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Does a concrete slab need footings below the frost line?

DIYGuy's picture

The slab would be for a storage shed, unheated and free standing. I would tend to go with concrete so it will serve as the floor, but if I have to set footings below the frost line, I think it would be too much work. In which case I would go with crushed gravel and frame the floor. My concern about that method would be animals getting in and making their home between the floor joists. The existing shed, ready to fall down on it's own, currently has a family of skunks living under it. 

What's the consensus on this? What would you do? I'm in the midwest and I believe the frost line is 36".

Thanks

In the absence of a contrary (post #191006, reply #1 of 11)

In the absence of a contrary opinion from a building inspector, I would go with the floating slab for a small structure like that.

 

There is a technique for larger structures also for slab on grade called frost protected shallow foundation. FPSF But anything les than 12x12 should be fine with just a 4" slab

 

 

Oh Well,

We did the best we could...

With or without a footing the (post #191006, reply #2 of 11)

With or without a footing the ground will freeze and heave in an unheated structure.  Though you often don't notice it because there's no fixed point to compare to, sometiimes the heave can be 5-6 inches (though about 2 is more the norm).

The heave will tend to be greater around the edges than in the middle, since heaving is dependent on moisture and its usually (but not always) drier near the middle fo the slab.  You can also get uneven heaving due to variations in soil type from one end of the structure to the other.  (Obviously, in a heated or even well-insulated structure the interior rarely freezes.)

It's this differential heaving that's the main issue.  The foundation and any slab needs to be strong enough to withstand the uneven heaving without cracking (or otherwise failing).  (There's also the problem, on sloped sites, of having the foundation creep downhill over multiple freeze/thaw cycles.  And one needs to consider the structure's relationship to other structures -- decks, patios, etc -- that it may connect to.)

You can reduce heaving with a proper substrate (stone/sand vs expansive soils), and with good drainage.  My parents used to have an 70-year-old farmhouse that was built on a stone foundation with the stone simply sitting on the soil -- no footer whatsoever.  (This was near Louisville, where winters are fairly mild, but still the frostline was 12-18 inches, and many roads took a beating from winter freeze/thaw.)  The house did have a few "seasonal" cracks, but nothing serious, since it was built on top of a hill (well-drained), on relatively poor (rocky) soil.  On a more normal site the foundation would have disassembled itself in 25 years.

And, obviously, having a footer below frost level will prevent the foundation from heaving, but you can still have some heaving of the slab in an unheated structure.  So in some ways it's almost better to NOT have a footer for an unheated, uninsulated structure.

That said, code trumps all.  In our neck of the woods sheds smaller than a certain size need not be permitted, but you need to check your local rules.


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

I'm going to disagree with you whole heartedly (post #191006, reply #9 of 11)

"With or without a footing the ground will freeze and heave in an unheated structure."


The idea of going to just below frost depth is to prevent  heaving. How is frost going to heave a whole building if it can't get beneathe it?


You CAN get heaving in the center of the slab in an unheated structure with footers to depth, but ONLY if you did not use proper soils and drainage,

But this is not that kind of a structure to need to deal with that.

 

 

Oh Well,

We did the best we could...

You CAN get heaving in the (post #191006, reply #10 of 11)

You CAN get heaving in the center of the slab in an unheated structure with footers to depth

That's what I said.


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

Same frost depth here (36") - (post #191006, reply #3 of 11)

Same frost depth here (36") - we had good luck doing this sort of construction (although it's more of an insulated/half-heated shed) with a turned-down slab edge monolithic pour down to around 30" below grade - much easier than spread footings/stem wall.   No problems through some fairly tough winters since 2001.

Floating slab is fine (post #191006, reply #4 of 11)

In my neck of the woods (Southern Michigan) a floating slab is code approved for up to a 24'x36' building. They do require that the perimeter be thickned to 12"x12".

For a smaller shed, just pour a slab about 5" thick and under most conditions, it should rise and fall with the frost with no breakup.

The other thought, if this is a small shed, the cost of the concrete in a minimum pour may exceed the framed floor in cost.

 

Terry

Thanks for the input, (post #191006, reply #5 of 11)

Thanks for the input, everybody. My customer (I'm contracting the work) is a lawyer who handles a lot of construction contracting cases, so I'll have him look into the code issues. The shed will be pretty small, probably 6' x 10'. The grade is flat and the soil has fairly good drainage. From what I read here, it' seems a 5" reinforced slab will probably outlast the house, much less the shed sitting on it. It will be completely freestanding, so no issues with adjacent structures.

I'm going to suggest going with a slab. He already has a concrete guy, so I can focus on what I do best, the carpentry end of it. 

Thanks again for the good advice.

Peace! :-)

It would be advisible to (post #191006, reply #6 of 11)

It would be advisible to throw a few pieces of rebar or wire into the slab.  That way, if it does crack, it won't go in two different direction and tear the shed apart.


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

Proper Drainage (post #191006, reply #7 of 11)

Like maybe 4" of ~ 1" crushed washed stone beneath the concrete and the proper drainage so that water will not collect in the stone.  Also you may want to beef the slab up a bit by either going with 5" of concrete, or putting a piece of #4+ rebar on each of the 4 edges.

Matt

No simple answer (post #191006, reply #8 of 11)

Wether or not a footing is needed below the frost line depends on the soil, the moisture content and the timing of water infiltration and the movement of the frost line. In very general terms, coarse grained soils frost heave less than fine grained soils. The pore space between grains is large enough for ice crystals to form in the void rather than pushing the grain to grain contacts apart. The question gets real complicated when you start to consider multiphase movement of water in the subsurface.

However, the US Army Cold Regions Research lab has bublished some good work on using grain size to get an estimate of the thaw stability of frozen soil. If you aren't willing to do some investigation on the soil the foundation is going to rest on, play it safe and go deep. Alternatively, have a plan B ready if the foundation fails.

Rules of Thumb (Often mashed)

When making decisions based on your gut, consider the end product.

For that applications, you (post #191006, reply #11 of 11)

For that applications, you are often allowed to do a thickened slab edge and use a monolithick pour. You have a perimeter that is like 16 inches thick and like 12 inches wide at the bottom tapering to the 4 inch slab in the center. Easy to prep and pour. Use reinforcing in the perimeter and WWM in the center. It's a standard thing. Some call it a floating slab I think. I did a large garage approved by the building official like this ... even had a second floor on it ... in a cool/cold climate.

There ain't NO free lunch. Not no how, not no where!