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Community and Q&A

Some basic porch questions…

jdchess | Posted in General Questions on

Rectangle envelope with a 8/12 trussed (raised heel) gable roof. Raised slab foundation with stem walls. Vented attic.

Maybe dumb questions, but…

What’s the normal detail for attaching a porch to the front/back assuming a lower pitch for the porch (4/12 vs 8/12)?

With a porch and vented attic, do the soffit vents sit in the normal place near the exterior wall of the house (and under the porch) or are they extended out to the overhang of the porch? If moved out to the porch overhang, are they still effective?

Should the rafters for the porch attach to the front edge of truss or attach further up on the truss top chord? Or is this a decision made based on the desired porch roof height? I’m attaching a couple of very crude sketches to show what I meant by these two options. I think the sketches are self explanatory, but if not please let me know. Hopefully you guys can let me know if I’m completely off on how this should be done.

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  1. Expert Member


    The choices are usually governed by aesthetics - within certain parameters.

    Choose how high you want the eaves of the porch to be, and whether you are g0ing to have a ceiling or expose the rafters. The latter decision should be informed by whether you may insulate the porch in the future - and that in turn informs where you locate the soffit vents. If the porch will remain open you can leave the vents as they are on the rest of the roof, otherwise move them to the eaves of the porch.

    Start with the assumption that the porch roof will be a minimum slope of 3/12, but can be increased. Draw elevations to see whether increasing the roof pitch or adjusting the height of the eaves makes it look better. As long as you tell the truss supplier where the porch framing will bear it makes no difference which option you choose. If you decide to go with a ceiling, you might as well include the porch framing in your truss package.

  2. jdchess | | #2


    Thank you for the input. Haven't decided on ceiling vs. open rafter for certain. I did assume that with a ceiling, you would move the soffit vents to the eave of the porch, but I'm not sure what the detail would look like with open rafters and the soffits in the normal place against the house. With open rafters on the porch, would you build a normal eave with soffits just as if the porch wasn't there, or is there a modification needed?

    1. Expert Member
      MALCOLM TAYLOR | | #3

      It depends where the rafters hit as to whether you continue the main roof as though the porch wasn't there. If they land quite a distance up the main roof, then yes the eaves and soffits would be continuous. If they hit at the exterior wall then you can stop them at the edge of the porch. In the latter situation you can mount vents on the exterior wall at the top.

      In terms of ease of construction, a porch framed with trusses and a ceiling is a lot simpler.

  3. jdchess | | #4


    If the porch has a ceiling and the rafters attach to the front edge of the truss (at the top of the heel), would the attached sketch be roughly correct?

    Basically, it's like the porch becomes a really deep eave overhang with everything else being the same as normal eave overhangs and no porch. Yes?

    1. Expert Member
      MALCOLM TAYLOR | | #5

      Yes. The only thing to look out for is the location of a ledger if one is provided. If it is flush with the porch rafters it will block the ventilation path.

      Can you drop the porch roof that far and still have headroom inside? From the sketch it looks several feet below the interior ceiling.

  4. jdchess | | #6


    Also, to your question about headroom..the interior ceilings are 9' so the porch ceiling in that drawing is about 7'6". So may ideally want a little more height, but it seems that moving the rafter up to where it lands on top of the truss top chord would not really change much in regards to the ventilation path or location of the soffit vents, yes?

    If I move the rafter up the top chord enough for an 8' porch ceiling, we get something like this maybe?

    1. Expert Member
      MALCOLM TAYLOR | | #7


      You are on the right track and both variations work. I'd step back and concentrate on making the porch somewhere so nice you want to be there as often as you can. Perhaps forgo GBA for a bit and look at Houz.

      On the last sketch you posted you don't need the vent-channels. Just cut back the sheathing on the main roof in between the porch rafters.

  5. jdchess | | #8


    Not entirely sure I understand why the vent baffle would be needed in one instance and not the other. Can help me understand why this is?

    Point taken on fleshing out the details. I will work out more of what I actually want the porch to be.

    1. Expert Member
      MALCOLM TAYLOR | | #9


      The baffles are only necessary if the insulation is deep enough that it would impede the air-flow at the exterior walls. Typically people choose the height of their raised-heel trusses so they have the same height there as the insulation they want throughout the attic, so the baffles keep things clear. In your last detail, the porch rafters hit the roof well above the level of the insulation. All you need there are large holes in the sheathing on the main roof in between the porch rafters to allow air-flow between the two spaces.

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