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Monolithic Slab

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  • Monolithic Slab

    Hi Folks- great forum. I am in the planning stages of building a 24 x 38 detached garage in Minnesota. Code allows me to pour a monolithic slab 4" thick with an 8" x 12" perimeter foundation. My plan was to maybe beef up the perimeter footing a bit (10 x 12) and pour it over 3 inches of compacted crushed rock and a vapor barrier. After reading here I see I may be better off with foam insulation ($$) of some type under the slab, but I am not sure if this is needed as the space will "probably" not be heated. There is a loft above which will, (saltbox style). My other concern is how to get a slope towards the garage doors in a single pour and still have level sills to frame on. Does one shim the sills, or make up for it in the framing elsewhere? I figure 1 to 1 1/2" of slope over 24 feet should be enough. Any comments suggestions? Sand vs. Rock? My best description of the soil below is "has some clay in it". I will use fiber reinforced concrete and rebar in the perimeter. Of course, my concern is dollars to put a 42" footing in as opposed to cracks/heaving. I should mention a lot of folks use floating slabs here on detached garages and most issues seem to be lack of vapor barrier and poor base compaction. Thanks for any response.


  • #2
    Re: Monolithic Slab

    Hi Joe,

    I don't have a lot of time to reply today, but I'd recommend you do a google search for "frost protected slab foundation". I think you can also search the JLC archives and the FHB archives for this same topic. If memory serves, both have published info about this type of foundation within the past few years. If you can find it, email me offline and I'll see what I can find and fax or scan and email to you. Frost protected foundations originated in Scandinavia (Well, duh!) and eventually came into favor here in extremely cold climates. The folks up in Alaska like 'em a lot.

    Mike O'Handley
    Bungalow Rescue, Seattle


    • #3
      Re: Monolithic Slab

      Joe, check your email, as I have sent you quite a few pictures, and information. The vapor barrier on a uninsulated garage foundation in my opinion, is a moot point. If there is living area above, then, I would say that it probably is worth the effort, but I would assume you will still get condensation on the floor during those first few weeks in the spring when the slab is still cold from winter, but the air temp is warming up. The cold slab will attract the moisture vapor in the air and it will condensate on the slab surface unless you can figure a way to warm the slab during this time.