Do You Need Gravel Under Concrete?

Do you need gravel under concrete?

The short answer

Yes, on almost every driveway, patio, walkway and shop floor we pour. A compacted layer of crushed stone gives the slab a uniform surface to bear on instead of whatever mix of soil, fill and clay happens to be under your yard, and it gives water a path to drain instead of sitting against the underside of the slab. Skipping it is one of the most common reasons a slab cracks or settles early.

If you are looking at a quote with a stone base line item

It is a fair question. Concrete is the expensive, visible part of the job, and a layer of stone underneath it looks like it should be optional. It is not, on the vast majority of what we pour. The stone is not filler. It is the part of the job that decides whether the concrete on top of it lasts.

What gravel does that bare ground cannot

Soil is not one consistent material, especially once you factor in old fill, clay pockets, and whatever organic matter is left in the topsoil. A slab poured straight on that mix bears unevenly, firm in some spots and soft in others, and that unevenness is what eventually cracks it. Crushed stone, compacted in place, gives the whole footprint the same bearing strength across the board.

The second job is drainage. Stone has air gaps between the pieces that soil does not, so water passes through it instead of pooling. On red clay ground especially, which holds water instead of letting it soak away, that drainage path is what keeps water from sitting against the bottom of the slab and working on it every freeze and thaw cycle all winter.

A uniform, compacted surface for the slab to bear on
A path for water to drain instead of pooling under the slab

The stone itself, and how it goes down

Around here the stone we use is commonly what gets called 21A, a crusher run mix of crushed stone and stone dust sized to lock together and compact tight rather than staying loose underfoot. It does not go down as one thick dump and get rolled over once. It goes in layers, and each layer gets its own compaction pass before the next one is added, which is what actually gives you a firm, even base rather than stone that looks compacted on the surface and stays loose a few inches down over the subgrade.

When people try to skip it, and why we do not go along with that

Every so often someone asks if we can just pour straight onto the graded dirt to save a step. We understand the instinct, since it is a real cost and a real day of work. We do not do it, because we have seen what happens on the slabs that skipped it, cracking and settling that shows up within a few years instead of decades. If a particular small, light pad on already firm, undisturbed ground genuinely needs less stone than a driveway would, that is a judgment call we make on site, not one we make sight unseen over the phone.

Terms on this page

Crusher Run Compaction 21A Stone All concrete terms

Good questions

How thick is the gravel layer supposed to be?

It depends on the soil, the slab, and what the slab will carry, which is exactly the kind of thing we look at on site rather than quoting blind. We will tell you what your ground calls for when we come out.

Can I use sand instead of crushed stone under a slab?

Sand does not lock together and compact the way crushed stone does, so it does not give you the same stable, uniform bearing surface. We use crushed stone for the base on the work we do.

Does a shop floor need the same stone base as a driveway?

The base still matters, and often matters more, since a shop floor carries heavier point loads than a driveway does. The details of how much stone and how it is compacted are part of what we walk through at the estimate.