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Slab in unheated garage

steveray

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Nov 25, 2009
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West of the river CT
There is no reference from R401.2 to frost protection to the loads in R301......THERE IS NO IRC SLAB THAT IS "SUPPORTED" BY A FOUNDATION...It is supported by the base per R506
 

redeyedfly

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Feb 22, 2021
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Minneapolis, MN
My understanding of the "why" it is allowed for a garage up to 600 Sq. Ft. is not that the building will not be subject to frost heave, it absolutely will (well, depending on your soils and temperatures), it is allowed because there is unlikely to be differential movement in the structure due to the small size. Even if there is an issue, because it is "only" a garage, it is unlikely to become a life safety issue or damage the structure to the point it is unable to be used for it's intended purpose. Basically, the risk assessment allows it.

This isn't to say that it is a good idea to do this everywhere, it is a minimum code after all. Certainly, if you have an area with vastly different soil types, you might want to ensure the foundation is below frost.

While there are sometimes where frost will come in underneath a garage slab that is independent of the building foundation and cause it to heave, it is relatively unlikely. Remember, for frost to heave the ground, there has to be water in it. The only way that the ground is going to have water in it with a building overtop is either really poor water management, or soil particles that are small enough to engage capillary action and hold water instead of letting it drain away.
I agree with all of this except the last paragraph. There must be moisture in the soil but not necessarily water. You also need clay in the soil to create the lensing which is what drives the force exerted in frost heave. You can certainly have frost heave below buildings with clay soils and all other water management done correctly.

To a different post, frost doesn't like or dislike anything, it's all simple thermodynamics. The only barrier to heat moving through a stem wall is the very little insulating properties it has. What you're actually seeing when there is no frost under a floating slab is the insulation of the structure above it reducing the heat radiating up. The roof is helping you, the stem wall is negligible.
 

redeyedfly

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Feb 22, 2021
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Minneapolis, MN
I'm trying to focus on what the IRC requires, not what I'd require or what makes sense or what some people believe the IRC requires but can't quite point to it.

As far as the driveway, if it were a load scheduled in R301 that R401.2 said had to be "accomodated" and thus supported by a frost protected foundation, I would say the code says yes. But all "passenger vehicle garages" are scheduled and no concrete driveway nor dirt nor any driveways are scheduled. As pointed out there are exceptions to the frost ptotection requirement but none apply to a >600 sf garage. I'm just reading the words in the IRC.
You're reading words and not definitions. Frost protection is required for structures, slabs are not defined as such unless they are elevated.

Are you purposely being thick? Your continued questioning is absolutely pointless. This topic has been explained to you by multiple posters but you continue to ignore their comments.
Get a dictionary and read Ch. 2.
 

Rick18071

Sawhorse
Joined
Nov 28, 2009
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Poconos/eastern PA
In PA you can build a garage up to 2,000 sq ft without a permit. And not need frost protection. This doesn't mean it's a good idea to do it.
 

bill1952

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Aug 12, 2021
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125
Location
Clayton NY
In PA you can build a garage up to 2,000 sq ft without a permit. And not need frost protection. This doesn't mean it's a good idea to do it.
Rick, my family is from Montrose PA area, great grandfather settled there I 1840, and we almost moved there for retirement.

I'm unclear if that is two separate amendments. In the IBC, the work exempt from permit is often misinterpreted to mean the work does not have to meet code. Not so. Sure, it isn't necessarily inspected and you will probably get away without meeting code if you so desire. I know of at least one instance in my very narrow area of speciality where permit exempt work was reported, subsequently inspected, and red tagged.
 

tmurray

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NB, Canada
I agree with all of this except the last paragraph. There must be moisture in the soil but not necessarily water. You also need clay in the soil to create the lensing which is what drives the force exerted in frost heave. You can certainly have frost heave below buildings with clay soils and all other water management done correctly.

To a different post, frost doesn't like or dislike anything, it's all simple thermodynamics. The only barrier to heat moving through a stem wall is the very little insulating properties it has. What you're actually seeing when there is no frost under a floating slab is the insulation of the structure above it reducing the heat radiating up. The roof is helping you, the stem wall is negligible.
What "moisture" are you seeing that is not water?

Water is all H2O, regardless of phase. In the world of building science, if we want to talk about a specific phase of water we say phase [vapour/liquid/ice] water. We rarely refer to moisture as it means either vapour or small amount of condensation.

With this in mind, I assume you are indicating that there would be phase vapour water contained in the soil that can cause frost, I would question how often this happens. Typically, there will be an intermediate phase change to phase liquid water before the ultimate phase change to phase ice water. There are conditions that can be present that causes deposition where phase vapour water undergoes a phase change directly to phase ice water, but the conditions are extremely demanding for this to be able to happen naturally.

The capillary force holding water is not just present in clay, but any soil particles small enough to ensure the adhesion of the water molecules to the soil particles overlaps sufficiently to defeat gravity.
 

tmurray

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Well I'm longing for the fantastic canoe routes of the truley great white north now but are you saying a garage over 660 sf has to have a full foundation where you live?

