Frost depth for unheated building with walkout basement

[MuddyWrench]

Hi everyone,

This is my first post here, though I've been a long-time reader!

I’m building a 24x32’ 1.5-story cape-style garage set into a hillside. The back of the site is about 6 feet higher, so the first floor is essentially a walkout basement. I’m in Connecticut, where the frost depth is 42".

I have a question about how to treat frost depth in this situation. The designer I hired has drawn a stepped footing that goes down a couple of steps to stay 42" below finished grade. The slab sits on the footing at the back wall, where it’s 6 feet below grade. But since this will be an unheated structure, wouldn't finished grade essentially be the basement floor? Am I overthinking this, or is there something I’m missing? I haven’t found anything in the code that addresses this specifically.

Thanks so much for any guidance!

 

[Mr. Inspector]

Is this an accessory building? What exception are you using in the IRC to not go down to the frost line?

 

[MuddyWrench]

@Mr. Inspector the footing is always below the frost line relative to finished grade. The thing that is confusing me is that if the building is unheated, then the slab will be the same temperature as the outside air. So should I treat the slab as finished grade? If that's the case, should the footing be 42 inches below the floor level?

This is for a building that will be my workshop.

 

[Mr. Inspector]

The footing for an unheated building of this size should be at least to the frost line if it has a slab or not per 2018 IRC. I don't know what the frost line is in your area.

 

[steveray]

Firstly....Welcome to THE forum!....

Secondly, watch out for this:

N1102.2.9​

Slab-on grade floors with a floor surface less than 12 inches (305 mm) below grade shall be insulated in accordance with Table N1102.1.3.

Thirdly....The "basement" generally wants to stay 55 degrees and frost generally does not want to go sideways....The insulation in the previous code section will give you a thermal break that will generally keep any frost from creeping under the slab...Make sense?

Wanna say what Town?

 

[mtlogcabin]

It is an un-heated garage walls and floors are not required to be insulated unless they are part of the building thermal envelope that is conditioned

BUILDING THERMAL ENVELOPE. The basement walls, exterior walls, floors, ceilings, roofs and any other building element assemblies that enclose conditioned space or provide a boundary between conditioned space and exempt or unconditioned space.

 

[Yankee Chronicler]

@Mr. Inspector the footing is always below the frost line relative to finished grade. The thing that is confusing me is that if the building is unheated, then the slab will be the same temperature as the outside air. So should I treat the slab as finished grade? If that's the case, should the footing be 42 inches below the floor level?

This is for a building that will be my workshop.

Your concern is valid. However, if the temperature inside the building is at or below 32 degrees F., how much work are you going to be able to do in it? Is this an occasional workshop, that remains unheated most of the time and you fire up a portable heater when you want to do some work?

If the space is unheated, then I agree that the slab should be considered "grade," and the bottom of footing should extend to 42 inches below that elevation all the way around.

 

[mtlogcabin]

ASCE 32 is prescriptive and part of it is in the IRC. I have seen engineers prescribe a couple solutions usually requiring wash rock and perimeter insulation when not meeting the frost depth requirements.

R403.1.4.1 Frost protection.
Except where otherwise protected from frost, foundation walls, piers and other permanent supports of buildings and structures shall be protected from frost by one or more of the following methods:

1. Extended below the frost line specified in Table R301.2.(1).

2. Constructed in accordance with Section R403.3.

3. Constructed in accordance with ASCE 32.

4. Erected on solid rock.

Footings shall not bear on frozen soil unless the frozen condition is permanent.

 

[redeyedfly]

Crazy how no one has answered his question.

Yes, the inside of the garage will be essentially the same temperature as the exterior and your frost footings should be 42" below the lowest point of exposure to outside temps, i.e. 42" below the garage slab.

Pay attention to how you detail the slab connection to the foundation, Your slab could heave from the soil beneath it freezing while your foundation stays put. Allow the slab to float within the foundation, no hard ties between the two.

 

[bill1952]

Thank you redeyedfly! I suggested the OP who had posted this elsewhere post here. Your answer is what I thought code would require and just what I did on my 900 SF unheated accessory garage.

Wouldn't same apply to an unheated crawl space (which I would never recommend but they seem permitted) where grade of crawl is lower than grade surrounding building? To be frost protected without insulation footing has to be below frost line measured from lowest grade?

OP will probably not listen to me again.

 

[MuddyWrench]

Thanks you all for the feedback!

@redeyedfly thank you for answering my question directly!

@bill1952 not at all! I'm not looking for validation to do it the easy way. I just want to get it right. Generally I like to do things the hard way.

