Detached garage - Michigan foundation question

[dtbingle]

Located in Michigan, planning for a 16x22 (352 sq ft) detached garage in the backyard. My city follows the 2015 Michigan Residential Code for building requirements.

The 2015 MRC specifies that detached accessory buildings (ie. garage) under 600 sq ft do not require frost protected footings. In other words, it is not a requirement for the footing to be below the 42" frost line.

For those in similar cold climates, have you built detached garages with footings shallower than the frost line? How many years has it been built and has there been any issues with settling or frost heave?

Trying to determine if I should go down to the frost line as a preventative measure or if it's just way overkill for a 352 sq ft detached garage.

 

[finn]

Foating slab with thickened edges under my 32’x54’ has been there since, I think, 2003.

No heaving or movement.

My 48’x75’ shop was probably built before that, and is of the same construction. It’s still fine, too.

Both are built on well designed, sandy soil. I suspect that the type of base and drainage makes a difference.

 

[dtbingle]

Foating slab with thickened edges under my 32’x54’ has been there since, I think, 2003.

No heaving or movement.

My 48’x75’ shop was probably built before that, and is of the same construction. It’s still fine, too.

Both are built on well designed, sandy soil. I suspect that the type of base and drainage makes a difference.

Thanks! Do you happen to remember any specifics on dimensions of the thickened edges (depth/width)?

That's a good point on type of base. I believe it's about 12" of fairly loose soil and then turns into harder clay if I'm remembering correctly. The area looking to build on remains pretty dry and doesn't get any standing water after heavy rains, so that's a plus.

 

[kbs2244]

this is where a local, experienced. concrete contractor is worth having

he will know local soil and the underlayer that is needed for a "floating slab"

 

[finn]

Thanks! Do you happen to remember any specifics on dimensions of the thickened edges (depth/width)?

That's a good point on type of base. I believe it's about 12" of fairly loose soil and then turns into harder clay if I'm remembering correctly. The area looking to build on remains pretty dry and doesn't get any standing water after heavy rains, so that's a plus.

I was 400 miles away when the garage was built. I sort of remember that the edges were 16”, but don’t run with that.

I agree that you should talk to a few local builders or concrete guys to see how they handle it in your county.

Might find that it’s not that expensive to have them do the job. Faster, too.

 

[ConCretin]

I have a 28 x 32 on a mono-slab that's 10+ years old. A couple years ago I added on a 14 x 36 addition for my boat, also on a mono slab. Both have a 4" slab with a 12" deep edge. I ran a couple #5 bars around the bottom of the perimeter and reinforced the slab itself with #4 bars at 18" centers. I formed the perimeter with 24" wide form panels and after the slab was in, I stood up a 2 x 12 on the slab and placed an 8" curb, which stiffens the perimeter even more and gets the wood framing up a bit from grade.

The original building was on 8" of gravel. The addition was on 6'+ of well compacted gravel. I pinned the two together but you can't even see the crack between them. I suspect they move around a little but not enough that you'd notice.

Check out my Guide to Floor Slabs in the link below for some additional thoughts.

 

[theoldwizard1]

The 2015 MRC specifies that detached accessory buildings (ie. garage) under 600 sq ft do not require frost protected footings. In other words, it is not a requirement for the footing to be below the 42" frost line.

I have lived in SE MI (Metro Detroit) for over 60 years. I don't think I have seen frost ever below 24". I would still want a footer. Remember the old timers used to call them a "rat wall" for a good reason (prevents varmits from burrowing under).

 

[Monza Harry]

My garage addition was October/November 1995 pour, addition is 14' X 28' 4 to 5" slab on "foundry sand" (sand and clay mix) my "rat wall" @ 18" ended up about 4" above grade at the far end, with the slope of my lot, so lots of fill brought in! Rebar between the existing floor and the new one. Machine tamped about 3 hours worth. No visible cracks yet but not much floor is visible. And I live about 10-15 miles from Dearborn MI. My underlying soil is a sand/clay/river rock mix really hard digging. Harry

 

[Stuart in MN]

Garages on thickened edge slabs are very common in Minnesota, and our frost depth is 48" or more. Any problems with settling or heave have more to do with the skills of the contractor building it.

 

[matt_i]

You could do a couple of things...build a "trench foundation" which is essentially an earth-formed pour. I work with a guy who basically cut his foundation with a ditch witch. One of my neighbors did the same thing except with a mini-excavator. I'd advise taking time off work and coordinate the digging and pouring into a short window of time. I self-suffered when I waited between these events and endured a monsoon summer which collapsed all of my trenches and required weeks of hand digging to scoop out the muck. I was in the best shape of my life lol.

Option 2 is to take pieces from a frost-protected shallow foundation design. It involves board insulation "wings" which go horizontally out from the foundation to trap the heat of the earth as well as crushed stone. If you google search "frost protected shallow foundation" and find the HUD.gov link with a .pdf you will find the recipe-book.

 

[Dogmeat]

Built my 16x24 garage about 6 years ago in s.w. Mi...only needed a "ratwall" slab.

 

[dtbingle]

Thanks everyone for the feedback.

Sounds pretty conclusive that going down to the 42" frost line is overkill in this situation.

I will likely go with a slab with thickened perimeter / trench foundation - roughly 16" wide by x 24" deep as @Dogmeat did. The city strongly suggests a 24" + deep concrete for rodent protection aka ratwall.

Just need to decide on division of labor.....hire out slab and then maybe pour/use concrete block to do stem wall myself. Or just hire it all out (I'm expecting this would end up being two pours with the stem wall, or complicated forms, and be a decent chunk of $$ more).

Option 2 is to take pieces from a frost-protected shallow foundation design. It involves board insulation "wings" which go horizontally out from the foundation to trap the heat of the earth as well as crushed stone. If you google search "frost protected shallow foundation" and find the HUD.gov link with a .pdf you will find the recipe-book.

This is a good suggestion, but wouldn't work in my situation. The 2015 michigan residential code that my city uses only allows this for heated structures maintaining a monthly average over 64F. I only plan to heat when bringing a car in for maintenance work.

Full code text:
"For buildings where the monthly mean temperature of the building is maintained at a minimum of 64°F (18°C), footings are not required to extend below the frost line when 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)."

 

[Jtrudel90]

Full code text:
"For buildings where the monthly mean temperature of the building is maintained at a minimum of 64°F (18°C), footings are not required to extend below the frost line when 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)."

this is a little confusing maybe someone could clarify the second part of this.

Coles notes:

If you heat the building you don't need need 4 foot frost walls (full foundation) as long as the slab is insulated with foam board as described in post #10

If you build a frost protested foundation you cant use it for an unheated space that doesn't have a minimum maintained temperature of 64°...?

 

[dtbingle]

this is a little confusing maybe someone could clarify the second part of this.

Coles notes:

If you heat the building you don't need need 4 foot frost walls (full foundation) as long as the slab is insulated with foam board as described in post #10

If you build a frost protested foundation you cant use it for an unheated space that doesn't have a minimum maintained temperature of 64°...?

That is how I understand it. The foam board insulated shallow frost protected foundation, can only be used for heated structures maintaining that minimum average temperature.


For those michiganders above who had garages built, do you remember pricing on your concrete work?
Just got pricing back from one contractor....$8500-9000 range. Honestly I was expecting $10-12k.

The first quote was based on:
18x24 structure
Includes 24" deep ratwall x 8" wide and has a stem wall extending 8" above grade.
Interior slab is 6" thick fibermesh with rebar to support future car lift.
Includes excavation and site prep work (flat area with part dirt part grass and ground is pretty easy to dig in)