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Floating Slab or Footings Below Frost Line

02/24/2017 2:54 PM

I have a question. First a little background.

My girlfriend wants me to build her a small 8’x10’ garden shed. We were looking into it the past 3-4 years. At first she was looking at the ones you purchase at the big box stores (Rubbermaid), and some that are constructed like at home Depot, Menards or Fleet Farm.

She never cared for that. She made a sketch which is quite cute that goes well with her garden, nice over hang, Copula, trim and scroll work. Kinda whimsical, and being whimsical, it will be quite an investment.

The problem is, she wants a cement floor.

I’ve build buildings before, more so on the farm. So, I would start at placing the footings below the frost line. Where we live the frost line is 3-4’ deep, and then a wall up for the bottom plate, (sill).

At first that’s what I was going to do, then I started thinking, if I could put drainage in and underneath the slab and above the existing grade it could work.

So,..,I started to search the internet and I find what I had in mind. It’s called Monolithic or Floating Slab Foundations, with a detail of it here.

With the building being as small as it is, my question is, has anybody have experience with this? And what are your thoughts.

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#1

Re: Floating Slab or Footings below frost line.

02/24/2017 3:08 PM

From the detail pdf....

"This foundation is intended for use in areas where ground frost penetration is less than 12" below grade and in colder regions where floating slab foundations are specifically permitted by code for use on garage and accessory buildings, lt should not be used over organic soils, soft soils, muck or expansive soil types."

I would find somebody with one of these...and make a trade agreement if I could....

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#3
In reply to #1

Re: Floating Slab or Footings below frost line.

02/24/2017 3:33 PM

That was the first design,... the link to the second design was;

2. PERMEABLE BASE MONOLITHIC FLOATING SLAB FOUNDATION – These details provide additional frost heave protection by under laying the entire monolithic foundation with water-permeable gravel.

I wanted to get some type of background on this first....

But this is called a Alaskan floating slab, even though it was first developed in some Scandinavian country.

An excerpt;

The Alaska slab foundation and other FPSF designs have proven to be effective for heated buildings, but what about unheated storage buildings like garages and barns? Well, the Alaska slab foundation was the inspiration for a good solution for small storage buildings too. Using a similar thickened slab, either floating on the ground, or at a slightly higher elevation than the ground around it, works well at keeping water away and preventing frost heaves.

Besides reducing the depth of trenches and the amount of concrete required, a floating slab foundation can be poured all-at-once or monolithically. That means just one concrete delivery and one drying period, for savings in both time and cost.

Basically, the design itself is having enough drainage to keep it from heaving. I just wanted some feed back and if someone I familiar with this so I can get some background information before I bring it up to the local board for a building permit..

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#5
In reply to #3

Re: Floating Slab or Footings below frost line.

02/24/2017 3:51 PM

10-4 ... I guess it all comes down to the soil type, and methodology.....I would just be concerned with possible sinking or tilting of the slab....On a gravel bed with proper elevation and firm soil, it should be fine....

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#6
In reply to #5

Re: Floating Slab or Footings below frost line.

02/24/2017 3:59 PM

I'm sure there's going to be settlings and movement as winters pass. And I wouldn't be surprised that after a few winters, it may not be at the original plumb.

That I can live with.

I'm leaning towards that if someone doesn't come up and point something out.

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#24
In reply to #6

Re: Floating Slab or Footings below frost line.

03/08/2017 12:35 PM

After the garden shed is (fully) completed, can you then negotiate for your own (man-cave) structure, to be built as a low-profile, anchored, (sled-shed)?...

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#25
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Re: Floating Slab or Footings below frost line.

03/08/2017 12:38 PM

We made shelters for our horses on sleds.... Its been pretty windy lately and still is, with wind gusts over 50 MPH... they can pretty mush tip them over at gust at 70 MPH.

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#2

Re: Floating Slab or Footings below frost line.

02/24/2017 3:28 PM

I am in Wisconsin too. I have several out buildings. Some were built with footings below the frost line. some on floating slabs. They both work well.

For a garden shed I would go with a floating slab with a grade beam around the outside edge.

The grade beam probably isn't even necessary if it is only ever going to be used as a garden shed but you can never tell how something will be used in the future and as long as you are pouring it you might as well add a couple inches of depth and some re-bar just around the perimeter.

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#4
In reply to #2

Re: Floating Slab or Footings below frost line.

02/24/2017 3:40 PM

I'm from Green Bay, WI

That's what I thinking,... and being a smaller building, and having a reinforce slab, I think it'll be fine.

