Footer or Monolithic?

[TractorJeff]

Has there ever been a discussion on Footer and Wall verses Monlithic Slab as far as quantity of materials and Labor cost? I was discussing with a neighbor down the road about it, he believes that the quantity of poured concrete in his footer and walls equals the cost of a monolithic pour.
The advantage I see is that the walls can be done for the Carpenters to start framing, then concrete floor can be poured after as time allows for their availability. Most Concrete guys are booked out a couple of months now.

 

[FANTM58]

The cost of my stem wall/ caisson is more than the slab cost.
4'x 10" walls plus 10 caissons. But this is due to the soul conditins
On my site. And true , my framer will be able to start, before the
Slab is poured. I have about 15 k into the foundation before
The slab. It's a 40'x60'. It will have a 5" slab with 2" foam
And hydronic heat in the floor.

 

[ForceFed70]

Having a stem wall that goes above the slabis handy for water/moisture control. This could be replicated with blocks.

Biggest reason for stem wall/footer vs monolithic is depth of frost protection. In my area, the footing needs to be a minimum 2'8" below grade - many areas require 4'. Can't really do a monolithic pour in that scenario.

 

[rburke65]

Your neighbor may be correct, but I would think that you Building Dept.....local codes.....would determine if you need one or the other. Wisconsin? I would think you'd need a footer n stem wall....but what the hell do I know.

 

[Kaizen]

My area requires 48 inches for a frost wall. As I wanted an economical garage I went with an Alaskan slab that has a thickened edge. After that I had them pour a 1 foot stem wall. Not much time between the two pours. A pro set up can frame and pour a stem with an Alaskan slab. My guy just preferred it this way. Doing a footer and wall would have at least doubled my price on the concrete and I would have needed more excavation work.

 

[matt_i]

"trench foundation" which is very common around here for garages/outbuildings involves removing sod + organics in the center, also a pile of washed limestone also can be dumped. Then excavating a trench ~8" wide to the frost depth. People I've talked to have done this with mini excavators and also with ditch-witches.

One forms with something like a 2x10 or 2x12 above ground, spread and compact gravel, install vapor barrier and rebar, and fill the entire thing in a monolithic structure, the slab is screeded to the top of the forms and finished flat at the end. Anchor bolts installed while 'crete is still wet.

If all the work can be lined up correctly (which is hard if you are coordinating all of the subs), it can be done very quickly, basically excavate one day, build forms the next, gravel and rebar the next, pour on the 4th day, saw the joints and start framing after that.

The only downside I see is that the outer surface of the concrete is "rough" meaning its just formed by the soil itself. If there were ever to be a significant frost event, there's more "tooth" for the frost to grip & lift the wall, as opposed to a smooth-poured stem wall with steel or plywood forms or an ICF wall with the foam barrier in between.

 

[ForceFed70]

How do you compact it when it's only 8" wide?

Regardless, it wouldn't pass inspection in my neck. Code calls for 16" wide footers. Not sure I'd be comfortable with only 8"

 

[matt_i]

One could of course dig it to 16" wide with a matching excavator bucket. However that's a significant amount of concrete in the ground. The general idea I believe is for a garage or shop the concentrated loads of a 2 story structure like a home aren't there.

As far as compaction, the foundation is poured into undisturbed earth. One would compact the gravel base in the center as well as possible but have to back off near the edges of the trench.

If you run some quick numbers, suppose you built a 1000 sf building 25 x 40 and loaded the attic up to 100 psf or 100,000# of total load (walls framing attic roofing snow loads, seems pretty high to me...), that's all going into 130 linear feet of foundation that's 8" wide. So 100,000/ (130 * 8/12) = around 1150psf bearing on the soil. Still approx one half of the 2000psf that standard soils are supposed to be able to bear.

 

[ForceFed70]

One could of course dig it to 16" wide with a matching excavator bucket. The general idea I believe is for a garage or shop the concentrated loads of a 2 story structure like a home aren't there.

