I have a post-frame shed, 12x16. there are posts at 6' on the short sides and every 4' on the long side. posts are set ~18" below grade, on a 2" cap of cement over a few inches of well compacted gravel, holes were all about 27" or so, 2x6 girder runs level at the top (very level all the way around) around base.

I have ~5.5" to grade from bottom of 2x6 girder to top of board, and the I've got posts set in concrete, at 24" from the top of that board so I want to fill that in with concrete, I could put gravel in, say 2" and 3.5" of concrete over that.

Its about 3.75 cu yards of concrete, or like $400 It will cost $100 for gravel to fill in 2", but my concrete is still only a little cheaper, and it will be a thinner slab.

I could put gravel in, say 2" and 3.5" of concrete over that or just full 5.5" of concrete, or I could do lot of gravel and a 2" top coat over that with a bunch of steel and fibers,

Its just a shed, so I'm not too worried about it, I'll put steel mesh in and probably fiberglass to control cracks and run a groover down the center.

Point is, can I get away without using gravel and just lay plastic down and pour a slab, or is the clay soil going to destroy it if I don't have a couple inches of gravel. I am not, going to dig this down deeper, or pour higher that the girder.

Its 5.5" of space, with a few low spots I can rake out. It much faster to do a one day pour so I can put my books back in and my tools to keep them dry, so full 5.5" concrete is good and fast, but a couple inches of gravel would be nice, but is a total extra set of work for little gain


(Yes, I should have done this in a different order--t was not an option at the time.)

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    Is the clay undisturbed? It's hard to say from across the internet. Not all clay is alike, and I have no idea what climate or landscape you're in. These things aren't black and white.
    – isherwood
    Commented May 1, 2017 at 19:36
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    Where is this? At what depth is the freeze line there? Do you mean fine gravel just above coarse sand? pea gravel? or larger than pea gravel? I take it there will be no plumbing under the slab, right? Commented May 1, 2017 at 19:47
  • frost depth is 18" it would be 1" gravel with all fines, its roadbase, it compacts down no plumbing Commented May 2, 2017 at 4:42

4 Answers 4


Summary: No, gravel is not mandatory in all cases; but it can be very important in others. It depends what you need: gravel can provide a capillary break (preventing water from wicking upwards), can facilitate drainage, and can facilitate gas movement (like radon).

So in some cases, these are not always important factors.

According to BSI-003: Concrete Floor Problems, Joseph Lstiburek, May 26, 2008:

(click through to freely read full article - entertaining as well as informative!)

The plastic sheet also acts as a capillary break. We can use crushed stone as a capillary break if there are no fines in the crushed stones—but the stones don’t act as a vapor barrier. So it is common to use crushed stones with no fines as a capillary break—and then add a plastic sheet over the top of the stones in contact with the concrete as a vapor barrier. Or just use the plastic sheet to do both—be the vapor barrier and the capillary break. So why not just use plastic and not stones? Well the stones can also provide drainage (i.e. be a “drainage pad”) and facilitate soil gas control (i.e. “radon venting”) by creating and extending a pressure field that can be coupled with a vent to the atmosphere. Depending on where you are you will often see just plastic or plastic with stones. Below grade slabs tend to get the stones and plastic approach. Slab-on-grades tend to get the plastic only.

These concepts are also discussed further in the case of basements in BSD-103: Understanding Basements, Joseph Lstiburek, October 26, 2006.

  • So if I'm just covering up a patch of dirt under a fence, do I need gravel? Commented Nov 1, 2023 at 12:24
  • @Merlin-they-them- the context of this question was concrete under a building... so I doubt for what you describe its the same. But you could post a new question with more detail. Commented Nov 1, 2023 at 12:32
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    I edited the title to state that and created a new question here: diy.stackexchange.com/questions/285760/… Commented Nov 2, 2023 at 3:48

Is gravel absolutely required NO but it is a good idea. Most the time i see slabs poured directly on earth the owners are doing it themselves and the base is not level or has holes. If a slab is not a uniform thickness this can cause cracks even with reinforcement of fiberglass, mesh or rebar.


I used to pour concrete working for a mason. Yeah we were standing in it BUT one guy would pull it up while another Screeded with magnesium 2x4. Another guy or 2 behind with come slings pushing Crete behind screed. Yes it is difficult if you don’t pay attention but it is possible to pull up the wire mesh as you work your way back from the screed. And I’ve jack hammered out a fair amount of old slabs and found the wire is not sitting on bottom of slab. You definitely can end up with this happening if your not attentive to it but it is possible to pull the wire up. Buy flat wire and tie ends together. Rolled wire is a royal pain to wrestle with.

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    Hello, and welcome to Stack Exchange. This is great info, but it doesn't answer the original question. Please check out our tour to learn how we work. And keep posting! Commented Jun 24, 2018 at 15:59

The clay won't do anything to the slab that it wouldn't do to the gravel. It's ok to pour on the clay, except.......

If you would have said anything but "installing it on clay", I'd say "no problem ". However, the problem with clay is water will not percolate down and away from the slab. We don't like water laying on the bottom of slabs because it will "wick" up through the slab...especially if there's heat in the building.

Your situation is a little different, I think, because it's a "shed". I doubt if you're going to install a floor covering. If you are, then the moisture wicking through the slab could cause the flooring to come loose. If you're not installing a floor covering, then you may see a damp spot from time to time if you live in a damp region.

By the way, I would not install the mesh in the slab. They never install it in the correct location (in the middle of the slab) and then it causes the slab to crack. They lay it down on the ground before the pour and then tell you they will use hooks to pull it up during the pour. It can't be done. So, then the steel is on the bottom and makes the slab shrink less than the top of the slab. That's a crack for sure.

  • Will the fiberglass substitute for steel reinforcement? Would properly located rebars along with fiberglass produce a better slab without an exorbitant cost? Commented May 1, 2017 at 20:19
  • Lee Sam, if steel rebars are used in a slab do you favor using spacers to locate the rebar vertically or simply hooking and pulling up. Commented May 1, 2017 at 20:29
  • @Jim Stewart No, I don't like fiberglass as a substitute for reinforcing steel. Also, they cannot hook and pull up rebar for the same reason they cannot hook and pull up wire mesh...THEY'RE STANDING ON IT. If steel is called for, it should be rebars (not wire mesh) and spacers used. I live in a mild environment and we pour exterior unreinforced slabs about 15' x 15' before we start to using reinforcing. If we use reinforcing, then it needs to placed in the middle of the slab and meet the Pe ratio of steel to concrete, or it will crack anyway.
    – Lee Sam
    Commented May 1, 2017 at 21:40
  • I have long wondered how the steel could be pulled up with people standing in the concrete! I figured they stood in the gaps in the steel rebar lattice. Commented May 1, 2017 at 22:08
  • Im going to put remesh in and center it myself. Im considering some bolts in the posts and wiring the remesh to that to keep it at height and taut. I may just use chairs. I have some rebar laying around, but I dont think its enough. At the end of the day, it is just much, much easier to pour 5.5" thick of concrete, but I do think 1.5-2" of gravel under it is a good idea, just a much more difficult bit of logistics. Commented May 2, 2017 at 4:45

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