Guest User
8/24/2014
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. Alex 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... I have more images, but the site will only let me upload one... Also, plywood on top of the curb - peel and seal the whole thing (except for the 7" hole, of course)...
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8/25/2014
Alex, Any chance to vent some of the stuff though a gable end wall? The larger penetration is well suited for a curb mount set up. Lets open it up to the larger crowd. Eric
Guest User
8/25/2014
The gable wall is a little far away. One of the fart fans might be a possibility, but then it would dump out right over a sliding glass door. This might be a possibility...thanks! One down three to go... Alex
Info @windowsonwashington.net
An informed customer is our best customer.
8/26/2014
Could could probably just build and flash one larger curb and vent all of them through the flat section.
Dick Bus
9/1/2014
another way would be to bend a flashing to 'cover" the corrugations from the ridge cap down to the penetration. This flashing would be bent up at the top side of the penetration. This flashing would need to be wider than the penetration. If you need a drawing let me know and I will get something sketched up.

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