Just finished my deck and installed the Trex Rainescape waterproof system. Planning on wiring it up and finishing it with come cedar ceiling planks. Since this setup is waterproof, do I need to use any conduit for the wiring or will the wet-rated wire through the joists be sufficient to meet code?

  • Is your deck close to the ground or higher up?
    – Machavity
    Commented Jun 8, 2022 at 13:57
  • 5
    Conduit isn't used to waterproof wiring. Your question doesn't really make sense.
    – isherwood
    Commented Jun 8, 2022 at 14:05
  • My deck is 8-9ft above ground.
    – Brian Hoag
    Commented Jun 8, 2022 at 14:24
  • 12
    You can't use Romex at all becasue Romex (NM/B) is not wet-rated. And UF is not Romex.
    – Ecnerwal
    Commented Jun 8, 2022 at 14:27
  • 3
    Consider low voltage lighting. Then you can use landscape wire and don't need to worry about conduit, UF, etc. If you want to use conduit to protect the LV wire in exposed or critical parts, you can use any kind of conduit, pipe or hose you choose, whatever seems to make sense to you. If you need an outlet, use proper conduit and do that properly, but you don't have to extend that scheme to the LV lighting.
    – jay613
    Commented Jun 8, 2022 at 15:58

3 Answers 3


If this is outside in any fashion, it's considered a wet zone, even if the area is totally dry. A deck with Trex is no different from a standard wood deck in that regard.

Generally speaking, conduit is seldom not a good idea for any exposed cables. You can then dispense with the UF cables and run THHN/THWN in the conduit (saves some money to offset the conduit). The other benefit is you can directly connect the conduit to your exterior metal boxes.

UF cable can work, but it's not as well protected. The risk is it gets damaged and you have to replace a whole run.

  • 1
    Agreed - conduit is basically always superior.
    – Ecnerwal
    Commented Jun 8, 2022 at 14:26
  • 6
    Working with UF is also a major pain.
    – FreeMan
    Commented Jun 8, 2022 at 14:33
  • 1
    Just an opinion. I dk if it would be visible or not, but given the fact you went thru the trouble to make it totally dry, you probably have space underneath you'll be occupying. (My son did this on the deck on his new build!). Anyway, IMHO just cable looks like a hack job. Hard to make it look good, esp, UF. And like @FreeMan said, it's a PITA to work with. PVC conduit is not expensive and looks professional. Commented Jun 8, 2022 at 16:07

Wire is not allowed without conduit. All exterior conduit is defined as wet. All wire in exterior conduit must be wet-rated. It usually is wet, from condensate if nothing else. A "waterproof deck" would make no difference at all to that.

Wet-rated cables are allowed without conduit, whether or not there's a "waterproof system" above them.

  • Gotcha. So just need wet-rated cables to run through the deck joists. However, for the double protection, use the conduit.
    – Brian Hoag
    Commented Jun 8, 2022 at 14:33
  • 3
    Conduit is also required for physical protection for cables below a certain height (I believe it's 7', check your local code to be sure). Since the OP's deck is 8-9' above the ground, if any cable runs down a wall or deck post, at some point it will need to enter conduit for that physical protection, and Schedule 80 conduit would probably be required.
    – FreeMan
    Commented Jun 8, 2022 at 14:35
  • 2
    @BrianHoag If using conduit, use THWN wire in conduit. It's better protected than exposed UF, and putting UF in conduit is an unbelievable and unnecessary pain in the rear (and requires larger conduit than you think it will.) If you have a UF run "exposed to damage" that might need a protective sleeve as Freeman mentions.
    – Ecnerwal
    Commented Jun 8, 2022 at 14:49

Be more concerned about rodents nibbling. They love the shelter of decking, and even if not living there, will visit for the odd meal - of plastic. Conduit will supply the necessary protection from that, so the waterproofing comes second - in my book.

  • Exactly what I was going to say, Rodents are likely more of a danger to your cable than moisture is! Commented Jun 10, 2022 at 1:26

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