@Yankee Chronicler you're right. The truth is, the building will be insulated, and heated for as long as I own it. The problem is, I'm building it as a two car garage and I can't be sure the next owner will want to pay to keep the heat on. I want to get it right, and I wasn't sure what the code had to say about it.

As mentioned by @mtlogcabin, I think I'll choose to follow ASCE 32. The thought of building a nearly 12' block wall isn't too appealing. Unheated FPSF comes with the added benefit of protecting the slab, and not just the footings, from frost. My plan (as of today), is to place 1.5" of XPS on top of 6" of 3/4" crushed rock covering the whole footprint of the building, with wings extending 36" from the front half. (I don't need insulation wings in the back because of the soil depth). I'll pour footings directly on the foam, then build CMU walls on three sides (front will be wood framed). Finally I'll pour the slab. Anyone else done this? I mentioned it to my inspector and he wasn't familiar with pouring footings directly on foam, but said he'll accept it if it's in the spec.

Thanks again!

 

[steveray]

Wouldn't same apply to an unheated crawl space (which I would never recommend but they seem permitted) where grade of crawl is lower than grade surrounding building? To be frost protected without insulation footing has to be below frost line measured from lowest grade?

We do ventilated crawlspaces all the time and do not go down below interior grade as it is not required.....It's ground surface, not interior concrete floor surface.

R403.1.4​

Exterior footings shall be placed not less than 12 inches (305 mm) below the undisturbed ground surface. Where applicable, the depth of footings shall also conform to Section R403.1.4.1.

 

[bill1952]

We do ventilated crawlspaces all the time and do not go down below interior grade as it is not required.....It's ground surface, not interior concrete floor surface.

R403.1.4​

Exterior footings shall be placed not less than 12 inches (305 mm) below the undisturbed ground surface. Where applicable, the depth of footings shall also conform to Section R403.1.4.1.

No crawls without slabs? Just dirt?

 

[redeyedfly]

We do ventilated crawlspaces all the time and do not go down below interior grade as it is not required.....It's ground surface, not interior concrete floor surface.

R403.1.4​

Exterior footings shall be placed not less than 12 inches (305 mm) below the undisturbed ground surface. Where applicable, the depth of footings shall also conform to Section R403.1.4.1.

That is not considering frost. It’s physics, not code semantics.

 

[SCBO1]

Is the additional 1/2 story being used for anything?
Assume storage.

When I get theses questions, I always revert to my crystal ball and ask " When will your mother-in-law be moving in?"

But in the end it's always about $$ or bitcoin, isn't it?

 

[bill1952]

Is the additional 1/2 story being used for anything?
Assume storage.

The issue is about a one story accessory building built or cut into a hillside. Mine is a large 2 car garage. At the north west corner, the floor - a slab on ground - is a few inches above adjacent ground. At the south east corner the floor is about 40" below the adjacent ground. Frost depth here is 48". Unheated building. The question is from what point is the 48" depth of footing measured - slab on ground or adjacent ground? I don't believe the IRC is clear.

 

[mtlogcabin]

That is not considering frost.

Frost is considered in the second sentence

"Where applicable, the depth of footings shall also conform to Section R403.1.4.1."

R403.1.4.1 Frost protection.
Except where otherwise protected from frost, foundation walls, piers and other permanent supports of buildings and structures shall be protected from frost by one or more of the following methods:

1. Extended below the frost line specified in Table R301.2.(1).

2. Constructed in accordance with Section R403.3.

3. Constructed in accordance with ASCE 32.

4. Erected on solid rock.

Footings shall not bear on frozen soil unless the frozen condition is permanent.

 

[redeyedfly]

Frost is considered in the second sentence

"Where applicable, the depth of footings shall also conform to Section R403.1.4.1."

R403.1.4.1 Frost protection.
Except where otherwise protected from frost, foundation walls, piers and other permanent supports of buildings and structures shall be protected from frost by one or more of the following methods:

1. Extended below the frost line specified in Table R301.2.(1).

2. Constructed in accordance with Section R403.3.

3. Constructed in accordance with ASCE 32.

4. Erected on solid rock.

Footings shall not bear on frozen soil unless the frozen condition is permanent.

So what was the point of your post?

 

[mtlogcabin]

I thought I was agreeing with your post that steveray's post #12 did not include the frost depth requirements that are reference in R403.1.4.

 

[redeyedfly]

I thought I was agreeing with your post that steveray's post #12 did not include the frost depth requirements that are reference in R403.1.4.

gotcha