Businesses specialising in suspended ceilings supply and install secondary ceiling systems beneath a room's structural ceiling. These systems usually comprise a metal grid or framework holding ceiling tiles or panels, creating a void that can conceal pipes, wiring, ductwork and other building services. They are used in offices, shops, schools, healthcare premises and other commercial buildings, as well as in some homes. Typical services include measuring and planning layouts, fitting grid systems and tiles, replacing damaged or stained panels, and altering ceilings to accommodate lights, air-conditioning grilles, sprinklers and access panels. Products may include mineral-fibre acoustic tiles, plasterboard, metal or other specialist panels, with options designed to improve sound absorption, resist moisture or meet specified fire-performance requirements. Some firms also undertake repairs, cleaning, refurbishment and the installation of bulkheads or other ceiling features.