What Size Crushed Stone Goes Under a Concrete Slab
Everything you need to know about what size crushed stone under concrete slab.
What Size Crushed Stone Goes Under a Concrete Slab
The short answer: most concrete slabs sit on a compacted base of #57 Crushed Stone, an angular stone that runs from about 3/4 inch down to smaller fragments, spread 4 to 6 inches deep and compacted before the pour. If you are asking what size crushed stone rather than smooth, rounded gravel, because angular pieces lock together and drain water away from the slab instead of letting it shift underneath.
That base layer does two jobs at once. It spreads the weight of the slab evenly over the soil below, and it lets water drain out from under the concrete instead of pooling there and freezing, which is what causes slabs to heave and crack over a few winters. A stone base is the standard sub-base for driveways, patios, walkways, garage floors and shed pads alike, with the depth adjusted for the load the slab has to carry.
Why Crushed Stone Size and Depth Change by Project
Crushed Stone is the general term for angular aggregate that has been mechanically broken and screened to a target size, and it comes in a range of gradations. For a base under a slab, contractors reach for #57 Crushed Stone because it is an open graded mix with almost no sand or fines mixed in. That open gradation lets water pass straight through the base instead of sitting under the concrete, which matters most on a slab that has to get through freeze and thaw cycles year after year.
3/4" Crushed Stone is close in size and does the same job. Some suppliers treat it as nearly identical to #57 stone, others grade it slightly differently, so confirm with your supplier which one they stock before you order. Either one beats pea gravel, bank run gravel, or plain sand for this job, because rounded stone and sand particles slide past each other instead of interlocking, which lets the base settle unevenly once the slab is on top of it.
Depth depends on what the slab carries and what sits under it. A standard 4 inch thick residential slab, patio, walkway or shed pad, usually gets 4 inches of compacted base stone. Driveways, garage floors, or any slab that will carry vehicle weight typically get 6 inches, sometimes more if the native soil is soft clay or holds water after rain. On sites with poor drainage or a high water table, contractors sometimes add extra stone specifically to keep water moving away from the slab rather than letting it sit underneath.
The base is normally placed in layers rather than one deep dump. A 4 inch base often goes down in one or two passes, while a 6 inch base is usually spread and compacted in two lifts of about 3 inches each. Each lift gets run over with a plate compactor before the next one goes down, which packs the angular stone tight and keeps the finished base from settling once the slab and its load are sitting on top of it.
Figuring the Amount You Need
The math for a slab base is straightforward. Multiply the length in feet by the width in feet by the depth in inches, then divide by 324 to get cubic yards of loose volume. A 20 foot by 20 foot slab base at 4 inches deep works out to 20 x 20 x 4 divided by 324, or close to 5 cubic yards before compaction. A 24 foot by 24 foot driveway pad at 6 inches deep works out to 24 x 24 x 6 divided by 324, or about 10.7 cubic yards, roughly double the volume for a modest increase in both footprint and depth.
Because crushed stone is sold by the ton rather than by the cubic yard, run your own length, width and depth through the crushed stone calculator to convert that volume into the tonnage to order, and add a little extra to cover compaction and an uneven subgrade. Compacting a loose base reduces its volume, and subgrades are rarely perfectly flat, so a small buffer keeps you from running short mid pour. It is far easier to have a little stone left over than to stop a pour and reorder.
Mistakes to Avoid
- Pouring directly on bare dirt or sod with no stone base at all, which leaves nothing to spread the load or drain water away from the slab.
- , rounded river rock, or plain sand instead of angular crushed stone, since rounded material will not lock together under compaction.
- Dumping the full depth of stone at once instead of compacting it in layers, which leaves loose pockets that settle later and crack the slab.
- Going too shallow on a driveway or garage floor, where vehicle weight needs the extra depth that a patio or walkway does not.
- Mixing two different gradations of stone in the same base, which can leave gaps instead of a tight, even surface.
- Skipping compaction equipment and just tamping stone down by hand, which rarely achieves the density a plate compactor gets in a few passes.
Related Materials and Next Steps
Once you know the size and depth, ordering is straightforward. Start at Crushed Stone if you want to compare gradations side by side, or go straight to #57 Crushed Stone or 3/4" Crushed Stone if you already know which one your project calls for. Other jobs on the same property, drainage trenches, French drains and paver bases, lean on the same open graded stone, while backfill and driveway topping often call for a different size entirely, so match the product to the job before you place an order. Getting the base right the first time is far less work than tearing out a cracked or settled slab later on.
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Frequently Asked Questions
Can I use pea gravel instead of crushed stone under a slab?
Pea gravel is rounded, so the individual stones roll and slide past each other instead of locking together. That makes it a poor choice for a slab base because it will not compact into a stable, even surface. Crushed stone, has angular faces that interlock when compacted, giving the slab a solid, well drained platform instead of a shifting one.
Do I need a vapor barrier under the crushed stone base?
Some builders add a plastic vapor barrier under the slab, either directly under the concrete or under the base stone, mainly to slow moisture moving up from the ground into an interior floor. That is a separate decision from the crushed stone base itself, which handles drainage and support. Check your local building code and the slab's use, since interior floors usually call for more moisture control than an outdoor patio.
How many inches of crushed stone does a shed slab need?
A shed slab carries light, steady loads, so 4 inches of compacted crushed stone is typically enough on stable soil. If the ground stays wet, holds water, or is soft clay, adding an extra inch or two of stone helps keep the base dry and stable under the slab year round. Compact the stone across the full pour area before setting forms and placing concrete.
What is the difference between #57 stone and 3/4 inch crushed stone?
Both are angular crushed stone is a size description some suppliers use for a nearly identical product. Confirm with your supplier which one they stock before ordering a slab base.
Does crushed stone under a slab need to be compacted?
Yes. Loose crushed stone settles unevenly under the weight of concrete and anything placed on top of it later, which cracks the slab over time. Spread the stone in layers of a few inches each and run a plate compactor over every layer before adding the next, rather than dumping the full depth and compacting once. Proper compaction is what keeps the base, and the slab above it, from shifting.
Can concrete be poured directly on crushed stone without sand on top?
Yes, concrete can be poured directly on a compacted crushed stone base without a sand layer in between. Sand is sometimes added as a thin leveling layer to smooth the top of the stone before forms go in, but it is not required if the stone base itself is graded and compacted flat and even. Skipping sand does not change the size of stone needed underneath.
Should the base stone be wet or dry before the concrete pour?
The crushed stone base should be damp, not soaked and not bone dry, when concrete goes on top. A little moisture helps the stone compact tighter and keeps dust down, while standing water or a soggy base will not support the slab properly. If the base got rained on right before the pour, let it drain and firm back up before setting forms and pouring.
What happens if the crushed stone base under a slab is too thin?
A base that is too shallow does not spread the slab's load evenly across the soil underneath, so weak spots in the native ground telegraph up through the stone and into the concrete. That shows up later as cracking, settling, or an uneven surface, especially under driveways and garage floors that carry vehicle weight. Matching the depth to the job is cheaper than repairing a cracked slab afterward.