And there were a few other concerns.

One other one is that the soil does have a light clay mix., I may need to drop some pilings for footings.

Its just that even though its a small building... but just the material investment I need to look at other possibilities.

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#7

Re: Floating Slab or Footings Below Frost Line

02/24/2017 5:05 PM

Around here the frost line can go to 5+ feet and there are 10 of thousands of old building, and slabs from buildings that don't exist any more, with many that are pushing 100+ years old that were made with bare bones floating slabs and very few are out of level to any noticeable degree.

Unless you have soil that's naturally prone to frost heaving due to naturally high water tables year round a basic floating slab is just fine.

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#10
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Re: Floating Slab or Footings Below Frost Line

02/24/2017 5:27 PM

We built a 80'x100' manure slab. And that was floating. The pencil neck government engineer said that will never last. Well, it lasted 25 years, until we had to expand and break it up for the expansion... and the government engineer... his design begin to fail after 2-3 years.

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#14
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Re: Floating Slab or Footings Below Frost Line

02/24/2017 7:48 PM

About a mile from my place there is old abandoned farmstead with a ~30' x 40' slab from a old barn still there. Best guess on age by remaining artifacts found around place is it's pushing the 90 - 100 year mark.

looking at where badgers dug under the one side it's about a ~ 5" thick floating slab.

Less cracks and shifting than most of the sidewalks and public works found around town made in the last 10 years (made to code) too.

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#19
In reply to #10

Re: Floating Slab or Footings Below Frost Line

02/27/2017 11:53 AM

... and that's why he works for the Public Sector (i.e.: ''gummint'') and not in the Private Sector...

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#22
In reply to #7

Re: Floating Slab or Footings Below Frost Line

02/27/2017 1:58 PM

I remember visiting a friend in Upstate Michigan, who described to me, in detail, the methods of slab construction, basically a " lens-type construction" as the soil was almost all sand. Thinner in the middle and thicker on the edges, and it basically floated on the subsurface like a contact lens. I would imagine this is specialized style , as the subsurface is very consistent, so the slab would not be subjected to different stresses.

I vote for post and beam, putting in caisson, or pier footings, below the frost line, and using beams to span the problem areas, standard joist spacing, 16" O.C., and a good , above grade , plywood floor, with whatever you choose, over that, like a house. Rigid foil backed foam in between the joists gives one and option for insulation. Slabs are awfully cold in the winter. The finish surface could be vinyl, tile, cement board etc. We normally put down a few inches of 3/4 gravel, then 2" of sand, w/ a vapor barrier (6 mil visquine, pond liner, or the like) , and an air gap between the floor and the plastic. This type of construction allows for flexibility and customization, does not need to be monolithic, and allows for a little movement with temperature and humidity.

I am sure there are many ways to approach this. I would examine all local sources of advice first, to see what has worked and what hasn't, before committing lots of time and money.

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#27
In reply to #22

Re: Floating Slab or Footings Below Frost Line

10/23/2017 7:32 AM

the methods of slab construction, basically a " lens-type construction" as the soil was almost all sand. Thinner in the middle and thicker on the edges, and it basically floated on the subsurface like a contact lens. I would imagine this is specialized style , as the subsurface is very consistent, so the slab would not be subjected to different stresses

Thought I'd add this. Sand is worse than clay as a substrate. very unstable.

I recall, when I was on the farm, from post #10 slab we laid it on a sandy type loam. we did it just the way you describe... only we did it in one pour. One solid slab. Against all convention and recommendations.

Never had a problem. until we expanded and had to break it up. the size was a little bit bigger that 100'x80'. Sometimes one is best to be lucky than good.

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#8

Re: Floating Slab or Footings Below Frost Line

02/24/2017 5:19 PM

Just in case: do you have to deal with any of those fee collecting code people?

Beyond "the code" I can't give much of an opinion. It has been 3 years since it got cold enough for me to bring in the orchids. In a REALLY BAD winter our frost depth is about 1/16".

At times I do wonder about foundations. About 20 years ago it was so wet that I could hit water at about 4 inches. I have a coquina shelf 6' down and the water was trapped on top of the shelf. I had a floating foundation and don't understand why none of us had our slabs crack.

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#9
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Re: Floating Slab or Footings Below Frost Line

02/24/2017 5:24 PM

If I were to build the floor like a deck with posts going below the frost line, I wouldn't even need a building permit. But when my girlfriend upset... everyone is upset.

