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Slab in unheated garage

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bill1952

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Joined
Aug 12, 2021
Messages
2,107
Location
Clayton NY
I've been involved InDesign and instruction since 1982 and active in code development since 1987 - ICBO, BOCA, SBCCI, ICC, and NFPA - but as a specialist in assembly. Now retired and just want to build a garage, so this is a NYS Residential Code question. Where does the code - IRC close enough - tell me a slab on grade garage floor in unheated building doesn't have to be protected from frost? Lots of unheated post frame/pole barn buildings with an independent slab I them that I'm sure is subject to frost heave. Thanks for helping this old dummy.
 
2018 IBC not IRC

I believe this would have to be a detached stand alone building from any heated structures.

1809.5 Frost protection.
Except where otherwise protected from frost, foundations and other permanent supports of buildings and structures shall be protected from frost by one or more of the following methods:

1. Extending below the frost line of the locality.

2. Constructing in accordance with ASCE 32.

3. Erecting on solid rock.

Exception: Free-standing buildings meeting all of the following conditions shall not be required to be protected:

1. Assigned to Risk Category I.

2. Area of 600 square feet (56 m2) or less for light-frame construction or 400 square feet (37 m2) or less for other than light-frame construction.

3. Eave height of 10 feet (3048 mm) or less.
 
Thank you. But if it doesn't meet the three conditions - specifically over 600 sq ft - how can there be so many pole barns where the poles are on a footing below frost line but the flor - the slab - is not?
 
What I found was some states distinguish ag buildings and lessen some code requirements but the definitions I saw were fairly strict - like you have to be in the agricultural business. I thought about planting a garden but don't think that would qualify me.

Now I'm not sure if a detached unheated garage serving a single family house is under IRC or a U use under IBC. Seemed odd IRC seems only to address attached garages.
 
bill1952 said:
What I found was some states distinguish ag buildings and lessen some code requirements but the definitions I saw were fairly strict - like you have to be in the agricultural business. I thought about planting a garden but don't think that would qualify me.

Now I'm not sure if a detached unheated garage serving a single family house is under IRC or a U use under IBC. Seemed odd IRC seems only to address attached garages.
Click to expand...
IRC covers unattached garages. R403.1.4.1
 
R403.1.4.1 on frost protection includes exception "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." but what about greater than 600 sf? Seems genius.

Thank you for pointing this out. I had seen it but dismissed it as not applicable for a 1000 sf unheated building.
 
It is for accessory structures, which I think includes garages. We had a 25 x 28 garage and definitely too small for two cars and usual assorted stuff. IRC is pretty silent on the subject.

And spell check got me. I meant to say it seemed tenuous, not genius, in previous post.
 
bill1952 said:
It is for accessory structures, which I think includes garages. We had a 25 x 28 garage and definitely too small for two cars and usual assorted stuff. IRC is pretty silent on the subject.

And spell check got me. I meant to say it seemed tenuous, not genius, in previous post.
Click to expand...
What else do you want the IRC to discuss about detached garages?

20'x30' will fit three cars but not much else. 25'x28' is actually a fairly large two car garage, 20'x20' is about minimum for 2 cars.
 
redeyedfly said:
What else do you want the IRC to discuss about detached garages?

20'x30' will fit three cars but not much else. 25'x28' is actually a fairly large two car garage, 20'x20' is about minimum for 2 cars.
Click to expand...

Only for city dwellers who drive tiny cars. My pickup will not fit in a 20' deep garage, and my wife's suv won't either.
 
we parked our Outback and Crosstrek on our driveway and set objects around and chalked out what we thought was right size. 28 x 28. (When I saw post frame system I liked was based an 12' module, I just went to 28 x 36, which easily allows my trailer and 3 canoes to fit.)

As for what else do I want from IRC, something about not having to frost protect the slab on grade floor even though the walls and roof do have to be. Odd but it seems you could build a house the same way, though the energy requirements - 2" xps or equal under slab - might provide a shallow frost protected foundation.
 
bill1952 said:
we parked our Outback and Crosstrek on our driveway and set objects around and chalked out what we thought was right size. 28 x 28. (When I saw post frame system I liked was based an 12' module, I just went to 28 x 36, which easily allows my trailer and 3 canoes to fit.)

As for what else do I want from IRC, something about not having to frost protect the slab on grade floor even though the walls and roof do have to be. Odd but it seems you could build a house the same way, though the energy requirements - 2" xps or equal under slab - might provide a shallow frost protected foundation.
Click to expand...
Why would you have to frost protect a floating slab?

In an attached garage the slab is not frost protected. There is no reason for it to be protected, it is free to move and does not affect the rest of the structure. The same applies to a pole barn; the poles are frost protected, the slab is not. Frost protection is for footings so the structure does not experience differential movement from frost heave and become overstressed. A slab is not the structure unless you have a very small (<600sf) light frame structure designed to float on the subgrade as it moves with frost.
 
Well, R401.2 Requirements:
Foundation construction shall be capable of accommodating all loads in accordance with Section R301 and of transmitting the resulting loads to the supporting soil. Fill soils that support footings and foundations shall be designed, installed and tested in accordance with accepted engineering practice.

and R301.5 Live Load lists a live load for Passenger vehicle garages

and R403.1.4.1 Frost Protection
Except where otherwise protected from frost,
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:....

I realize that garage floor slabs on grade are commonly not frost protected but not seeing that the live loads of a garage floor are exempted from the frost protection requirement - except by tradition. I'm just missing where garage floors are not a part of "foundation walls, piers and other permanent supports of buildings and structures". Is a s.o.g. not a permanent support of garage floors?
 
bill1952 said:
Well, R401.2 Requirements:
Foundation construction shall be capable of accommodating all loads in accordance with Section R301 and of transmitting the resulting loads to the supporting soil. Fill soils that support footings and foundations shall be designed, installed and tested in accordance with accepted engineering practice.

and R301.5 Live Load lists a live load for Passenger vehicle garages

and R403.1.4.1 Frost Protection
Except where otherwise protected from frost,
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:....

I realize that garage floor slabs on grade are commonly not frost protected but not seeing that the live loads of a garage floor are exempted from the frost protection requirement - except by tradition. I'm just missing where garage floors are not a part of "foundation walls, piers and other permanent supports of buildings and structures". Is a s.o.g. not a permanent support of garage floors?
Click to expand...
Probably best to admit you've dug yourself in a hole than continue to dig.
 
Yeah. Unwritten exception to the requirement for supporting all required live loads with frost protected foundations. Easy enough to ignore.

Does make me wonder if I built two identical one story buildings with s.o.g. floors, stored cars in one and lived in the other, if both slabs could be not protected from frost.
 
bill1952 said:
Yeah. Unwritten exception to the requirement for supporting all required live loads with frost protected foundations. Easy enough to ignore.

Does make me wonder if I built two identical one story buildings with s.o.g. floors, stored cars in one and lived in the other, if both slabs could be not protected from frost.
Click to expand...
You couldn't do that because the exception for floating slab foundations is only for accessory structures which are certainly not dwellings. If you lived in one with a floating slab and frost footings (a very common design) you would also be required to heat the structure and insulate the foundation which prevents the soil below the floating slab from frost.
What you are suggesting is not supported by code.
There's nothing confusing, ambiguous, or unwritten in the code about this topic. It all follows rational engineering principals.

But keep digging your hole...
 
Where is the "the exception for floating slab foundations is only for accessory structures" over 600 sq ft? That's all I'm asking for.
 
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