Does ground under exposed concrete have a deeper or shallower frostline?
if you have a 10x16 concrete slab or even a unheated shed will the frostline be as deep under the concrete as it would be 10 feet away. Ground is mostly sand
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The frost line below a slab on grade depends on the climate zone, what is on top of it, whether it’s heated, and if there is insulation under the slab.
A simple slab on concrete with no building on top, and that has no insulation underneath would have the same frost line as the surrounding ground. If it had a warm building or was insulated underneath the concrete, then the ground would be warmer underneath. What would change the ground temperature under a slab compared to land 10 feet away is insulating or heating it in anyway. If you explain what you are hoping to build, whether you planned to insulate under the slab and the climate zone you live in then we may be able to offer more help.
I have a basement at -4 feet below grade and ceiling height + 4 feet above grade. One of the basement walls has a walk-out door to a patio at 3 inches below the basement concrete floor. Where is the frost line considered to be in the patio? I.e. do I need to lower the footing below that wall separating basement and patio in order for footing to be under frost line ?
Hi Omar,
Frost lines vary by climate. depending on where you are it could be just 6 inches or it youre in the far north it could be 6 feet. I would ask your local building department to learn what the regional frost depth is, or even a local general contractor perhaps. Whatever it is, it would be consistent, so to avoid cracking and frost heave you would be smart to put a perimiter skirt of EPS foam insulation. As a general rule, an inch of EPS is about the same as a foot of dirt for its insulation values. And one would hope that whoever built your home took that into account for the footings on the walkout side.
Thank you for your response,
Sorry to have given the impression the house is already built. it is not. it is still on paper still. I am in the process of designing it.
The frost line in my area is 18 inches below grade. Being 4 feet below grade the base of the continuous strip footing under all exterior walls easily meets that condition. So no problem there.
The question is the footing on the walkout side.
Do I need to drop it on that walkout side so that its base is below 18 inches
Or not drop it and insulate the the patio concrete slab
When you say Perimeter skirt of foam insulation
Hi Omar,
No I didn't realize you were in the design phase still. This is more a question for your designer or engineer, I don't know your local building code and I would not be serving you well to try to give you specific recommendations for foundation frost protection here in a discussion forum. But conceptually speaking, you will need to make sure that the footing is kept warm enought to not be subject to frost damage, and yes there are different ways to do that, putting it deeper is one way.
And by perimeter skirt I mean laying a sheet of insulation flat extending out from the foundation at least 2 feet, at least 2 inches thick. That is another option you might want to discuss with your design team. If it covered the footing and allowed sufficient height still to allow for your patio that might be a simple option. Patios themselves can be protected from frost heave as well by insulating underneather as well as with a perimeter skirt, but exactly how much insulation and how far out would be required is something an engineer or designer should determine.