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Free-standing porch deck and roof with mineral wool exterior insulation

Pat Kiernan | Posted in Green Building Techniques on

I’m looking at building a one-story porch with a deck and shed roof connected to a new two-story house with 4″ of mineral wool exterior insulation, 1×4 battens screwed into 2×6 studs 24″ OC , and siding.

I agree with the preference often expressed on this site of not attaching the deck to the house structure, both in terms of water issues and thermal bridging.

My current thinking:

The deck will be supported adjacent to the house with piers that rest on an extension of the house basement footing. The outer edge of the deck will be supported by piers on footings below frost depth. The deck is about 2 feet above grade. The roof load could be carried by posts that are supported by the deck piers below.

The roof ledger or beam adjacent to the house would need to support 100 pounds per linear foot (20 lbs. structure, 80 lbs. snow). The floor system is open web trusses with a 2×4 or 2×6 ribbon board — so no rim board. The porch roof will need to be flashed to the house siding for water control.

Does anyone have a detail for the flashing/connection of a free-standing deck and roof?

The integrity of the roof/siding flashing would seem to require some attachment between the porch roof and house wall.

Lastly, is it reasonable to try to support the porch roof ledger from the house wall (and skip the posts that carry the load to the deck piers)?

Thanks for any responses.

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Replies

  1. Expert Member
    Malcolm Taylor | | #1

    Pat,
    It's quite reasonable to support the roof ledger from the house wall. That's the standard method of doing so. It's also quite reasonable and simple to support the deck too. Water management issues are significantly reduced when the deck is under a roof.

    You might also find this discussion useful:
    https://www.greenbuildingadvisor.com/community/forum/energy-efficiency-and-durability/99493/i-am-building-home-de

  2. GBA Editor
    Martin Holladay | | #2

    Pat,
    Most porches are exposed to wind-driven rain and wind-driven snow, so the decking and floor joists need to be weather-resistant. Most builders use pressure-treated joists, although you could substitute a rot-resistant wood like red cedar or white cedar if you want to avoid PT lumber. Open-web floor trusses -- the floor framing you have chosen -- wouldn't be appropriate unless the truss manufacturer approves them for this purpose.

    When it comes to flashing the floor of this porch, there is no need to provide flashing at the joint between the floor board closest to the house and the house. This can be a free-draining slot.

    The shed roof will need to be flashed to the house, of course. Ideally, you will include flashing that is integrated with the wall's water-resistant barrier (WRB).

  3. Pat Kiernan | | #3

    Malcolm,

    Thanks for the post and link. It is standard to support the deck and roof ledger from the wall, but this isn't a standard wall. I haven't seen a detail for the type of construction I'm considering.

  4. Pat Kiernan | | #4

    Martin,

    Thanks for your post.

    My apologies on the floor system description. I guess I wasn't clear -- the open web truss is the floor system for the house.

    The WRB will be at the exterior of the sheathing. Have you seen any details that minimize thermal bridging?

    1. Alderwiser | | #7

      Late reply/question. Pat, what did you end up doing for the flashing detail for the roof? I have the exact same scenario, where I'm hesitant to break the CI for a sliver of metal flashing!

      Kevin

  5. Expert Member
    Malcolm Taylor | | #5

    Pat,

    I don't know how you are insulating your concrete foundation walls, but if it is on the interior you might think about adding several pilasters to the outside wide enough to support a beam. That would satisfy the lateral support requirements and insure the structures acted as one.

    Supporting a roofed deck on piers will take a bit of design work to develop enough sheer resistance. A quick talk with an engineer might be a good idea to see how complicated it is or isn't. It would be great if you return to this thread and tell us what you end up doing. Good luck!

  6. Pat Kiernan | | #6

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