Floating Foundation?

[Michael Masney]

I had a client call regarding an inspection and he said the home had a "floating foundation". The house is 1/3 basement and 2/3 crawlspace. I have never heard of a floating foundation. Any clues to what he was talking about?

[ccoombs]

In my area a floating foundation is typically poured at the garage. The footings and stem walls around the garage are poured first. Then the forms are removed and the garage slab is poured. Because there are no ties between the stem walls and the garage slab it is called "floating." This detail can also be used at a basement.

Based on the information you provided, the "floating foundation" could be the garage slab and/or the basement slab.

[Keith Swift, PhD.]

Michael, I agree with Curtis, and often refer to "floating" slabs in garages, or "non-strucural" slabs, which means that the cracks that one commonly finds in them have no real significance.

[rbunzel]

I think a more appropriate term was floating slab or floor. We had these in northern colorado where we had expansive soils. The foundation walls were poured and piers pours for the support columns. The floor was poured separately using asphalt strips as a buffer to foundation and piers. The idea was that if the soil moved the floor would float independently. It only worked about 50% of the time. The rest of the time the slab would bind and crack alarming the home owner even more.

Ultimately the soils won and builder gave up on concrete floors and started suspending wood floors off the foundation walls.

//Rick

[Claude Lawrenson]

http://www.geotecheng.com/papers/fou..._and_risks.htm

http://www.homeinspectioncourse.com/...undations.html

http://home.howstuffworks.com/house4.htm

[Russel Ray]

As described in the above posts. We called them floating foundations along the Gulf Coast in Texas and Louisiana.

[David P. Valley]

FLOATING SLAB


MONOLITHIC SLAB

[Michael Masney]

Thanks for all the help, I believe the client meant it was a floating slab basement floor. Not so sure I would use the term floating foundation!

[Joe Farsetta]

Be advised that some municipalities allow what may also be knows as a floating foundation.

This is where pre-cast concrete foundation walls are set atop of tamped gravel. No footings are required. Foundation walls float. Slab components are then poured, and the concrete mix pulled in between vertical support sections built into the pre-cast, and spaced out at 3 or 4 foot intervals.

I dont like it too much, but the NY State Building Code allows it, and I'm seeing more and more of it, expecially with modulars.

[Robert J. OConnor]

As others indicated there is a big difference between a "floating slab" and a "floating foundation"

A true floating foundation is typically only found in areas with expansive clay soils (some areas of TX are a prime example). This is another good link on those types of foundations ... http://www.houston-slab-foundations.info./

As Joe indicated there are what could be called "floating foundation walls", but I tend to just think of them as precast thickened foundation walls. A similar situation is where the foundation wall width is made equal to the minimum footing width (there is no separate pour for footing and wall). Works well when there are good bearing soils, and have specified this myself on several jobs (mostly crawl space additions).

JMO & 2-nickels ...

[Marcel R. Cyr]

Boy I would be happy to see floating foundations in this neck of the woods.

The building I am doing right now had footing under the steel columns 10' x 10' x 2 feet and bearing on ledge.

The last Hospital Building I built, had footings 15' x 24' x 4' thick at all the corners of the steel columns of the Building.

I asked the Structural Engineer as to what the hell he was doing, and responded that it was designed for Ballast, I could not help but ask if he was afraid the Building was going to fly away. ha. ha. He was not amused, but that is alright. I just based my observations on having never seen a building fly away in all the years I have been around in this World, but that has only been about 39 years, right.?

I believe, the pics David Valley introduced would be more appropriate to define what floats and what does not and subjected to external forces due to the Geographic Locations of the U. S. might introduce special design concepts for Mother Nature at it's work.


Marcel


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[Cynthia Daugherty]

If anyone would like to view the consequences of placing a "floating slab" on expansive clay: please visit www.justanotherlemon.com. This home was built by Pulte Homes in Lenexa, KS on grundy silt loam (expansive clay). Within months of construction, the slab showed signs of movement and over the course of 2 years the heave is unbelievable. The floating slab on support pads with isolation rings (the pads were formed/poured directly on the expansive clay) is a good theory, but eventually the support pads themselves began to "float". The structural damage is beyond words and the cost to cure is well above the original purchase price. This home is a good "lesson learned" for many.

[Brian A. MacNeish]

Originally Posted by dvalley

FLOATING SLAB


MONOLITHIC SLAB

david:

In the above, drawings the bottom of the footings/slab perimeter base appear to be from about 12" to 20" into the ground. What protection is recommended to prevent frost heave in areas where frost can pentrate deeper than the above #'s?

[Richard A. Hetzel]

Tough luck, Marcel...actually, a building flew away just over 40 years ago, so you just missed it. WE had the opposite problem once...10,000-gallon buried fuel tank, under a driveway...the contractor couldn't understand why we needed that big concrete anchor underneath it, so he left it out. The tank didn't STAY under the driveway for very long! One morning, there it sat, completely above grade...like a chick that hatched out of an asphalt egg. =)

[David P. Valley]

Originally Posted by Brian A. MacNeish

david:

In the above, drawings the bottom of the footings/slab perimeter base appear to be from about 12" to 20" into the ground. What protection is recommended to prevent frost heave in areas where frost can pentrate deeper than the above #'s?

If the slab is below a heated area, there is no concern as the frost shouldn't affect this slab.

However, no slab is immune to frost heaving, as I've seen it happen in a few homes in Massachusetts. The builder should be familiar with the frost table in your area (if not, you've got a lousy contractor). It is very important that the correct type aggregate be installed at the correct depth. This will be decided upon by your builder and the deciding factor will determine the long-term condition of a slab in freezing geographical areas.