Attic venting with porch detail
Building in NY state, Climate Zone 5A.
Metal roof, blown in cellulose in the attic and planning to use LP soffit for eaves.
I’m in the middle of reviewing my truss shop drawings and can’t figure out how to properly vent my attic with the way the porch roof meets the building.
According to code, I only need 1/300 = approx 4sqft of venting which would be covered by the soffits on the back of the building and above the garage door, but I worry that the air flow will be uneven.
The main roof on the building is 6/12 pitch, which dies into a 5/12 porch roof. The problem is that the ledger for the porch rafters is going to be where the soffit normally would be for approx 20′ of the building. I explored dropping the porch roof so I can add a soffit into the truss roof, but the height of the porch opening gets too low. Don’t want to lower the pitch as it won’t shed snow as well.
Are there any low profile vents I can cut into the ledger in between rafter bays that won’t overly weaken it? Another concern is: if I put a ceiling on the porch, I will need to install vents there too. Is that going to adequately vent the truss roof?
Stumped and appreciate comments!
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Replies
yhuynh,
- The angled cut ends of the 2'x10" rafters are already 3/4" longer than a 2"x10" ledger would be. If you cut another 3/4" off the depth of the ledger you will have an 1 1/2" gap for a vent inlet.
- The Front Elevation drawing is a bit off. The fascia to the left won't line up with intersection of the two roof pitches, it will be about 5" lower.
- Any reason the ledger is Pt?
Thank you Malcom!
The engineer specified 2x8 rafters (not shown in this drawing). I don't know why the ledger is PT, that's what the engineer specified. Is there a concern there?
Are you suggesting I hold the rafters high on the ledger? Having the vent below the ledger wouldn't work I assume as it will be blocked by insulation. Would leaving a gap in the sheathing there and just stapling some screen material do the job? This seems like a simple and elegant solution if I understand correctly! (Drawing attached)
Ok to install ledger/girt behind the sheathing?
Any concern about venting through that porch roof if it gets a ceiling?
Thanks for pointing out the error in the elevation drawing - the overhang will bring that fascia down.
yhuynh,
That looks good. You should probably run the change by your engineer.
No problem with the ledger being Pt, I'm just not sure why it's necessary.
If you are running a ceiling on the porch, provide a continuous vent gap close to the wall as an inlet for the roof above. I prefer a pre-made vent strip, as screening tends to get damaged. https://www.menzies-metal.com/metal-flashings/plastic-soffit-strip/
That's just the product I was thinking must exist! Thank you again