Monolithic slab vs. "real" foundation

[OriginalSterm]

Got quotes for materials for stick built and for pole barn. I'm looking at a 40'x 32' x 12' garage with 3 doors and a 2 post lift. Went to the town for my permit and long story short, he's not a fan of monolithic slabs in this part of the country. His feeling was the monolithic slab would either crank with the ground heaving, or could be washed away with the swale near the proposed site. I've assured him the guy doing the excavation and concrete work knows what he's doing (lots of experience, lots of good recommendations), he still is having heartburn over that type of floor.

Are his concerns founded? The frostline here is ~42", the ground is clay, and I'm anxious to get started!

 

[dirttracker18]

Ha

Of course we don't know where you are since it's not in your profile.

However, I am in northern Ontario and mono slabs are the norm here. We get serious frost as well.

My slab is 30 X 36 mono with no cracks or washout and mine backs onto a hill. I have seen bigger mono slabs around here with no issues. It is really all about proper and thorough prep work.

All that being said, he's the inspector and what he's says goes. Right or wrong.

 

[Bender78]

My post & beam 28 x 32 has a monolithic slab; been in place for 15 years with no issues at all.

 

[mtwaterguy]

Doesn't really matter whether he's a "fan" or not. What does the towns law say about permitting a mono.

 

[KenC]

I like them. But, I also like lots of rock fill under the edges, say 4' back from the edge, to drain water away. With out water, frost heave shouldn't be an issue. But if water is trapped under the edges of the slab, above the frost line, it can push it up.

 

[Stuart in MN]

Slabs are pretty common in Minnesota and it gets plenty cold here. My garage is over 15 years old with no issues; the one car garage it replaced was built on a slab in 1948, and the floor was still in good condition on that one as well.

 

[kwb]

My shop is a Monopour - which is fine. Decade later and no issues with the structure but we rarely get frost that deep in PNW.

What I wish I had was a stem wall to let things hit before hitting the wall and even with a PT plate I really don't like hosing out the shop because of wood & water.

 

[OriginalSterm]

Guess I need to state where I am for this to be a helpful question. I live in upstate NY and normally have pretty rough winters as far as snow and cold go.

Is any sort of heaving going to happen and what can I expect if I have a lift installed?

 

[Gary S]

Mono slabs are used a lot here without problems. Frostline here sinks to 72" in a cold winter. Depth of frost isn't a real concern, but type of soil is. Sandy soild handle any kind of building better than clay soils.

 

[ConCretin]

Monolithic slabs work fine as long as they are not connected to other structures and they are built on well drained, frost resistant materials.

I'm in Maine where we get 4' or more of frost. I have a 32x28 on a mono slab and it's worked perfectly. I went a bit heavy on the rebar and suggest you do the same. I have 2, #5's in the bottom of the haunch and #4's @ 12" oc, ew for the slab itself.

 

[brownbagg]

mono slab are 99% of all slab here, but one thing to remember. an Inspector cannot by law, alter, revise an engineer plan, all he can say is, does it meet code. So bascially you can tell him to blow it out his …. you dont care if he like it or not. If it has an engineer stamp. shut us and issue the permit

 

[Steevo]

I am in Eastern Idaho, near Idaho Falls (43° 28' 0" N ), and here they require a "real" foundation footer to avoid frost heave.
My footers are 36" below grade, with 18"w x 18"h footer topped with 12" wide foundation walls that extend up 18" above grade, with the slab poured separately inside the foundation.
It was not cheap.
I think I spent $9500 for the foundation and slab on my 24' x 40' shop.

 

[ForceFed70]

42" frost line? Wow that's deep. So the edges of the slab are actually dug 42" deep?

While I do think that the standard slab/foundation design is better, there is no reason why a monolithic pour wouldn't work. Especially for something like a garage.

 

[TurnipTruck]

If the ground drains water away, monolithic is great. Just ask the interstate highway builders.

 

[ConCretin]

42" frost line? Wow that's deep. So the edges of the slab are actually dug 42" deep?

While I do think that the standard slab/foundation design is better, there is no reason why a monolithic pour wouldn't work. Especially for something like a garage.

Monolithic slabs basically float on the frost. If constructed on a proper subgrade they move up and down uniformly. Just don't hook em to another building.

 

[Stuart in MN]

42" frost line? Wow that's deep. So the edges of the slab are actually dug 42" deep?

Thickened slabs float on top of the ground, they're generally 4" thick in the middle and 12" at the edges.

 

[ForceFed70]

Thanks guys. I know the design of the monolithic slab, but I always thought that the idea was to have the edges go deeper than the frost line.

So if it's above the frost line and frost heaves the slab, how does it not crack? It can't possibly heave evenly across the entire slab.

 

[ConCretin]

Thanks guys. I know the design of the monolithic slab, but I always thought that the idea was to have the edges go deeper than the frost line.

So if it's above the frost line and frost heaves the slab, how does it not crack? It can't possibly heave evenly across the entire slab.

A mono slab typically has a haunched edge of around 12" - no need to go below the frost line.

 

[Sureshot]

I built a 26x26 in town years back and had to put pilings under grade beam for a foundation. Then poured the floor a year later. Way overkill for a garage on flat ground but it was what the a@#!le wanted. The rules were changed shortly after. My current 42x60 is a polebuilding with a slab but is heated all year.

