted (L-cut). Boards are usually installed in a single layer. Double-layer application is desirable for panels with square edges (L), thus preventing the formation of thermal bridges in the joints. Thermal insulation in panels should be placed on inverted flat roofs in a single layer. XPS thermal insulation boards are resistant to cold, rain and snow, but not to long-term exposure to ultraviolet radiation, such as that caused by direct sunlight. Therefore, the panels should be handled with care and the protective packaging should be removed just before installation.