As far as compaction, the foundation is poured into undisturbed earth. One would compact the gravel base in the center as well as possible but have to back off near the edges of the trench.

I wouldn't be comfortable with that. The edges take all the weight. It's where you need the compaction the most. I suppose virgin soil would work if you trust your excavator operator. I'd think you could compact a 16" trench tho regardless.

Don't you have to deal with snow loading in MI? Snow load typically represents much more weight than a 2nd story.

 

[n20junkie]

The only advantage to a monolithic slab is price, possibly.

People talk about frost protected slabs and the sort, but they never mention that these are designed for HEATED buildings. Not sporadically heated, or unheated structures. This means that without extending below the frost line, the building will certainly move if your in the north. It all depends on how much water is under the building, but some will move way more than you think.

Maybe the movement is fine for the owner. For me, I chose to do a full foundation with drainage and not worry. I am not building my one and only garage to have the worry of such things.

 

[Always_Thinkin]

If you think about it, considering a typical wood frame garage the walls would sit on the perimeter of the monolithic pour. Every case i have seen where a monolithic pore is used the floor cracks. This is because more load is carried by the outside perimeter of the monolithic slab than the middle of the slab. So the middle does what is called anchoring and the soil beneath the perimeter footing settles because of the higher load it sees versus the middle of the slab so then it cracks your floor. In my opinion, a stem wall poured and then a floating slab poured inside is the only way to go...

 

[dbabicky]

I live in NE Wisconsin and had my own concrete business for years. Most garages and pole barns up here were just poured with an 8" X 10" grade beam around the perimeter. Never had an issue with any of them. I have people come up to me all the time yet and compliment me on our work and how well everything stood up to the test of time. Last slab I poured was 7 years ago.

 

[matt_i]

People talk about frost protected slabs and the sort, but they never mention that these are designed for HEATED buildings. Not sporadically heated, or unheated structures. This means that without extending below the frost line, the building will certainly move if your in the north. It all depends on how much water is under the building, but some will move way more than you think..

If one reads the .pdf there are FSPF design guidelines for always-heated buildings, and un-heated buildings (see Page 10 for details). The heat is in the earth, its just a matter of entrapping it with insulation to protect the bottom of the foundation, and making sure that water which collects has free conveyance out of the area underneath so it doesn't pool and freeze solid (by using washed stone)

https://www.huduser.gov/Publications/PDF/FPSFguide.pdf

 

[finn]

I live in NE Wisconsin and had my own concrete business for years. Most garages and pole barns up here were just poured with an 8" X 10" grade beam around the perimeter. Never had an issue with any of them. I have people come up to me all the time yet and compliment me on our work and how well everything stood up to the test of time. Last slab I poured was 7 years ago.

We're north of you, and footings are rarely used for utility buildings like garages and pole buildings, or even commercial construction.

I assume the people from other areas that have problems with slabs have other issues , like poor soil or drainage.

 

[dbabicky]

I agree.

 

[NUTTSGT]

Whether it's from poor construction or the screwed up Winter weather we get with inconsistant temp swings, I've seen plenty of stuff move over the years. My sidewalk tends to heave in the Winter beside the house garage.

That alone has made my mind up to always have a footer and block/stem wall to build on. I'd hate to build something nice only to have it heave during the winter weather and create problems.

 

[TractorJeff]

It will be close to a 5 inch slab with bulldozers and tractors parked all over it. In my lifetime I don't believe it will heave.
Dbabicky, which is cheaper a small foundation wall with a 6" stem wall on top or a mono-slab with the same 6" stem(curb) wall?
Want to come down and do it?
Can't find anyone here till later in the year.

 

[rburke65]

I've never heard of a stem wall poured without a footer.

 

[mxz1998]

I live way way up North of you guys. In the Northern part of Alberta(directly East of Juneau Alaska). Up here, 90% of garages are a monolithic slab with a thickened edge. They work very well. But that is probably to do with the fact that all the garages here are heated in winter time.

