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Waterproofing fascia board before flashing

Jamie B | Posted in General Questions on

Hey GBA,

I have a low slop invented roof, it has an overhang and I will be inatalling the soffits and flashing the fascia board soon.

I plan to use Hardie board for he flashing. I originally was going to get the metal guy to flash it all with metal, but since it’s 22inches high, I was concerned about oil canning or otherwise warping from expansion/contraction. So I decided to do Hardie. I’ll screw the hardie board tight to the fascia board without furring.

Now, everytime I see roofers or sheet metal guys do fascia work, they’re all applying blueskin peel n stick, or otherwise waterproofing the fascia.

Does anyone have any knowledge about this? Are they worried about water or moisture getting in there and causing the facia to rot?

Any thoughts are appreciated.

Toronto – climate zone 6

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  1. GBA Editor
    Martin Holladay | | #1

    I assume an "invented" roof is really an "unvented" roof.

    Hardie board is not flashing. You can use Hardie board for your fascia, but you shouldn't call it flashing.

    Most roofs have drip edge (metal flashing), and the drip edge needs to transition properly with the fascia. It's hard to know whether you understand the basic principles without a sketch showing your detail for the fascia and the drip edge.


  2. Jamie B | | #2

    Thanks Martin,

    Yes "unvented" is what I meant. I'm typing on my phone today.

    As I don't have my laptop fired up, here's a quick hand sketch. Haven't hand drafted in a while.

    Again, just wondering why roofers waterproof their fascia board.

  3. GBA Editor
    Martin Holladay | | #3

    Your detail shows a rake, not an eave. Detailing at a rake may be the same as detailing at an eave -- but not always.

    I would label the Hardie board as the "fascia" or "rake trim." Underneath the fascia is your rake rafter (or perhaps, your sub-fascia).

    The two main ways to detail the rake to manage water are the following:

    1. The fascia should be wide enough to project beyond the soffit (so that water doesn't wrap around the corner).

    2. The drip edge at the top of the fascia should be wide enough (should have a wide enough lip) to keep most of the water from dribbling down the fascia. (Obviously, the fascia will get wet during a driving rain, but the upper flashing still needs a good wide lip.)

  4. Expert Member
    Michael Maines | | #4

    Martin, I think you missed the "low slope" part of Jamie's description.

    Jamie, if your details for an unvented low-slope roof are done properly, there should be no vapor drive from interior to exterior (if there is then you have other things to worry about). You just need to keep water from getting from the exterior to the framing. Hardie is not waterproof so a WRB of some sort is a good idea, though most roofs don't have anything below the trim because it can dry easily.

  5. GBA Editor
    Martin Holladay | | #5

    I didn't miss the reference to "low slope" -- but I couldn't find a clear image on Google Images to illustrate my vocabulary point (namely, the difference between the fascia and the flashing or drip edge). You're right, of course, that the image shows a steep-slope roof, not a low-slope roof.

    However, I stand by my answers.

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