Why is the regional frost line considered inconsequential when pouring a concrete slab house foundation or a driveway, but of paramount importance when pouring a footer for a “conventional” building foundation? Up north, no builder would consider constructing a house on top of any less than a two foot deep footer, yet would not hesitate to pour the driveway on grade…50 years after the fact, the driveway is just fine. What gives?
Because with a footer, you are basically just pouring anchor points for the structure to attach to. If one footer shifts due to frost, the whole structure will be put under stress and bend.
If you pour a slab of concrete, it is part of the structure and because it’s one unit, if part of it rises due to frost it will rise in an even manner that doesn’t put the rest of the structure under stress (well, it’s supposed to anyhow – you can build on ground that’s not stable enough for it and crack the slab, and hoo boy is that expensive). Same thing with driveways…the cement pad as a whole will move. With driveways, though, the cement is not typically thick enough to fully resist cracking and degradation, but it’s inexpensive enough to repair that the massive additional cost to make it more crack-proof is seen as not worth it.
I used to live near a subdivision of small houses built on slabs. One fellow mmentioned that he had gotten a contractor to come in and level his slab - he had a water bed (yes, that long ago!) and the water was an inch below the frame at one end and above the frame at the other end. But… the slab did not crack.
I had a new house built a decade ago, and the concern was soft ground - the drilled down IIRC about 20 feet and poured concreted pilings to base the foundation on. The contractor said the alternative was a concrete foundation the perimeter of the house, belove frost line, about 2 feet by 2 feet in that area.
Slab on grade buildings still generally need foundations and footings (footer is a citation at the bottom of the page, and its use here is technically incorrect). There are ways to do a floating slab without foundations/footings, but usually they require a web of rebar and perhaps insulation such that the ground under the building can’t freeze and heave, and by tying the slab together with the rebar it resists any heaving around the perimeter. Even so, I think it’s more typical to turn down slabs at the edge not only to get to frost depth but also because that’s where the walls are going to be, so good bearing is needed.
That depends on how much frost, doesn’t it, and the distribution?
Where I live, we never have slab foundations. It’s always a basement, built to get below the frost line, so at least 6 to 8 feet.
In these parts a foundation has to run at least 40" below the grade to prevent movement. The foundation from ground freezing. I don’t think simple slabs are allowed at all. Deck posts and other supports all have go down 40". A driveway doesn’t have to do that because it’s not holding anything else up but its needed for a garage floor.
It never occurred to me that one would do a slab of concrete without rebar reinforcing. IIRC from watching my garage and driveway being made, a grid about every foot or so. Driveway still cracked a little bit, because now the ground underneath was drying and shrinking more. There are businesses that will drill holes in the driveway, and inject a non-clay slurry to fill any voids and boost the slab back up.
My old house, the garage slab was a DIY project and fairly rough finish. The guy who built it, said there’s an important point to building a garage with a bunch of friends helping - “Don’t break out the beer until the job is finished.”
I think you would find that a lot of commercial and light industrial is built slab on grade. This is the case in Alberta, so I expect it is similar for Saskatchewan. As jjakucyk said, the perimeter is insulated and since the slabs are large frost heave is not an issue. Also I believe
they are a lot thicker than the mere 4" for a garage. I could be wrong, my expertise is residential construction.
It also has to do with what the slab is connected to. An attached garage has footings at frost depth, or same depth as main structure. A detached garage slab can just have thickened edges.
Slabs are not going to behave as a rigid single structure unless engineered to do so - which generally is unnecessarily expensive. A concrete slab floats - like a raft or lily pad. If portion heaves or subsides the slab will only flex to a point and then crack. Most cracking you see is just from contraction, but some is because things are moving. Any structure on top is going to move with it, and a certain amount is just not a big deal. Right angles, plumb walls and level floors in a mature residential structure are nice to work with, but not to be expected, even with proper footings.
While a slab doesn’t have (nor need) a footer, there’s a lot of prep work that goes in to it - you can’t just pour the cement on the grass or whatever. Around these parts (Ohio), there’s usually around three to four inches of sod/dirt, and it needs to be removed. You then need to level it, install forms, spread fill, compact it using a plate compactor, etc. etc.
The day before my barn floor was poured, I told the contractor I wanted a slump of 4. He was upset: “A slump of 4?? I’m going to have to have two more helpers if you want a slump of 4!” I told him, “Well, you better get them. And I’ll pay extra for it.”
You would think that most contractors would jump at the opportunity to up sell and build a better product, but no. I think the everyday grind of fighting with all the customers that don’t want to pay for the right product grinds them down. That and the economics of keeping your systems simple. Glad you spoke up and he was willing to adapt.
You have to be really careful with concrete contractors. For many of them, their goal is fast and easy. This means they make the cement really wet. Heck, if they make it wet enough, it’s practically self-leveling. You will realize you got screwed in a year or so. By then the contractor is long gone.
The typical residential or small commercial slabs around here use fiberglass fibers for reinforcement. Short fibers for crack control and longer fibers for even higher loads.