I need to look more at IBC and type U buildings perhaps, treating a 1000 sf building for vehicles as something other than an accessory structure to an IRC dwelling. Or just submit for permit and assume (safely I'm sure) reviewer will not care the 1000 sf garage floor is not supported on a frost protected foundation.
Sorry, Typo. 600 Sq. Ft. and yes they have some sort of full foundation under them. Typically we see a frost wall, just a 4' wall with footings to get them below the frost line, but we have people provide an engineered design for a slab on occasion as well.
 

mtlogcabin

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Joined
Oct 17, 2009
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8,128
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Big Sky Country
Decades ago we had a local engineer design a mono slab that could be used on an unheated garage/shed. His design is 6" of washed gravel and install gutters to direct water away from the foundation. He limited his design to a 1080 sg foot building with no plumbing installed. There have been hundreds installed in the past 30 + years and no failures or frost heave problems.
 

bill1952

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Aug 12, 2021
Messages
125
Location
Clayton NY
I noted in first post that this was a post frame/pole barn. I learned post frame is not subject to the prescriptive requirements of the IRC. It's an IBC type U building.
 

redeyedfly

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Feb 22, 2021
Messages
215
Location
Minneapolis, MN
What "moisture" are you seeing that is not water?

Water is all H2O, regardless of phase. In the world of building science, if we want to talk about a specific phase of water we say phase [vapour/liquid/ice] water. We rarely refer to moisture as it means either vapour or small amount of condensation.

With this in mind, I assume you are indicating that there would be phase vapour water contained in the soil that can cause frost, I would question how often this happens. Typically, there will be an intermediate phase change to phase liquid water before the ultimate phase change to phase ice water. There are conditions that can be present that causes deposition where phase vapour water undergoes a phase change directly to phase ice water, but the conditions are extremely demanding for this to be able to happen naturally.

The capillary force holding water is not just present in clay, but any soil particles small enough to ensure the adhesion of the water molecules to the soil particles overlaps sufficiently to defeat gravity.
Moisture meaning adsorbed water instead of free water. You assume incorrectly. You can try to talk down to someone else; discuss phase of water all you want in building science but this is a geotechnical issue, not vapor drive. In geotechnical engineering there is certainly a difference between bulk water and moisture. But, hey, thanks for a bunch of information from 6th grade science completely irrelevant to frost heave. You have a building science hammer so it must apply to a geotechnical nail!!!

Capillary force is present in all fluid/solid interactions. But clays are different from granulars. The quick and easy answer is the size and shape of the particles. Clays particles are flat like a playing card and stack closely. Frost heave occurs when adsorbed water between the playing cards freezes and creates a frost lens driving the particles apart. Granular soils have relatively enormous spaces between particles which allow free draining of water and also allow the frost somewhere it can expand (kinda like air entrainment). It does get much more complex, see your local geoengineering professor for a more detailed answer. Try not to mansplain to them, they, understandably, don't like that much.
 

tmurray

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Moisture meaning adsorbed water instead of free water. You assume incorrectly. You can try to talk down to someone else; discuss phase of water all you want in building science but this is a geotechnical issue, not vapor drive. In geotechnical engineering there is certainly a difference between bulk water and moisture. But, hey, thanks for a bunch of information from 6th grade science completely irrelevant to frost heave. You have a building science hammer so it must apply to a geotechnical nail!!!

Capillary force is present in all fluid/solid interactions. But clays are different from granulars. The quick and easy answer is the size and shape of the particles. Clays particles are flat like a playing card and stack closely. Frost heave occurs when adsorbed water between the playing cards freezes and creates a frost lens driving the particles apart. Granular soils have relatively enormous spaces between particles which allow free draining of water and also allow the frost somewhere it can expand (kinda like air entrainment). It does get much more complex, see your local geoengineering professor for a more detailed answer. Try not to mansplain to them, they, understandably, don't like that much.
You're probably right. I don't know nearly enough about geotechincal engineering to be considered an expert. I'm sorry you misunderstood what I was referring to in my original post. I was referring to water as being the molecule and assumed most would come to the conclusion I would be talking about water absorbed into the soil since trying to place a concrete slab over a body of open water (bulk water as you put it), doesn't sound like your typical residential garage. I'm glad we were able to clear up the miscommunication.
 

bill1952

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Aug 12, 2021
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Location
Clayton NY
This is a fickle business. No way will my building department permit a slab in an unheared garage to not be frost protected.

On the other hand, what they'll accept for a shallow frost protected foundation for unheated building is much less than what I read the code requiring. A 30x36 haunched slab 8" thick and 12" wide at perimeter with 8" vertical and 24" horizontal 2" XPS, nothing like any of the guidelines.

Paraphrasing, "...we don't need to go minimum 12" deep here.. "

At the end of the day, the ground is all coarse sand for many feet and site is a gentle slope, so I don't have real concerns about frost heave, so will accept making it simple as possible.

Frustrating interpretations vary so wildly from jurisdiction to jurisdiction.
 

ADAguy

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Sep 11, 2013
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5,985
Location
California
Moisture meaning adsorbed water instead of free water. You assume incorrectly. You can try to talk down to someone else; discuss phase of water all you want in building science but this is a geotechnical issue, not vapor drive. In geotechnical engineering there is certainly a difference between bulk water and moisture. But, hey, thanks for a bunch of information from 6th grade science completely irrelevant to frost heave. You have a building science hammer so it must apply to a geotechnical nail!!!

Capillary force is present in all fluid/solid interactions. But clays are different from granulars. The quick and easy answer is the size and shape of the particles. Clays particles are flat like a playing card and stack closely. Frost heave occurs when adsorbed water between the playing cards freezes and creates a frost lens driving the particles apart. Granular soils have relatively enormous spaces between particles which allow free draining of water and also allow the frost somewhere it can expand (kinda like air entrainment). It does get much more complex, see your local geoengineering professor for a more detailed answer. Try not to mansplain to them, they, understandably, don't like that much.
No one has mentioned water table depth or perculation of soil yet.
 
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