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Square Penetrations in Corrugated Metal Roofing - Flashing Details

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  • Square Penetrations in Corrugated Metal Roofing - Flashing Details

    Mornin' Gents...

    I'm stumped by how to properly install/flash a couple of penetrations through an existing corrugated (wavy) metal roof. I have a fart fan hood/vent and a range hood roof vent - actually two of each - two separate. B&B units in the same structure. Obviously it would be more desirable not to go through the roof in the first place, but that is unfortunately not an option.

    The fart fan hood (see photo) has a 4" round pipe stub with a flat square s/m base. It seems like it's wanting a small curb, and yet also crying to have a decktite boot. It seems like a simple solution would be to use a decktite boot (high temp silicone) and then do a small curb out of 2x4's (the base of the hood is 9" x 9" square, and let the whole thing sit above the roof (the height of the decktite boot). I'm struggling with the hillbilly aesthetics of this workaround. I would flash the (treated) 2x4's to protect them from the elements, but essentially the rainfall would just flow under and through the hood/curb and depend on the decktite to keep the water out. (2x4' sitting on top of corrugations, but not attached, maybe with caulk/adhesive, but otherwise letting the water pass underneath. Maybe call it a "faux curb".

    Also note that both of these hoods have built-in backdraft dampers - so a "pipe vent" type vent (as another solution) is really not an option. Trying to keep outside air from coming into the building, obviously. Plus I'm hell bent for leather to figger out how to do this properly.

    The range hood roof vent has a 7" round hole for the duct to stub up into the hood. The base s/m is 15" square. This one definitely wants a curb. I was thinking I would cut a 15-1/4" square hole in the corrugated metal for the curb and then just flash the thing. I can wrap my head around flashing the bottom side - simple. And the sides, too - slightly trickier. But the top has me stumped. Honestly the whole thing has me stumped.

    Here's a little more info: 7/8" corrugations, 24ga fwiw. The fart fans will be on the new addition roof - 2:12 with 1/2" OSB. The range hoods would be on the original building roof - 6:12 on 1x4 slats on 2x6 rafters. Everything is currently accessible from the underside as well.

    Back to the range roof vent - I would put in 3/4" plywood under the roof - filling up the 1x4 slat space and giving me a good base. Maybe 21" x 21" (with a 7" dia hole in the middle) - pieced or notched in between the slats if/as needed. This will likely end up being a bear to make happen. Then install the curb (made from treated 2x4's on the vertical), then get to flashing. On the bottom side, up and over the curb, then on top of the corrugated roofing - riveted in with closure strips.

    I would also peel & seal the corr metal to the plywood all around the opening. There's my crux - peel & seal all around (and across the entire plywood pc) to seal the metal to the plywood "deck". Does this solve my problem at the corners of the square hole?

    So then I flash the sides - again up and over the top of the curb, then 4"-5" over the top of the corr metal, with a kick and a hem. Sealant and a couple of screws. Also peel & seal the flashing to the top of the curb (under the hood).

    So then I get to the top piece of flashing. Immediately, my side pieces are exposed at the top - sitting on top of the corrugations, extending 4"-5" past the square cut opening, with water running underneath them.

    So here's the dilemma. My top pc of flashing wants to go under the corr metal - no problem to do "at" the hole, but then it wants to come out and on top of the corrugations and on top of the side flashing pcs, but then there's still water running underneath everything - at least on the sides. Even if I cut/slice the corr panels even with the top of the square hole - then I can slide my side pcs in to that slot and under the panel at the top ( a bear, but I can remove screws to raise it up, maybe) - but then I have a little water catcher there. And, at the end of this new "slot" that I've cut, I have a direct vulnerability to rain getting in - with caulk being the only option that I can see to keep it out.

    So gents, thanks for sticking with me thus far in this verbose explanation (I hope). I've used the "what if this were a new roof how would you do it" analysis technique. The "square hole" in the corr metal panels would be 6-8-12" larger than the curb and the whole thing would be double-flashed, but I'm still drawing blanks at the four corners - under the flashing at the bottom - and on top of the flashing at the top.

    Especially the top. My brain is starting to hurt trying to figger it out. 36 years in the construction business (builder/gc) and this one has had me stumped for a month now. Now I've got to figger it out and get 'er done. Note that I'm new to the area and don't have a relationship with metal roofer yet. I've talked to a couple of guys about it, but they got the "deer in the headlights" look pretty early in the conversation.

    Any and all help/advice/counsel/sketches/photos much appreciated in advance.

    Driftwood, Texas

    P.S. I've also thought about putting a square patch pc of corr metal with the square hole for the curb in it, over the top of everything (with sealant) and then flashing the curb over the top of that, too. That may be a solution, but again, not thrilled about the prospect of depending on that sealant (or butyl tape) for years to come...

    Also, plywood on top of the curb - peel and seal the whole thing (except for the 7" hole, of course)...

    photo 3.jpgphoto 1.jpgphoto 2.jpgphoto 4.jpg

  • #2
    Re: Square Penetrations in Corrugated Metal Roofing - Flashing Details

    Master Flash Boots work great on corrugated roofs, but you'll have to come up with different caps.. As you've discovered, the caps you have won't work.


    • #3
      Re: Square Penetrations in Corrugated Metal Roofing - Flashing Details

      Don't penetrate that roof
      Sure there are ways to go about penetrations but if you can avoid that it is the best way.
      Do you have a gable?
      Mark Parlee
      BESI(building envelope science institute) Envelope Inspector
      EDI Certified EIFS Inspector/Moisture Analyst/Quality Control/Building Envelope II
      EDI Seminar Instructor
      Level one thermographer (Snell)
      You build to code, code is the minimum to pass this test. Congratulations your grade is a D-


      • #4
        Re: Square Penetrations in Corrugated Metal Roofing - Flashing Details

        Too late! It's got 300 screws through it already. :)
        But did you see the pics? the pitch change may or may not be flashed corectly, it's exposed fastener corrugated with no sheathing and no underlayment on a 2&12 pitch. Might be ok for a carport...I think cutting a hole in it is the least of his problems..
        Last edited by larryh; 08-29-2014, 08:45 PM. Reason: to add a smiley


        • #5
          Re: Square Penetrations in Corrugated Metal Roofing - Flashing Details

          I agree with Mark, head for the gable ends, or the soffit/bird blocks, instead of the roof. Poking holes in a 2/12 seems particularly risky, around here that roof would leak already, whenever there was snow or ice, much less with extra holes in it.

          If you do go through the roof, a couple of thoughts.

          First, you can use an aftermarket, separate backdraft damper along with a vent you do know how to flash. A couple I've used are the fantech RSK and the Tamarack cape damper. That could take care of the bath vents, though really those small 4" ducts should be very easy to re-route away from the roof.
          (I believe Fantech's dampers can be used on range hoods too if you want to go that route on it as well.)

          I have seen penetrations done two ways in metal roofs. The first is to cut the entire panel above and below the penetration, and work in overlapping layers around the penetration. They overlap the lower roof panel and tuck under the upper roof panel. You still have to find a way to make corners that won't leak, which usually involves solder or a factory-made welded piece. Can you cut the panels all the way across?
          The other method is to screw/rivet and glue/seal a piece into the middle of the field of the roofing panels. These make me very uneasy, you're relying on the seal, but I have seen some manufactured seals with screws every inch or two, which seemed likely to hold water out for a while.

          Good luck

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