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How do you mount an exterior light fixture on siding with a rainscreen?

grmp945 | Posted in General Questions on

Hi all,

I have just finished adding a 3/8″ rainscreen (Cor-a-Vent strips) to a small building I’m working on. I used zip sheathing and I have not yet added the siding. The rainscreen gap is 3/8″ and the siding is .315″ (LP smartside) for a combined depth of about .69″.

I want to add a porch light by the door. The porch light requires a 4″ round electrical box. How would you integrate an electrical box to the zip sheathing so that the box is flush with the siding?

Thanks for reading!

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  1. Expert Member


    You use a mounting block.

    You can make your own, or buy them in a variety of materials at a BB store or lumberyard. It needs a cap-flashing, although some of the pre-made ones come with integral flashing.

  2. grmp945 | | #2

    Hi Malcom,

    Maybe I'm misunderstanding something simple. How would a mounting block help with integration into the zip sheathing? I could mount the electrical box directly on top of the zip sheathing and have the front of the box fit flush with a mounting block, but the problem would still be the back side of the electrical box not being flashed to the WRB.

  3. Expert Member


    You flash the mounting block, not the electrical box. That's the case whether the box is installed proud of the sheathing in a mounting box, or as is more typically the case fastened to a stud and just proud of the exterior sheathing. The assumption is the light fixture and gasket keep moisture out of the electrical box. It's the same situation with exterior outlets where the cover-plate provides protection against moisture. That's the big advantage of using a mounting box over just relying on the siding. It gives you a smooth, flat surface for the fixture gasket.

  4. Expert Member
    BILL WICHERS | | #4

    Ye olde electrician’s trick is to caulk the top and sides of the fixture-to-mounting interface with a releasable caulk like silicone, but leave the bottom unsealed so that it can drain. Don’t trust the gaskets on anything on outdoor electrical things — they never seal very well.


    1. Expert Member
      MALCOLM TAYLOR | | #5

      Most pre-fab mounting have have a cover that stops any water getting to the box. I've never had trouble either way, but I also usually locate exterior lights under some overhang.

      1. Expert Member
        BILL WICHERS | | #6

        I’ve always seen the most problems when fixtures are placed such that water can sheet down the wall. Sometimes this is just due to how the building is, other times it’s from occasional wind-blown rain.

        Protecting the fixtures under overhangs is always a good idea when possible.


  5. Expert Member
    CARL SEVILLE | | #7

    Flash the wire to the WRB, then install a block with a hole cut in it the size of the electrical box that the siding will run into like a corner board.

  6. grmp945 | | #8

    Thanks all for your replies!

    After some searching I found a couple of good options that allow for integrating the electrical box into the WRB very well:

    Quickflash products:

    Arlington FR series flanged fixture boxes:

    I also found a flanged mounting block that easily integrates into the WRB and won't require using a sealant/caulk to flash it:

    Any opinions on which of these options you would prefer?

    1. Expert Member
      Peter Engle | | #9

      I've been specifying QuickFlash products for years. They can be very helpful in solving the scheduling issues with utilities penetrating the envelope. The trades can rough-in their utilities, or a conduit sleeve for the utilities, and the carpenters can use Quickflash to seal the WRB/Air barrier to the sleeve. Then the sleeve is available to the trades to run their stuff through the wall at the end and simply caulk the interior of the sleeve. I've done some work with production builders who have been having good luck with this approach. Nice products.

  7. grmp945 | | #10

    One advantage I see with the Arlington flanged fixture panels is that it combines the flashing panel and electrical box into the same unit, whereas the Quickflash panels require purchasing specific compatible electrical boxes - and from looking at the list of compatible boxes, they are not always readily available.

    1. Expert Member
      Peter Engle | | #11

      I generally use the quickflash to either flash the cable or a PVC sleeve for later installation of the cable. Both of those are readily available. I don't love the Quickflash options for the actual junction boxes.

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