Ice & water UNDER or OVER the drip edge? - GreenBuildingAdvisor (2024)

I did a little searching before asking this, thinking that the topic
*must* have come up in the past, but didn’t find anything coherent.
We’re having this bit of disagreement between a GC and a roofer as
to whether ice & water shield over decking should flash over or
under the drip-edge piece at the connection to the fascia. The
roofer’s claiming that putting the drip-edge metal over the I&W
isn’t a reverse flash because water in an ice-dam situation would
flow upward, and the GC [who I’m tending to agree with] points out
that a small roof leak [or, say, inevitable condensation runoff
under metal roofing panels] would hit the I&W and come down safely
over the drip-edge and be ejected instead of soaking into the
plywood underneath.

*Note*: this is a situation where the ice&water was trimmed to
stop just shy of the edge of the deck, leaving a little lip of
the deck and/or the shadow boards at the rakes exposed. This
deck is the over-roof structure atop a roof&walls polyiso retrofit,
where everyone seems to assume the taped foam underneath is yet
another water control layer. [Hint: it isn’t, because tape laid
down in a hasty fashion has numerous fishmouths.] The GC’s guys
must have wanted to leave a “clean slate” of nailable wood for the
roofer, but personally I thought not leaving a little one-inch flap
of I&W *over* the edge was crazy — especially when it took another
month for the roofer to get to the site, leaving only the full wrap
of I&W on the deck *as* my roof in the interim [with occasional water
leaking into the soffit space, I might add, which didn’t seem to
concern them too much]. The question arose when it finally came
time to finish off these details and git ‘er done.

It seems to me that you’d want some way to flash against *both*
potential scenarios, which we kinda wound up doing by putting the
drip-edge over the I&W but then rolling out Triflex slip-sheet
under the [metal] roofing that lapped over the drip-edge piece.
But Triflex or even generic roofing felt isn’t really supposed
to be a water barrier, as I understand it, esp. since it isn’t
self-adhesive at all and gets *stapled* down.

_H*

Answer