Code Q&A: Protection Requirements for Wiring Under Roof Decking

Aug. 25, 2022

Test your knowledge of the NEC.

Based on the 2020 NEC.

Q. What are the Code rules relating to the protection of wiring under roof decking?

See answer below.

A. Cables, raceways, and enclosures under metal-corrugated sheet roof decking are not permitted to be located within 1½ in. of the roof decking measured from the lowest surface of the roof decking to the top of the cable, raceway, or box [Sec. 300.4(E)] (Figure).

wiring methods protection against physical damage

Notes:

A similar requirement applies to luminaires installed in or under roof decking [Sec. 410.10(F)].
Raceways or cables installed under metal roof decking may be penetrated by screws or other mechanical devices designed to “hold down” the waterproof membrane or roof insulating material.

Exception: Spacing from roof decking does not apply to rigid metal conduit and intermediate metal conduit.

These materials are provided to us by Mike Holt Enterprises in Leesburg, Fla. To view Code training materials offered by this company, visit www.mikeholt.com/code.

About the Author

Mike Holt

Mike Holt is the owner of Mike Holt Enterprises (www.MikeHolt.com), one of the largest electrical publishers in the United States. He earned a master's degree in the Business Administration Program (MBA) from the University of Miami. He earned his reputation as a National Electrical Code (NEC) expert by working his way up through the electrical trade. Formally a construction editor for two different trade publications, Mike started his career as an apprentice electrician and eventually became a master electrician, an electrical inspector, a contractor, and an educator. Mike has taught more than 1,000 classes on 30 different electrical-related subjects — ranging from alarm installations to exam preparation and voltage drop calculations. He continues to produce seminars, videos, books, and online training for the trade as well as contribute monthly Code content to EC&M magazine.