Fixtures, Water Heaters, and Special Wastes
This chapter covers everything you set, hang, or hook up at the end of the rough-in. Fixture clearance dimensions are pure memorization and pure exam points. Water heaters carry the highest life-safety stakes in residential plumbing, so the temperature and pressure relief valve, its discharge piping, the drain pan, seismic strapping, and combustion air are tested heavily. Indirect waste, air gaps, and grease interceptors round it out because they are the rules that keep food service and appliance waste from contaminating a potable system or a public sewer.
Fixture Clearances and Minimum Fixture Counts
A water closet needs at least 15 in from its centerline to any side wall, partition, or vanity, so a compartment must be at least 30 in wide for a centered closet. Two water closets side by side need at least 30 in center to center. In front of a water closet or bidet you need at least 21 in of clear space in the IPC and 24 in in the UPC, measured to the nearest obstruction. A urinal follows the same 15 in from centerline rule with at least 21 in of clearance in front. A shower must have a minimum 1024 square inches of interior floor area and be able to contain a 30 in circle, and its door or opening must give at least a 22 in clear passage. Minimum fixture counts come from an occupancy table: the number of water closets, lavatories, drinking fountains, and service sinks is based on occupancy classification and occupant load, with separate facilities for each sex above a threshold occupant count. Accessible fixtures add their own dimensional requirements, including grab bar locations and a 60 in turning space, which come from the accessibility standard rather than the plumbing code itself.
Flow Rates and Fixture Setting
Federal and code limits cap fixture consumption. A water closet may not exceed 1.6 gallons per flush, with high efficiency models at 1.28 gpf and dual flush systems averaging lower. Urinals are capped at 1.0 gpf with high efficiency models at 0.5 gpf or less, and waterless urinals are permitted where approved. A showerhead is limited to 2.5 gpm measured at 80 psi. A residential lavatory faucet is limited to 2.2 gpm and a public lavatory faucet to 0.5 gpm, while a metering faucet delivers no more than 0.25 gallons per cycle. These numbers make easy savings arithmetic: replacing one hundred 3.5 gpf flushes with 1.28 gpf saves 222 gallons. Setting a fixture is equally testable. A water closet flange must be secured to the structural floor and set at finished floor height, sealed with a wax ring or approved gasket, and anchored with closet bolts, never caulked or grouted in place of a proper flange. Standard rough-in from the finished wall to the closet flange centerline is 12 in, with 10 in and 14 in variants. Fixtures must be set level, with connections accessible, and slip joint connections must remain accessible for service. Every fixture must be supplied with an individual stop and connected so it can be removed without cutting into the drainage system.
Water Heater Relief Valves and Discharge Piping
Every storage water heater must have a temperature and pressure relief valve, and its rating is not negotiable: it opens at 210 degrees F or 150 psi, whichever occurs first, and its Btu relief capacity must equal or exceed the input rating of the heater. The sensing element must extend into the top 6 in of the tank where the hottest water sits. No valve of any kind may be installed between the tank and the relief valve or in the discharge line, because a shut valve turns a water heater into a pressure vessel with no way to vent. The discharge pipe must be the full size of the valve outlet with no reduction and no bushing, must run by gravity with no traps, must be of a material rated for at least 210 degrees F, must not have threads on its terminal end, must not be directly connected to the drainage system, and must terminate between 6 in and 24 in above the floor, ground, or an approved indirect waste receptor where the discharge is visible. The visibility requirement is deliberate: a dripping T and P is the only warning an occupant gets that something is wrong. Where a leaking heater would damage the building, a pan is required, drained by a minimum 3/4 in line to an approved location.
Water Heater Installation, Combustion Air, and Venting
In seismic zones a water heater must be strapped at two points, in the upper third and the lower third of the tank, with the lower strap at least 4 in above the controls so the strap cannot bear on the gas valve. In a garage the ignition source of a fuel-burning heater must be at least 18 in above the floor unless the appliance is listed as flammable vapor ignition resistant, and the heater must be protected from vehicle impact by a barrier. Fuel-burning heaters need combustion air. Using the standard indoor-air method, a space is considered to have adequate volume when it provides about 50 cubic feet per 1000 Btu/h of total appliance input, so a 40,000 Btu/h heater needs about 2000 cubic feet of space. Where the space is too small, two permanent openings are required, one within 12 in of the ceiling and one within 12 in of the floor. Drawing outside air through vertical ducts sizes each opening at 1 square inch per 4000 Btu/h, horizontal ducts at 1 square inch per 2000 Btu/h, and direct openings through an exterior wall at 1 square inch per 3000 Btu/h under the two-opening method. Vent connectors must rise at least 1/4 in per ft toward the vent, be supported, and maintain their listed clearance to combustible material, with single wall connectors requiring 6 in clearance and Type B double wall typically 1 in.
Indirect Waste, Air Gaps, and Grease Interceptors
Equipment that holds food, drink, sterile goods, or potable water may never be directly connected to the drainage system. Instead it discharges through an indirect waste pipe that terminates above the flood level rim of a receptor with an air gap of at least twice the diameter of the indirect waste pipe and never less than 1 in. A 1 in indirect waste line therefore needs a 2 in air gap. An air break, where the pipe terminates below the rim but above the trap seal, is permitted for some applications such as commercial dishwashers where the code allows it, but an air gap is always the stronger protection. A clothes washer standpipe must be at least 18 in and not more than 42 in above the trap weir, and the trap must be within the code's height limit above the floor. Receptors must be accessible and located so they are not in a closet or other inaccessible space. Food service establishments that produce fats, oils, and grease need an interceptor sized to the fixture flow rate and the required retention time, and the classic maintenance benchmark is the 25 percent rule: pump the interceptor when the combined grease and solids reach one quarter of the total liquid depth. A flow control fitting keeps the flow into the interceptor at or below its rated capacity, and a garbage grinder discharging into a grease interceptor is prohibited in many jurisdictions.
Last updated: July 2026