Clarity on foundation

[Rick D]

Hi all.
I’m looking to build a 23x24 two bay single story garage. It’s not clear to me the type of foundation that I need to pour for this. I live in CT where the frost line is 42”. I read in a number of locations that a simple slab pour is sufficient. Elsewhere I read I need to have footings. I also read I should do I minimum of a 4” pour but around the perimeter do a minimum of 6”. Can anyone provide guidance on this?
TIA

 

[BillK]

The best guidance will be provided by your city / county building department. They will be issuing your permit and you will need to do it the way they prefer.

 

[kbs2244]

This is not a project for an inexperienced DIYer

Get a good, local, concrete contractor
He will have done it before and will know he local codes and
site needs

Have a floor plan sketch for any lift or other heavy equipment
reinforcement needs

 

[billconner]

I believe CT use the IRC (International Residential Code) but I don't know if they amend it.

The IRC may exempt your building from having to have frost protected foundation.

1. 1.Protection of free-standing accessory structures with an area of 600 square feet (56 m2) or less, of light-frame construction, with an eave height of 10 feet (3048 mm) or less shall not be required.

So if you're stick building the garage and walls do not exceed 10', you don't need to go below frost line (or build a shallow frost protected foundation).

So a 4" slab with a thickened edge should be fine. The minimum size of the edge is 12 wide and 6 thick except the bottom has to be 12" below grade. Might have to be a little bigger based if really terrible soil or design snow load greater than I thought - but a couple ofvonches. Factor in slopes and a curb to get the framing above dirt, it could vary some. If you're planning to heat and/or cool, you'll need insulation under slab, and that would affect my recommendation. So a lot of pieces to put together and figure out the exact detail right for your project, but it is not that hard.

 

[462867aa]

Look on you tube for kens carpentry. It looks like his builds are mostly on Alaskan slabs.

 

[ConCretin]

There are basically three foundation options for a detached structure but your local codes could limit which of these is available to you. After that it's dependent on your preference, soil conditions, budget, etc.

Frost walls bearing on footings below the frost line with a separate slab on grade is a common option but is also the most expensive. Multiple placements and additional excavation and backfill add to the cost. Frost walls are appropriate for most soil conditions and provide a high level of protection from frost movement.

The second mid priced option is a frost protected shallow foundation. This could be either a monolithic slab with thickened edges or shallow footings a stem walls with a seperate slab. Rigid insulation is used to trap the natural heat from the earth and block frost penetration to make the structure frost resistant. These work fine but covering/hiding the insulation around the perimeter requires some thought.

The final and least expensive option is a simple monolithic slab that basically floats on the frost. As long as the soils underneath a relatively free draining, frost movement is not usually an issue. I have a 32 x 28 garage with a subsequent 14 x 36 addition that performs just fine with our 48" frost depth

If considering options two or three, I'd strongly recommend a concrete or masonry curb on top of the slab to lift the wood framing above interior and exterior grades to protect it from moisture. Frost walls can easily provide this protection by extending the wall height above grade.

 

[billconner]

Good summary but leaves out my favorite for 600 SF and under: shallow footing (1' minimum for all buildings), block stem wall to get framing above grade, floating slab inside block stem wall. Easy to insulate slab for heating and not have foam exposed at edges plus easy for diy.

 

[ConCretin]

Good summary but leaves out my favorite for 600 SF and under: shallow footing (1' minimum for all buildings), block stem wall to get framing above grade, floating slab inside block stem wall. Easy to insulate slab for heating and not have foam exposed at edges plus easy for diy.

It was easy to miss but I included it under Option 2 where I said "This could be either a monolithic slab with thickened edges or shallow footings a stem walls with a separate slab". I included it under FPSFs because that's the only way I've ever seen it done. Have you successfully used this method without frost protection insulation?

 

[billconner]

Apologies, missed that.

Sort of. I did a rubble filled trench with drains to daylight, then footer and block stem wall on top of it. Not insulating building so slab will be floating. (I say will be but am considering 12" x 12" pavers - less than $2000 versus slab at over $8000 a few months ago - and base is in for either.)

I just find the dry lay surface bonded block stem wall to whatever depth to be very diy friendly. If I didn't have the time and had the money, formed and poured stem wall, but I'd still do floating slab with foam, if required between slab and stem wall. I just very much dislike the foam exposed and having to cover it somehow.

For the OP, if not heating and staying under 600 sf, I think it's a good option. If heating or if over 600 sf, I would probably still separate slab from stem wall with vertical foam, but put footer on foam and extend apron for complete shallow frost protected foundation. (I say foam but really insulation - would consider Rockwood and gravel, and just read two articles on closed cell spray foam under slab - trading time for material cost - and XPS has gotten very pricey.)

 

[bb29510]

you got to have footing