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Cust. wants garage on floating slab

happyframer's picture

Customer wants a 24x36 16 o.c. garage.

To minimize costs, he wants a floating slab. I'm trying to convince him to get a full 4' foundation.

he insists it can be done so movement and cracking is minimized.

I know builders sometimes put huge pier footings in the middle of garages. Perhaps 3 of those under a 24x36 slab would work. How would that slab look?

Thanks for consideration.

BTW medium clay soil 25 mi. W of Minneapolis

(post #103502, reply #1 of 16)

What does the AHJ say about floating slabs? If they don't allow them, there's your out.

There are methods to frost heave proof floating slabs. Involves underground insulation. Called . . . . . . . . frost protected shallow foundation?

Well heck, a severe case of CRS just hit.

Other wise tell him "I sorry sir, but I refuse to build a mistake." and reccommend Shlock Builders, Inc, to him (|:>)



Anyone who doesn't take truth seriously in small matters cannot be trusted in large ones either. [Einstein] Tks, BossHogg.

A Pragmatic Classical Liberal, aka Libertarian.

I'm always right!
Except when I'm not.

(post #103502, reply #7 of 16)

Second method - use drains and hard compacted mineral fill to eliminate the water that freezes and cuases heaving.



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Oh Well,

We did the best we could...

(post #103502, reply #2 of 16)

SamT got it, do a search for frost protected shallow foundation.  Proven and accepted way to build.  You can also just do a floating slab without the extra step of "frost protection", but if you're in a northern climate you should expect some movement.  4' frost wall is always a safe bet.

"This is a process, not an event."--Sphere



(post #103502, reply #3 of 16)

I have done a couple. Vancouver island BC

Well draining base material and compaction is critical.

I have used a 5" slab thickness and 10" x 16" primeter footing with 5m rebar on a 16" grid with fiber.Keep it out of the sun too.

Engineer inspected and approved.

Cheers Rik

(post #103502, reply #12 of 16)


called it a "thickened edge slab"

If you didn't have time to do it right the first time, how come you've got time to do it over again?

(post #103502, reply #13 of 16)

The local garage contractor built my garage last year. Kansas City MO area. I always thought you had to have a 3'-0" stem wall foundation down to below frost here, but apparently not.

He used a thicked edge slab with 6 or 8 3'-0" deep piers. (Our slab was 22'x36', so he had to have the piers).

Passed city inspection. I know that doesn't mean anything, but this guy builds A LOT of garages around here.

He was half the price of my other 2 bidders, and we're happy so far. Like I said, one year old and no problems yet.

Garage in KCMO (post #103502, reply #16 of 16)

Hi Rasher - I know you wrote this original comment almost 10years ago, but I was wondering if your garage is holding up well still? 

We're located in KCMO as well, and we are looking at demolishing and rebuilidng our detached garage at our home. 

Right now some of the bids are pretty high for the concrete foundation, and wanted to know if you would recommend this peir method? 

also - if you remember the garage contractor you used, I would be interested in giving them a call for a bid. 

Any help would be much appreciated. Thanks! 

(post #103502, reply #4 of 16)

I have a 24x40 garage in Minneapolis that was built on a floating slab.  It's about 15 years old now, I have it loaded to the gills with cars and motorcycles (plus a four post lift) and the slab is holding up just fine.

(post #103502, reply #5 of 16)

In the midwest we use foundations down to frost line if the esterior is masonry. If it's frame with siding, then typically we do a slab with a thickened edge.

(post #103502, reply #6 of 16)

36' implies three bay garage.

You would have cracking so use control joints.

Structurally no problem.

What you do is remove organic top and lay drains in stone, then minimum 18" if compacted mineral fill, then form and build



Welcome to the
Taunton University of
Knowledge FHB Campus at Breaktime.
 where ...
Excellence is its own reward!



Oh Well,

We did the best we could...

(post #103502, reply #8 of 16)

In Michigan if you go over 20x20 you have to have footings/foundation. Unless you can build the whole thing over the weekend when city hall is closed.:>)

Even under 20x20 you have to at least have a rat wall.


Family.....They're always there when they need you.

(post #103502, reply #9 of 16)

I've never heard the term.

What's a rat wall?

(post #103502, reply #10 of 16)

Its like a baby foundation. Its a wall of cement that goes about 18" into the ground. Its designed to keep critters from tunneling under the slab.


Family.....They're always there when they need you.

(post #103502, reply #11 of 16)

Funny, I've never heard that term either, but we do call a thin sloppy slab a "rat slab" for similar reasons.



"This is a process, not an event."--Sphere

And I'm a legitimate certifiable Tool [JOBSITE WORD].--Dieselpig



(post #103502, reply #14 of 16)

built a 30x45 2 storey garage 15 years ago on a floating slab and a couple of years latter made an appartment out of the top storey. gyproc on walls and jointed. no joint cracks as yet! the key was to have good drainage under the slab so that the water that causes the heaving is not present. all we have is about 2 feet of packed stone under the slab and it seems to be working.

(post #103502, reply #15 of 16)

 Floating slabs are a great way to go when building single story structures.

My garage is 36 X24 on a floating slab. My slab is thickened on the edges ( about 10 inches) and the remaining slab is 8 inches thick. I have 1/2 inch dia rebar placed in a mat formation; 12 inches OC.  I had a compacted gravel base ( after first removing grass /topsoil) of about 16 inches before I poured. Garage is 12 years old, and no cracks.

As an industrial ( steel mill) carpenter in Pittsburgh, PA region, we built loads of floating slabs for small utility type buildings and for  even larger storage buildings...all buildings are in great shape.

Trick to stopping frost heave is to give water a place to drain, instead of a place to  saturate. You can and should place drain lines underneath compacted fill and "lead" drains so to carry water away from site. Rebar mat will give plenty of strength and you can add chopped fiberglass in the mix for good helps but no substitute for rebar.

 Don't be afraid of the "unknown" and actually, floating slab technology is well known...been in use for many, many years. Infact, according to history books  I have  read about Frank Loyld Wright, many of his midwestern homes were placed on floating slabs of his  own design.


Edited 3/22/2007 2:12 am ET by Davo 304