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New Addition Foundation


We are building an addition to our house in Minnesota. Our house has an approx 5 foot frost footing with a concrete crawl space.

My question...I'll need a frost footing for the addition; however I can't visualize how this new footing and foundation will butt up against the existing footing and foundation . On my existing foundation I have a 2x6 sill and 10 inch flloor joists with flooring. I would like the new addition floor to be level with the existing floor; however, I don't want a crawl space in the new addition. Does anyone have a cross section detail?...thanks in advance...j

Re: New Addition Foundation

I would recommend you consult the Yellow Pages under "Foundation Contractors" to obtain several local on-site professional opinions as to the best way to go on this project.

Since this involves a deep frost line, considerable excavation and some form-building, you will get a wide variance as to the design plan & cost of such a project.

One method that is commonly used is to install a a "monolithic slab foundation", which often requires the least amount of excavation, form-building and labor; generally, a 3-sided perimeter trench (for all load-bearing walls) is excavated with a backhoe, 1/2" steel rebar is used to tie in the level portion with the vertical deeper portions, extensive 3" styrofoam sheets are installed along the top, a minimal amount of peripheral concrete form boards are used along the top; any plumbing lines, electrical, drain lines, radiant heat lines, etc., are installed, then a vapor barrier, and a single concrete pour is then done; it is noted that some towns don't allow a monolithic pour in deep frost line localities, but most do (town permits required), and the styrofoam insulation reduces the depth of the required concrete pour to a depth much less than the local frost line.

Google "you tube video monolithic slab" for various videos of this procedure; also "you tube video monolithic slab hayes concrete"

The site below has a diagram section called "Slab on Grade" that illustrates this style of foundation; the slab on grade/monolith pour has 2 legs (sometimes 3) holding up a top slab, much like the small letter "n" or the new staples you install in your stapler when it runs out.


Re: New Addition Foundation

i just did an addition wiht a similar situation. we used installed a icf foundation for the walls which extended up just above the height of the existing floor then on the inside it gets backfilled with gravel which is kept down 6" below the height of the existing floor. by doing this you eliminate the crawl space . 2" foam goes down over the backfill along with the icf it creates a thermal break so their you dont loose heat through the slab

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