Special Systems: Storm Drainage, Fuel Gas Piping, and Specialty Plumbing
Storm drainage
Storm systems are sized from the horizontal roof area and the local rainfall intensity, not from fixture units. Convert roof pitch area to its horizontal projection, apply the design rainfall rate, and then select leader and conductor sizes from the code table. Gutters, leaders, and storm drains must be sized together; a proper system includes overflow scuppers or secondary drainage for roofs. Subsoil drains and area drains discharge to storm, not to the sanitary sewer, and sump pumps need discharge to an approved location with backflow protection where cross-connection is possible.
Worked example: A 20 ft × 30 ft roof is 600 ft² of horizontal projection. At a 6 in/hr design rainfall, flow ≈ 600 × 6 × 0.0104 ≈ 37 gpm. (One inch of rain per hour on one square foot equals about 0.0104 gpm.)
Fuel gas piping
Size gas pipe by load (BTU/hr) using the longest-length method: measure the longest run from the meter or regulator to the farthest appliance, add equivalent lengths for fittings, then read the table for that total length and the allowable pressure drop. Every appliance needs a shutoff valve, a sediment trap, and a listed connector where one is used (commonly limited to about 6 ft and never concealed in a wall). CSST must be bonded; gas piping is pressure tested before being placed in service, and the test must be witnessed per the AHJ.
Specialty systems
Recognize specialty categories and their special rules: medical gas (installer certification required), pool and spa piping, irrigation with backflow protection, fire sprinkler connections with backflow assemblies, and rainwater harvesting with approved treatment and air gaps.
Trap the exam sets: sizing storm drains by fixture units, running a gas connector through a wall, or omitting the sediment trap at a furnace or water heater.