Metal roof ridge decking gap
eorr
| Posted in General Questions on
Hello,
I’m using Zip roof decking. My contractor put the panels on from the bottom up to the ridge. There is about a 6 inch gap at the ridge, which he covered with membrane. I realized after he put the metal on, he cut the membrane away at the top of the metal sheets. So there is about a 2 inch gap underneath the ridge cap.
Should I be concerned about condensation from the ridge cap dripping into my ceiling?
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Replies
eorr,
That's how all vented roofs are built. You can't have a continuous ventilation channel with a roof underlayment covering the outlet.
With a metal roof, any condensation will run down the underside of the cap and drop onto the panels well beyond the gap.
Thanks, I realized that was probably the case after I posted this.
eorr,
One less worry!
I'm encountering a similar situation ( but for an unvented roof) in Climate Zone 5 during a gut renovation. We're installing new Zip sheathing on one side of a gable roof to tie into existing double-layer sheathing. To match the old condition, the framer left a gap at the ridge—contrary to my goal of a continuous air barrier.
The roof cavity will be filled with CCSF, and a self-adhering ice and water shield will cover both the new and existing sheathing, topped with a double-lock standing seam metal roof.
Given this assembly, how critical is the gap at the ridge? Should I insist on a butted sheathing condition for continuity?
Your ice and water shield will cover it, so it's probably not a big deal. It will be sealed. In warmer climates people stop the ice and water shield short of the ridge and install a smart vapor retarder instead to create what the code refers to as a "vapor diffusion port". You might want to think about humidity in your new conditioned attic. Moist air will migrate to the top. You are installing closed-cell which is good since it will stop the water vapor from migrating through the foam and condensing on the underside of the roof decking. Even still you might need to plan for some air flow or a small dehumidifier in this attic space depending on your climate.
Olanjide,
If you're filling all the rafter bays up to the ridge with CCSF, that will give you an air barrier at the roofline. Zip sheathing with seams taped & rolled is a nice secondary air barrier at that point, but the CCSF should be doing the air barrier work. There's no reason to leave a gap at the ridge if the roof isn't going to be vented, but it probably won't hurt anything.
Thanks Matthew & Paul,
Another concern is that the metal roof ridge detail requires a solid substrate for secure fastening. With the current gap at the ridge, I’m unsure how well the ridge cap will be anchored, as the fasteners would land near the edge of the sheathing. This raises questions about long-term durability and proper detailing. Is this an reasonable concern?
olanjide,
That gap would be a standard one for a vented roof. I wouldn't worry about the installation of the ridge cap at all.
The only situation I can think of where the gap is consequential is in high seismic zones where the two roof planes need to be structurally connected