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Community and Q&A

FPSF for raised porch

NorthJersey | Posted in Green Building Techniques on

I’m building a 9×16 enclosed entry porch. The entry is about 30 inches above grade. I’m using some Buildblock ICFs that are left over from my basement.

I intended to frame a wood floor structure for the porch. What would be a good approach for an FPSF with a crawl space?

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  1. GBA Editor
    Martin Holladay | | #1

    I'm confused.

    A frost-protected shallow foundation (FPSF) is a slab on grade.

    If you have a slab, you don't need a wood floor structure.

    Which do you want: a slab or wood framing?

    One final comment: I don't see how you can have a frost-protected shallow foundation with a crawl space.

  2. NorthJersey | | #2

    I would like wood framing supported by shallow footings.

  3. GBA Editor
    Martin Holladay | | #3

    If you want a crawl space and wood framing, you'll need to extend the frost walls down to footings that are placed below your region's frost depth.

  4. NorthJersey | | #4

    Wouldn't wings and interior insulation work without the slab in the middle? I could put in a 2-inch "rat slab" over the internal horizontal insulation if necessary.

  5. GBA Editor
    Martin Holladay | | #5

    You're right -- if you build frost walls and a crawl space, you don't need a slab.

  6. GBA Editor
    Martin Holladay | | #6

    Why not support the wood framing of your porch floor with a couple of Sonotubes?

  7. NorthJersey | | #7

    I'm not clear on the definition, but I understand "frost wall" to mean the traditional construction method used to extend the bearing surface below the frost line.

    If I can avoid using frost walls, I won't have to dig to a depth of about six feet (necessary based on how the yard is graded) compact the soil at the bottom of the trench, breach and then repair the dimpled membrane and bituminous membrane on my ICF foundation to a significant depth, and buy more Buildblock ICFs, concrete, waterproofing, and rebar. I also won't have to place concrete in a deep trench, which is a pain in the neck unless I call a pump truck for $1000 or so. I could probably also avoid an excavator rental and dig the footings by hand.

    I was hoping adding wings to the outside and horizontal insulation underneath would help me avoid these things.

    Oh yeah, I have a bunch of Fastfoot fabric I'll be using with the Buildblock. I did a monopour for the basement, and I'll probably do it with the porch, too.

  8. NorthJersey | | #8

    Oops. Here's the picture.

  9. NorthJersey | | #9

    Granted, the one I'm building is larger, and the main floor on the pictured house is much closer to grade than that of my house, otherwise I'd just do a slab.

  10. NorthJersey | | #10

    I'm going for a variation of the porch pictured in the attachment. I'll be continuing a bluestone veneer skirt from the rest of the house.

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