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Community and Q&A

Subfloor for new covered 3 season porch?

cabinflyer | Posted in General Questions on

I am planning a new 12’x12′ 3 season covered porch attached to the house in far northern Minnesota. The footings are pinned to solid ledge rock, so there is no chance of this thing moving. I’m using pressure treated ledger and 2x10s joists.

The porch will be about 4′ above the grade. The underneath of the structure will be protected from critters by heavy screening and a trellis. However, it will be exposed to outside air movement and temperature and humidity swings.

Question? Should I put in a pressure treated subfloor? I can’t seem to find any pressure treated that has tongue and groove edges. Or is a normal 3/4″ T&G subfloor good enough?



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  1. GBA Editor
    Martin Holladay | | #1

    Will the walls be open, so that the porch floor receives wind-blown rain? Or will this be an enclosed room with glass windows -- one that is protected from rain?

  2. Andrew_C | | #2

    You might want to reconsider screening the porch, if you are talking about vertical screens near the perimeter of the porch. In my experience, screens just provide shelter for animals and make the space more attractive to them, and they seem to be able to get in by breaking screens, burrowing, or just fitting through holes that seems really small. Plus, if it's covered, you'll be tempted to store junk under the porch where it will inevitably be forgotten and/or decay, provide more shelter for animals. And, a screen makes it harder to get in there to do maintenance, like spray wasp nests, put moth balls down the skunk's den, etc. YMMV, that's been my experience.

    I do like three season porches, hope you enjoy yours.

  3. cabinflyer | | #3


    Enclosed with windows.

  4. GBA Editor
    Martin Holladay | | #4

    This subfloor is indoors. There is no reason to use pressure-treated plywood.

  5. cabinflyer | | #5

    Would this still apply if the underneath of that subfloor is exposed to the elements?....and by "elements" I mean exposed to bare ground below. Snow and water may occasionally blow under there. Just checking...

  6. GBA Editor
    Martin Holladay | | #6

    Yes, my answer would still apply. Don't forget to cover the dirt in the crawl space under this floor with polyethylene, weighed down with a few bricks or rocks.

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