 

[Farmall450]

It will be close to a 5 inch slab with bulldozers and tractors parked all over it. In my lifetime I don't believe it will heave.
Dbabicky, which is cheaper a small foundation wall with a 6" stem wall on top or a mono-slab with the same 6" stem(curb) wall?
Want to come down and do it?
Can't find anyone here till later in the year.

I'd want more than close to 5" for any real weight.

 

[ForceFed70]

I live way way up North of you guys. In the Northern part of Alberta(directly East of Juneau Alaska). Up here, 90% of garages are a monolithic slab with a thickened edge. They work very well. But that is probably to do with the fact that all the garages here are heated in winter time.

Funny how the code differs. I'm in south central BC and code said 8" thick footers that are a minimum of 2' below grade. 2'8" total depth.

Could only get away with a "slab on grade" if it was for a temporary structure/shed. Anything bigger and they made me follow same code requirement as a house.

 

[Farrier]

Funny how the code differs. I'm in south central BC and code said 8" thick footers that are a minimum of 2' below grade. 2'8" total depth.

Could only get away with a "slab on grade" if it was for a temporary structure/shed. Anything bigger and they made me follow same code requirement as a house.

State by state, county by county; it all varies. Love to read the arguments on here about what is or is not code

 

[TractorJeff]

Well?
As I need to go forward, I may just end up pouring a Footer and laying 4 courses of Block. Layed lots of block when I was younger, don't want to but if I can get the building up not having to wait for the Concrete guy, I can always get the floor poured later.

 

[mxz1998]

Funny how the code differs. I'm in south central BC and code said 8" thick footers that are a minimum of 2' below grade. 2'8" total depth.

Could only get away with a "slab on grade" if it was for a temporary structure/shed. Anything bigger and they made me follow same code requirement as a house.

That is odd. I just built a 26x32 garage and the code was for a 16"x16" thickened edge with 6" thick slab(which is way overkill in my opinion)

 

[ishiboo]

I wouldn't be comfortable with that. The edges take all the weight. It's where you need the compaction the most. I suppose virgin soil would work if you trust your excavator operator. I'd think you could compact a 16" trench tho regardless.

Don't you have to deal with snow loading in MI? Snow load typically represents much more weight than a 2nd story.

How do you think pole barns are held up? I'm over-building mine with 24"x8" footing pads, with posts 9' on center for a 50x63' building. That means the weight of the building sits on 18*3.14 = 56 square feet of concrete. With a 8" footing all the way around it, it seems that would be (8/12)*(50+50+63*63) = 150 square feet of concrete it'd be resting on.

Despite being the frozen north, lots of these type of buildings are built on monoslabs.

 

[forAK]

Most I've seen on single story buildings or garages are a 16"x16" thickened slab. If there's going to be a crawl space or basement or second story, then a footer with stem wall is utilized. If the dirt work is done correctly, a mono is all you need for a shop. Compaction is key along with proper drainage and insulation around the perimeter will keep things from moving.

 

[ForceFed70]

How do you think pole barns are held up? I'm over-building mine with 24"x8" footing pads, with posts 9' on center for a 50x63' building. That means the weight of the building sits on 18*3.14 = 56 square feet of concrete. With a 8" footing all the way around it, it seems that would be (8/12)*(50+50+63*63) = 150 square feet of concrete it'd be resting on.

Despite being the frozen north, lots of these type of buildings are built on monoslabs.

Yeah, I think you misunderstood my concern. I don't think the building will sink into the ground (more than an inch or 2 anyway). I do think there's a good chance a monolithic slab will crack if the perimeter isn't compacted. The poles in a pole barn don't rest on the slab so that concern doesn't apply. I also suspect that you're planning to compact the ground under your footers are you not?

 

[ebonyswan]

Up here (Mission, BC, Canada) if I do stem wall and slab, no engineer. If I want to do monolithic floating slab, geotech eng has to sign off.