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Manufactured Home Footing and Slab

J

jaybrown

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Joined
Jun 29, 2022
Messages
14
Location
Delaware County, PA
Has anyone come across a new manufactured house build with a perimeter frost footing and 4" slab (4-6" stone w/ 6 mil VB) mono poured with no interior pier footings?
 
A double-wide would have interior bearing walls but a single-wide would be unlikely to. Why would it need interior pier footings?
 
I believe in NY - based on what the building department told me - a slab with frost protection is required, or perhaps is most common.
 
Mr. Inspector said:
The state use to let them use a slab but now piers are required per manufactures installation directions.
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Yes, we've done thick slabs before, not just 4" along with piers where required by the manufacturer. I don't see them just put on a 4" slab even if the perimeter is frost protected. There are always piers somewhere in the middle.
 
jar546 said:
Yes, we've done thick slabs before, not just 4" along with piers where required by the manufacturer. I don't see them just put on a 4" slab even if the perimeter is frost protected. There are always piers somewhere in the middle.
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No piers needed for a single-wide mobile home. This is a trailer, with a totally self-sufficient structure. It's a different matter entirely from a modular, which meeds supports where the modules intersect.
 
Yankee Chronicler said:
No piers needed for a single-wide mobile home. This is a trailer, with a totally self-sufficient structure. It's a different matter entirely from a modular, which meeds supports where the modules intersect.
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We've had manufacturers specify piers in the center before, so it happens regardless of how self-sufficient the frame is perceived to be. Don't ask me why they spaced them out that way; I just reviewed the specs from the manufacturer when I was in Pennsylvania, and that is exactly how they were shown. This was routine for the specialty mobile home installers. Your mileage obviously varies.
 
Not sure how this will apply to you folks, but up here, single-unit modular homes (minihomes) are usually built to a standard (Z240) that exempts these buildings from footings at depth. In other words, these single-unit modular homes can be set on blocks or slabs-at-grade.

I don't see a back-reference to any U.S. standards/Codes, though, so the above is probably of little use. (Some of our standards do - our CGSB 12.1-M safety glass standard is harmonized with ANSI Z97, for example).
 
I have zero experience with this, so how does a frost protected slab work on a mobile home? What's keeping the soil warm below the slab? Is there a heater under the floor of the mobile home? Just curious.
 
A little weird as I don't know that the floor is the envelope...

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R403.3​

For buildings where the monthly mean temperature of the building is maintained at not less than 64°F (18°C), footings are not required to extend below the frost line where protected from frost by insulation in accordance with Figure R403.3(1) and Table R403.3(1). Foundations protected from frost in accordance with Figure R403.3(1) and Table R403.3(1) shall not be used for unheated spaces such as porches, utility rooms, garages and carports, and shall not be attached to basements or crawl spaces that are not maintained at a minimum monthly mean temperature of 64°F (18°C).

Materials used below grade for the purpose of insulating footings against frost shall be labeled as complying with ASTM C578.
 
redeyedfly said:
I have zero experience with this, so how does a frost protected slab work on a mobile home? What's keeping the soil warm below the slab? Is there a heater under the floor of the mobile home? Just curious.
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It's not the heat in the building but the heat from the earth that keeps it frost free. A big insulating blanket that "catches" the heat from below. You can use frost protected shallow foundations for unheated buildings. Some different criteria - basically longer or deeper apron - but still works. And instead of extending out, that insulation at perimeter can go straight down - but you loose the simplicity of shallow excavation.

ps: 403.1.4.1 allows using ASCE 32 which includes the criteria for FPSF of unheated buildings.
 
Last edited:
redeyedfly said:
I have zero experience with this, so how does a frost protected slab work on a mobile home? What's keeping the soil warm below the slab? Is there a heater under the floor of the mobile home? Just curious.
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Threefold strategy typically:

Minimize/reduce water
Minimize soils subject to frost heave
Foundation resistant to deformation from freezing
 
bill1952 said:
It's not the heat in the building but the heat from the earth that keeps it frost free. A big insulating blanket that "catches" the heat from below. You can use frost protected shallow foundations for unheated buildings. Some different criteria - basically longer or deeper apron - but still works. And instead of extending out, that insulation at perimeter can go straight down - but you loose the simplicity of shallow excavation.

ps: 403.1.4.1 allows using ASCE 32 which includes the criteria for FPSF of unheated buildings.
Click to expand...
I think you still need significant insulation under the slab in ground freezing climates such as MN, not just at the perimeter.
 
redeyedfly said:
I think you still need significant insulation under the slab in ground freezing climates such as MN, not just at the perimeter.
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Yes, of course. Didn't mean to imply otherwise. In the ballpark of 2" if XPS under slab and 4' out. I guess you can leave out some underslab in heated but seems false economy.

I agree that Is expect the slab with a mobile home on it to have foam under whole slab. I guess you could enclosed and hear it like a crawl space.
 
redeyedfly said:
Point taken ;)
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Bills note here is part of what I was thinking:
bill1952 said:
heat it like a crawl space.
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If it is inside the envelope and an empty floor it might work...But that is a weird one where technically we only have authority over the foundation and connections:

AE101.1​

The provisions of Appendix AE shall be applicable only to a manufactured home used as a single dwelling unit and shall apply to the following:

  1. 1.Construction, alteration and repair of any foundation system necessary to provide for the installation of a manufactured home unit.
  2. 2.Construction, installation, addition, alteration, repair or maintenance of the building service equipment necessary for connecting manufactured homes to water, fuel, or power supplies and sewage systems.
  3. 3.Alterations, additions or repairs to existing manufactured homes. The construction, alteration, moving, demolition, repair and use of accessory buildings and structures, and their building service equipment shall comply with the requirements of the Connecticut State Building Code.
These provisions shall not be applicable to the design and construction of manufactured homes and shall not be deemed to authorize either modifications or additions to manufactured homes where otherwise prohibited.
 
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