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Community and Q&A

Venting a roof framed over a roof

Brian Ducharme | Posted in General Questions on

I have a ventilation question I have just thought of while designing a home. The house is a 1.5 story home (framed as a cape style but turned 90 degrees so the front entrance is the gable wall).

I realized that when the garage roof is framed and attached perpendicular to this roof line there may be a venting problem for the bays of the house that now have a roof framed over them. I wouldn’t think this would create a problem except when the garage ceiling gets insulated and sheetrocked it may interfere with these bays being able to draw air. Obviously there is no soffit venting possible within this space now and the garage ceiling insulation may be blocking the bottom of those rafter bays.

Would the correct solution here be to leave off 1′ or so of plywood along the bottom edge of the main house roof to expose the ventilation channels to the air space within the over-framed roof?

Sorry if I didn’t explain this correctly. Thanks for your help.

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  1. Expert Member
    Malcolm Taylor | | #1

    Any chance of a sketch?

  2. GBA Editor
    Martin Holladay | | #2

    If you want to have a vented roof assembly, your soffit vents need to be able to pull outdoor air into the ventilation channels.

    If you can't do that, you have to re-design your roof assembly so that it is an unvented assembly.

    For information on creating vented and unvented assemblies, see this article: How to Build an Insulated Cathedral Ceiling.

    --Martin Holladay

  3. Brian Ducharme | | #3

    here is a quick sketch showing the area im concerned about. thanks

  4. GBA Editor
    Martin Holladay | | #4

    First of all, I urge you to update your user profile so that your real name is displayed instead of a hard-to-write number. Here is a link to an article that explains the easy step: How the GBA Site Displays Readers’ Names.

    Concerning your question: You have two choices:

    1. Create ventilation channels that connect the air in your garage to the ventilation gaps in each rafter bay.

    2. Convert these rafter bays to unvented assemblies, following the rules in the article I linked to.

  5. Expert Member
    Malcolm Taylor | | #5


    Thanks for the sketch. If the garage ceiling is insulated so you have a vented attic above, the rafter bays can draw air from that attic as long as you don't sheath the entire main roof, or if you do provide openings in the plywood at each bay.

  6. Brian Ducharme | | #6

    Thank you for the response. I was also wondering if anyone could recommend a reputable insulation contractor within range of Eastern, CT. I am looking to get a quote on dense pack cellulose and I know it is a process that needs to be done correctly.

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