Can EMT be installed on a roof?

Yes. NEC 358.10 permits EMT exposed and in wet locations when raintight listed fittings are used and corrosion protection is provided. Conductors must be rated for wet locations because 300.9 makes the interior of the raceway a wet location.

What is the rooftop temperature adder in the NEC?

NEC 2023 310.15(B)(2) requires 33 degrees C, or 60 degrees F, to be added to the outdoor ambient temperature for raceways and cables in direct sunlight that are less than 7/8 inch above the roof surface. XHHW-2 conductors are exempt.

How do you avoid the rooftop sun adder?

Support the raceway so its underside is 7/8 inch or more above the roof surface, or use Type XHHW-2 conductors, which the exception to 310.15(B)(2) excludes from the adder.

Are you allowed to run EMT conduit on a rooftop?

Yes. EMT is permitted exposed outdoors, on a roof included. NEC 2023 Section 358.10(A) permits electrical metallic tubing for both exposed and concealed work, and 358.10(B) permits it in wet locations where the fittings are listed raintight and any dissimilar metal contact is protected against galvanic corrosion. Two things then decide whether the install passes: how the conductors are rated for the heat, and how the raceway is supported off the roof deck.

The rooftop temperature adder

This is the rule that fails rooftop jobs. Section 310.15(B)(2) says that where raceways or cables are exposed to direct sunlight on or above rooftops and are less than 7/8 inch above the roof surface, a temperature adder of 33 degrees C, or 60 degrees F, shall be added to the outdoor ambient temperature when determining the correction factor. In a 40 degree C design ambient that pushes you to 73 degrees C, and the 0.58 correction factor from Table 310.15(B)(1) takes a 12 AWG THWN-2 conductor from 30 amps down to 17.4 amps. A 20 amp circuit no longer works on 12 AWG.

There are two clean ways out. The first is the exception to 310.15(B)(2), which states that Type XHHW-2 conductors are not subject to the ambient temperature adder. The second is to get the raceway up: mount it on rooftop supports so the bottom of the conduit is 7/8 inch or more above the roof surface and the adder does not apply at all. Rooftop pipe blocks and strut stands do this and also keep the raceway from being walked on and from damaging the membrane.

Wet location conductors

Section 300.9 says that where raceways are installed in wet locations above grade, the interior of the raceway is a wet location. Rooftop EMT is a wet location inside, so the conductors must be listed for wet locations. THHN by itself is not. THWN-2 and XHHW-2 are, and virtually all building wire sold today is dual rated THHN/THWN-2, so check the print on the jacket rather than assuming.

Support and fittings

Section 358.30(A) requires EMT to be securely fastened within 3 feet of each outlet box, junction box, cabinet or fitting, and supported at intervals not exceeding 10 feet. Compression fittings listed as raintight are required outdoors under 358.42; setscrew fittings are not raintight and are a common inspection failure on roofs. Expansion fittings per 300.7(B) matter on long rooftop runs, since a 100 foot run of steel EMT moves roughly half an inch across a 100 degree F seasonal swing.

Access and clearances

Keep the raceway out of the path required around rooftop equipment and off the working space in front of any rooftop disconnect under 110.26. Many jurisdictions also apply local roof access and solar pathway rules that limit what may be run across a roof plane, so a call to the AHJ before layout is worth the time.

Field takeaway

EMT on a roof is fine. Use raintight compression fittings, wet location rated conductors, and either raise the pipe 7/8 inch off the roof or use XHHW-2 so the 33 degree C sun adder does not gut your ampacity.