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#11

Re: Floating Slab or Footings Below Frost Line

02/24/2017 5:36 PM

Living in Michigan, North of Detroit, when I was a kid, my father built my mother a slab for a 12' x 8' green house, no foundation, just a 8" rat wall, it never cracked. I live in a mobile home, I have a 10' x 12' shed on a slab, again with no foundation, just a 6" rat wall around the edges, It was poured in 1974, and is intact, no cracks, above the frost line (I know this because I dug under it and drilled through it, to run electricity to it.) So my shed heaves up and down with the frost, but is intact. I would guess floating is ok as long as your local building codes allow it.

By the way, our building codes say our foundations (like my deck that I built) need to be 42" below the surface, but do not include slabs like sheds, greenhouses, ect...

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#12
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Re: Floating Slab or Footings Below Frost Line

02/24/2017 6:01 PM

Sheila wants a cement curb also. And i like it, it keeps the sill dry.

I'm leaning more towards the slab. And looking forward to starting it. It's going to take the whole summer, since I was just told, I'd be traveling quite a bit for work.

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#13
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Re: Floating Slab or Footings Below Frost Line

02/24/2017 7:07 PM

8' x 10' here you can get away with a slab, and I say said slab will be ok, but your local building codes will decide that. We have had extreme weather in the last few years. Since the beginning of 2017, we have had -5 to +70F in a couple week period. It is almost 70F right now, tomorrow morning is supposed to be in the 20's, then back to the 50's by Tuesday. We have had frost heave and my 43 year old shed slab is fine.

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#17
In reply to #13

Re: Floating Slab or Footings Below Frost Line

02/25/2017 9:39 AM

Yes, because it will be a cement floor, its a considered a permenent structure and I need a building permit.

making a sketch of it now for approval.

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#15

Re: Floating Slab or Footings Below Frost Line

02/25/2017 4:44 AM

You've managed to string it out for 3-4 years already without doing any work. With luck researching the slab could push it back another couple of years. Do you really want and answer or do you just want to make it look difficult.

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#18
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Re: Floating Slab or Footings Below Frost Line

02/25/2017 9:43 AM

I really wouldn't call it stringing it out, it's a list, it's just now at the top of it. The items beneath it I would get to after this.

It's nice of you to at least show up and show your a smart ass.

Is there anything else you can add?.... of use I mean. If not,... then make yourself useful, and move on and enjoy yourself...

thanks

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#16

Re: Floating Slab or Footings Below Frost Line

02/25/2017 6:45 AM

In High school I worked for a guy that did concrete, and we did some garages that were floating slabs. I do remember key points:

  1. Has to be freestanding. You don't want to attach it to a building with foundation as the one is supposed to move and the other isn't.
  2. if you go more than 15' x 15' it's best to form it so there are basically grade beams under near where it will get saw cuts.
  3. He always keyed the edge down anyways so it had a beam around the perimeter.
  4. Drainage is #1! If you don't have water you don't have problems, excpet maybe in perma frost.

I wouldn't do anything else. A good gravel bed, 4" of concrete with the edges tapered down closer to 8" and make sure the ground slopes away for at least 5' in all directions.

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#20

Re: Floating Slab or Footings Below Frost Line

02/27/2017 1:09 PM

Seems like 8' x 10' is small enough to still be built as a sled (sic), with diagonal bracing, that could then be moved around from low, wet spots to high, dry spots...

...and when the lowest wood started to (go ''bad''), and with proper design, it could still be readily jacked up, and replaced, indefinitely, thereby totally avoiding the frost line...

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#21
In reply to #20

Re: Floating Slab or Footings Below Frost Line

02/27/2017 1:16 PM

On the farm, I've build horse shelters (8',10' x 20') like that.

Unfortunately, my girlfriend wants a cement floor because its going to be permanent. Our yard isn't that big, on top of that,... she has quite of bit of perennials as well as annuals planted around where the building is going.

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#23

Re: Floating Slab or Footings Below Frost Line

02/27/2017 2:43 PM

In the mid '70s I built a 10'X10' storage building on a monolithic slab with integral footer and crushed stone insulation under it. Still fine today. This is in south central Pennsylvania where we get some sub-zero temps in the winter.

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#26

Re: Floating Slab or Footings Below Frost Line

10/21/2017 11:05 AM

To bring you contributors to this thread up do date... I like to share with you the final result from my initial question.

I posted it in Workshop Creations where it also has a link to more complete details.

thanks

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