Roof and siding with steel
We’re trying to finalize details with our builder for the metal siding and roofing and there are a couple details I’m not quite sure of…
1. The siding is vertically hung corrogated sliding with a 7/8” depth, installed over coravent. The gap behind allows airflow behind but I’m trying to figure out the best way to allow airflow above and below the siding for air movement. I asked a couple manufacturers about using a perforated sill or j channel to allow airflow, but the only thing they offer is a solid option. They suggested if we want airflow to skip the j channel and space the corrugated siding up off the foundation by about an inch. But I’m wondering if that would look unfinished. Does anyone have pictures of details I should be requesting here for proper air flow? I tried to describe it, but they said it doesn’t matter if the top and bottom are closed off because there’s space behind. Maybe I’m missing something?
2. What is a good bug screen option for vertically hung corrugated siding that still allows airflow?
3. The roofing will most likely be Ultra-Snap standing seam panels from CMG Metals https://www.cmgmetals.com/product/ultra-snap-panels/. The attic is ventilated, LP Weatherlogic is our sheathing. It will not have exterior rain screen or insulation on the roof. What would be the best way to prevent condensation from the steel roof being a moisture issue? I’m thinking an impermeable membrane or ice shield installed over the entirety of the roof? Or maybe skip the impermeable membrane and use a product like Cedar Breather Roof Ventilation Mat instead? Our contractor said they typically use synthetic felt (no other membrane over the sheathing), but they can install a specific product if we know what we want. In our region, building for airflow and green design is not common so I am looking for something fairly straight forward, easy to source, and will do the job well of preventing moisture and condensation issues.
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Replies
Emel,
1. If you mount the panels on 3/4” horizontal strapping @ 32” oc, and use Cor-a-Vent or a perforated U-channel for the bottom and top ones, you can use a solid drip-edge flashing to close off the corrugations.
2. For perforated flashing below corrugated siding I get a metal shop to bend up the profile I want, along with any custom flashing I need elsewhere. Luckily my lumberyard has a shop. You might try: https://www.menzies-metal.com
3. Use a synthetic roof underlayment rated for metal roofs. It doesn’t need to be permeable as the sheathing can dry into the vent channel below.
You don’t need to worry about condensation under the roof panels if they are installed directly on the underlayment. No air gap means no moist air to condense.
https://www.greenbuildingadvisor.com/question/zone-6-a-straight-forward-and-affordable-metal-roof-assembly-does-it-exist
Thank you Malcom,
Our only weather stripping, in addition to the 7/8” depth of the corrugated metal, was going to be the horizontally placed coravent sturdi battens https://www.cor-a-vent.com/sturdi-battens.cfm which are 7/16” thick. We did not currently plan to use wood also. I was thinking this would be enough air flow if we could get perforated flashing. Is that not right?
Thank you for the perforated flashing recommendation, I’ll look them up. For airflow behind metal siding to be so important, I’m wondering why no metal roofing company I have talked to yet has known what I’m talking about and no one seems to make it? They all insist it’s not necessary and to just butt the siding right up to the unperforated flashing to stop bugs. The idea that air needs to be able to move from above and below makes sense it just feels like it’s a battle to get anyone to consider it here since even the siding companies don’t offer it. Would you happen to have a photo of what you’re recommending for this?
For the roof underlayment, when you say the synthetic roof underlayment, do you mean it’s fine to go with the synthetic felt they typically use, or use a synthetic vapor closed underlayment like a self adhered rain and ice barrier? Or maybe you’re saying either would work?
Emil,
If you are using Cor-a-Vent to mount the panels on you don't need any perforated flashing. Just attach an L-flashing with a hemmed drip-edge to the bottom Cor-a-Vent. (see attached sketch).
No need for something like I&WS, regular synthetic underlayment is fine. Just make sure it is rated for metal roofs, which are hotter than asphalt shingle ones.