Raceways and cables are being installed in the space directly beneath a metal-corrugated sheet roof deck. What minimum clearance must be maintained, measured from the lowest surface of the roof decking?
- A1 inch
One inch is less than the required clearance and leaves the raceway inside the range that a roofing screw reaches.
- 1-1/2 inches
- C1-1/4 inches
Three inches exceeds the requirement. Candidates choose it by confusing this rule with the clearance rules for recessed luminaires and insulation.
- D3 inches
Six inches is not a figure used anywhere in this section and would make routing in a shallow ceiling space nearly impossible.
Why B is correct
Roofing fasteners are the hazard. Screws driven through the deck penetrate a predictable distance below it, and the 1-1/2 inch clearance keeps electrical work out of that zone. Because the measurement starts at the lowest surface of the decking, hanging a raceway tight to the underside of a flute does not comply even though it looks clear from below.
What this question is testing
The item tests a specific numeric clearance and whether the candidate knows the reference surface from which it is measured, a detail that changes the answer on a corrugated deck.
On the job
Any building with an exposed steel deck, which means most retail, warehouse, and light industrial construction, has this condition. It bites when a re-roof crew arrives years later and drives new fasteners through the deck into conduit that was strapped tight to the underside. The result is a dead circuit and a search that starts in the panel and ends on the roof. Keeping the run 1-1/2 inches down, or routing along the joists instead, avoids all of it.
Memory technique
Stay an inch and a half below the deck so the roofer's screw finds nothing.
Exam tip
Read what the measurement is taken from. On corrugated deck, that is the lowest surface, not the flute.
Where to look it up
Look in the protection against physical damage section of the general wiring requirements article, in the subsection on cables and raceways installed under roof decking.