 

[Mr onetwo]

Monolithic or "alaskan" slabs depend on good drainage.You mentioned clay soil...you've got to bring in lots of gravel and make sure it drains properly...don't create a "bathtub".The frost level in Maine is 48".I put an plain slab under my 24 x 32 garage.Full 6" thick; 4000 PSI concrete and 1/2" rebar on 12" centers with 4 bar ladders around the edge..No thickened edge and no control joints.Gravel was compacted in 12" lifts before pour.1" blueboard under.Not one crack of any kind after 3 years!All these extra details cost around $500 extra....well worth it IMHO

 

[OriginalSterm]

The guy doing the concrete work said he'd have to deal a bit with the swale, was going to dig it down a little and put in a culvert pipe with wash stone and cover it up with crusher run. Also mentioned needing about 10 tons of stone for under the monolithic slab.

 

[joes169]

From the sounds of it, you may be better suited with frost walls under the garage. I take it the existing elevation from front to back (or side to side) is a few feet different? If so, a frost wall assembly is the more efficient approach in many cases.

- Have footings dug & topsoil stripped from center of garage, leave on site for backfill & to raise grade at back of garage.

- Pour continuous strip footings one day,

- Set & pour walls the next.

- The third day, the forms get stripped, and it sits a day or two before backfill. You can start building on it at this point and pour the floor later. The best part of it is that the floor can wait until the heavy const. work is done, meaning less abuse to it.

 

[OriginalSterm]

The swale will be in front of the garage, running parallel to it. There is ~18" of natural grade from the back of the proposed garage to the front as it sits today, before excavation.

 

[joes169]

The swale will be in front of the garage, running parallel to it. There is ~18" of natural grade from the back of the proposed garage to the front as it sits today, before excavation.

That's far simpler than I expected. I see no reason why a monolithic slab, with a culvert in the driveway in front of it, wouldn't work. If the inspector continues ot give you flack, you may want to ask him for an example of "a monolithic slab that slid into a swale"...........

Don't hold your breath though, as the example doesn't exist on such a small grade change.

 

[financer83rs]

I live in Maine i have a monlithic slab that i use as a half court baskeball court. It is 28'X50' poured it in 2006. I put 1" foam board under the whole thing. No cracks or any issues. This is the second one i have done. The first my X wife has From what i can tell that one looks good still and it was poured in 2000. That one was 26'X48'... Had to go bigger on the second one

 

[FATBOYINMAINE]

are you planning on framing the walls at grade, or with a knee wall? a lot of garages have a 2 foot wall to keep the rain water from splashing onto the wood and rotting out the bottom of the walls. with just a slab you could go with concrete block, or use the footers/wall and bring it up a few feet above the slab. compare the prices of each. i would and will be going with a mono slab and concrete block on my 32x48 build in Maine, just my 2 cents

 

[ddawg16]

I've been learning a lot about foundations over the past 3-4 years....interesting how they vary from different parts of the country. This is basically what I have figured out...

If you live in earthquake country (AKA California)...slab or crawl space....your going to be down about 2' with a min 15" footing...

Somewhere between the East and West Coasts and below the serious "Freeze your *** off states"....pier and beam or slab using those tension cables.

Up north where it gets really cold.....I pretty sure they do basements because they have to go down so far....might as well make a room out of it...

 

[buening]

I don't think it has been mentioned (sorry if I missed it), but is this a detached garage?

 

[OriginalSterm]

detached

 

[p_mori7]

Go with a slab over 12 " to 18" of compacted gravel.

Mine is 5" thick in the middle, 12" along the sides.

Excavate all organic material away first until you hit undisturbed soil, then build up your gravel base on top of that. The gravel base once compacted should bring you to the same level as you were before.

See the slab pics in my build thread. Virtually all detached garages around here are built this way, and we get serious frost. Reinforced quality concrete is the key.

A long time ago, I had read a document from Ontario gov't about frost heave on such slabs. With the right base, the heave was less than 1/2" IIRC. Without the base, the heave could be 1"+.

The key here is weight. All that gravel & concrete is ****** heavy, and water will want to find the easiest route away.

Proper roof watershed drainage is a must, and the downspouts must carry the water at least 3' from the slab. Since you are on an incline, any uphill watershed will need to be diverted away from the slab as well.

 

[KPSquared]

Depends on where you live and what local building code requires.

I live in Alberta, Canada and we're not allowed a thickened slab on anything over 760 sq.ft.

My 30x36 has 4' frost walls on 24" footings. It was either that or an engineered grade beam.

Local codes are there for a reason. . .check what is required and stick with that.

 

[buening]

You might find this article helpful: http://www.concreteconstruction.net/Images/How to Prevent Frost Heave_tcm45-341398.pdf

 

[StingRay]

I'm in Saskatchewan and it can get really cold here. Water lines are at 8 feet to stay below the frost line. My 28 x 44 is on a floating slab and is over 10 years old. The floor has nothing more than a few hair line cracks. I would agree that a well prepared base and good drainage are significant but the same can be said for avoiding problems with most foundations. I've seen basement floors here that are in wet areas that heave like crazy and they are 8 feet down. My parents house in such an area. Dad made sure that the basement had great prep, extra gravel and extra drainage and has no problems with heaving.

 

[6768rogues]

The slab will work but I, too, am concerned about the clay soil. Clay is very expansive and it should be removed to a suitable depth and engineered compacted fill installed.
By the way, 12 feet high is cutting it close for a lift. I would go a little higher. My 14 foot ceiling has been close when putting a truck or van on my lift.