Pouring concrete on grass/roots/rocks etc.

[Gregory Mosher]

Hey folks,

I'm going to be building a new shop shortly, i'll be building the forms soon, and then soon after the concrete will come.

What i've read is even if I pour down 4" of compacted gravel, and then pour a 4" or so slab reinforced with mesh rebar, that eventually the grass & roots of the trees cut down etc will decompose and leave voids underneath and cause shifting and cracking in my slab. the building will be 16x26.

Now, if I have the compacted gravel of 4" underneath the concrete, will this still happen, and if it does, is it going to be a big deal? I don't mind a split or anything running through, I can smooth it off, etc later on in the years, I don't mind anything cosmetic so long as it doesn't affect the structural integrity of the building. Any advice here folks? Thanks!

[Paul F Franklin]

Ideally you remove all organic matter from underneath the slab, including large pockets of loamy soil. Easy to do if you have excavation equipment. Not so easy if you are doing by hand. As a practical matter, smaller roots and the like are not going to cause any problem. I would not put a slab over cut stumps of any size without removing them and backfilling with sand or gravel.

[Jim Becker]

Paul is 110% correct...remove the organics first to get to a stable subsoil level, fill with compacted stone and then do your pours.

[Gregory Mosher]

Thanks gentlemen.

Just to be clear, smaller roots and the like (grass, blackberry bushes etc?) won't be an issue, other ones like small trees of 1-2 " diameter should be up and backfilled?

[John K Jordan]

Just to be clear, smaller roots and the like (grass, blackberry bushes etc?) won't be an issue, other ones like small trees of 1-2 " diameter should be up and backfilled?

Small tree roots can grow into large tree roots and move and crack concrete. Blackberry bushes won't do this.

However, I would remove all organic matter including grass and roots. I like to dig down past the top layer of topsoil, tamp the ground (a hand-held tamper is great for small slabs), add gravel and tamp it, then put down the concrete. One problem with allowing voids to develop under the slab is they can provide a path for water to move and erode the soil. A continuous footer around the outside can help with this unless you are unfortunate enough to have underground water near the surface.

Note that the concrete can span voids if the maximum load is not too much IF there is enough rebar in the concrete and the concrete is thick enough and the span is narrow enough and it is otherwise supported along the edges (e.g. good footers) and with piers to undisturbed subsoil as needed.

JKJ

[Brian Backner]

Unless I'm misunderstanding your intent, pouring a slab that big in an area with such cold winters, i.e., Nova Scotia, without a perimeter foundation wall that gets below the frost line guarantees problems down the road.

[Julie Moriarty]

All the jobs I worked on required zero grass, roots or other compostable debris. Rocks are fine.

[Robert Hayward]

Why do you people up north put gravel under a concrete slab ? Only time I ever did it here in Florida was when the building specs were done by a northern engineer/architect.

[Jim Becker]

Why do you people up north put gravel under a concrete slab ? Only time I ever did it here in Florida was when the building specs were done by a northern engineer/architect.

Unlike the typically sand/sandy soil you have, we have things like clay under the organics that get removed. Compacted stone provides a stable base that helps insure that the poured slab is less prone to seasonal issues, etc.

[Jim Andrew]

Gregory, what is the frost line in your area? In my area, Kansas, we are required to put in a 30" deep perimeter at least 10" thick to pour a slab for a building. I prefer to pour a stem wall, and then put the slab between the walls, what they call a floating floor, as if the floor goes bad, you don't lose the whole building, can just break out the floor and replace it.

[Joe Jensen]

Where I live we had to have the soil tested by an engineering firm. They specified how to prepare for the foundation and the concrete specs. We had to remove down 5 feet and compact back up before digging footings. Then they put down compacted gravel before pouring the footings. The compaction crazy effective as they had to add a small footing after the pour and it took a guy a couple of house with a jack hammer with a spade blade. to dig a 2 ft wide by 5 foot long trench.

[roger wiegand]

Our inspector requires removal of all topsoil or other potentially compostable material, but then we also have to go down 48" to be below frost line, so it's not typically an issue. If you're filling very far the gravel needs to be compressed in ~4" lifts

I think an added function of the gravel is to provide a means for the water to drain away from the slab so it doesn't freeze and heave in the winter.

All that said, my 115 year old barn is on a "foundation" that extends down no more than 12-18" and is still standing, albeit on very sandy (quick draining and not very compressible) soil.

[Ole Anderson]

Even if you choose to not do a full foundation below the frost line, at least deepen the slab at the edge by going down 12" thereby constructing a "rat-wall". This will also cut off any live roots and keep critters from burrowing beneath the slab. My son put a shed on a simple slab, the inspector came by later and made him dig in and pour an after-the-fact rat wall.