Obstructions Installed Entirely Below Sprinklers (The 4-Foot Rule for Overhead Ducts, Decks, and Trays)

Look, every greenhorn on the job thinks if a sheet metal HVAC duct is hanging a couple of feet below the branch line, they can just run their pipe over top of it and call it a day. Then they take the Minnesota Journeyman Exam, get slapped with a question about a 4-foot-2-inch supply duct, and burn three easy points because they answered based on what their lazy jobsite foreman told them on a Friday afternoon. Today we’re tearing down NFPA 13 Section 8.6.5.3.1 so you don't choke on this exact trap when the state examiner grades your test.