Flat roof DWV Vent with Modified Bitumen

Roofing/Construction Questions
1

I’m running a 2" DWV pipe up through an almost flat roof (~1/12). The roof is granular asphalt (10 yrs, will be replaced in next year +/-). Because the DWV is within 10 feet of an operatable skylight, the pipe will extend 36" above the roof.

  1. What is an appropriate, cost-effective (will be removed in next year) means to flash the pipe?

Possibly Oatey 2" Galvanized flashing https://tinyurl.com/y6o4gpfx with caulking on the underside near edges, nailed near edge into place and acrylic flashing and mesh on top of the edges?

  1. Is there an effective means to seal around the penetration of the roof? The roof system from top to interior is

Modified bitumen (granular asphalt)
plywood
foam board
plywood
2x6 joists

2

Is the “granular asphalt” Modified Bitumen?

3

Yes, it is modified bitumen. (Original question modified, thanks)

4

I’d use mod-bit flashing cement. Scrub it in good with a trowel to be sure it bites to the membrane. Make an 8x8 fiberglass mesh patch and cut the hole tight. Bed it in an 1/8" of mud, then cover that with another 1/2" and feather the edges.

1 Like
5

Perfect. I overlooked the cement specifically for/compatible with modified bituman.
Thanks!!

6

ALWAYS match your substrate if possible. If the hole is oversized, foam the void and cut a hole in a scrap of aluminum to seal it.

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7

Thanks again Darkthirty! Great suggestions.