NFPA 13 is the standard for the installation of fire sprinkler systems, adopted by the IBC and most building codes. Fire sprinkler systems consist of a water supply, control valves, sprinkler piping, and automatic sprinkler heads. The system is designed to suppress fires by discharging water in a pattern covering the area below the sprinkler. Water supply is provided by municipal water mains or by private reservoirs. The supply must be capable of delivering the design flow rate for the system for at least 30 minutes. Control valves regulate the flow of water into the sprinkler system and permit isolation during maintenance. Check valves prevent back-flow of water into the supply. Pressure switches activate alarm systems when the system is activated. Sprinkler piping is sized based on the hazard classification and design flow rate. Wet-pipe systems (most common) have water-filled pipes and heads that discharge immediately when activated by heat. Dry-pipe systems used in unheated spaces fill with air that escapes when a sprinkler head is activated, allowing water to flow. Deluge systems open all heads simultaneously when the system is triggered. Pre-action systems require two separate activations before water flows. Sprinkler head design varies by hazard, with standard pendent heads for ceiling installation and upright heads for open areas. Design density for ordinary hazard is 0.15 gallons per minute per square foot over a 1,500 square foot area. High-hazard areas require design densities of 0.30-0.40 gallons per minute per square foot. System testing is required after installation and annually thereafter.