Coarse, washed concrete sand is the standard choice for a layer under a concrete slab, but in most cases, a crushed gravel base is actually the better material for the job. The distinction matters because the wrong base material can lead to cracking, settling, and moisture problems that are expensive to fix later.
Concrete Sand vs. Masonry Sand
If you’re specifically using sand under a slab, you want concrete sand, not masonry sand. These are two different products, and using the wrong one causes real problems.
Concrete sand has coarse, angular grains ranging from 0.2mm to 4mm. Those angular edges interlock under pressure, creating a more stable surface. Masonry sand, by contrast, has fine, rounded grains (0.075mm to 2mm) designed for smooth finishes like stucco and bricklaying. Masonry sand is too fine for use under concrete. It holds moisture, compacts poorly, and weakens the support your slab needs.
When you order from a landscape supply yard, ask for washed concrete sand. The washing process removes clay and silt particles that would otherwise trap water and create soft spots under your slab.
Why Most Pros Prefer Gravel Over Sand
For driveways, garage slabs, patios, or anything that needs to handle weight, crushed gravel outperforms sand as a base material. Three-quarter-inch crushed stone is the industry standard because the angular pieces physically lock together when compacted, creating a rigid base that doesn’t shift.
Sand particles, even coarse ones, remain relatively loose. They shift under vibration and weight changes, and fine sand traps water between particles. Crushed gravel contains 30 to 40 percent void space, allowing water to drain through freely instead of pooling against the bottom of your slab. Sand creates smaller pore spaces that slow drainage considerably, and any clay contamination can block it entirely.
Sand does have a role in lighter applications. Small decorative walkways, thin patio slabs that won’t bear heavy loads, or leveling layers on top of a gravel base are all reasonable uses for concrete sand. It also works well as pipe bedding or as a final leveling layer for above-ground pools and paver patios. But as the primary structural base under a load-bearing slab, gravel is the stronger choice.
The Real Cost Difference
Sand costs less per ton than crushed stone, which makes it tempting for budget-conscious projects. But a sand-only subbase often requires thicker concrete to compensate for the weaker support, adding $2 to $4 per square foot in extra material. Factor in higher repair frequency from settling and cracking, and the upfront savings disappear quickly. A proper gravel base lets you pour a standard-thickness slab with confidence that it will hold.
How Compaction Prevents Settling
Whatever base material you use, compaction is what makes it work. Poorly compacted fill is one of the top causes of slab settlement. When loose material compresses over time, it creates gaps beneath the concrete. The slab loses support, cracks, and sinks into the void.
A plate compactor is the most common tool for the job. For small areas, a hand tamper (a flat metal plate on a heavy pole) can work, but anything beyond a few square feet calls for a mechanical compactor. The general rule: for every 1,000 pounds of compaction force, the machine effectively compacts about 1 inch of material depth. A compactor in the 3,000 to 5,000 pound centrifugal force range handles most residential slab projects well.
Before compacting, clear the area of rocks, roots, and debris, then level out any uneven patches. Dampen the material lightly before you start. Moisture helps particles slide together and bond more tightly. You want the material damp, not saturated. Each pass of the compactor should overlap the previous one so you don’t leave loose strips.
Where the Vapor Barrier Goes
Moisture migrating up through the ground and into your slab causes problems ranging from flooring adhesive failure to mold growth. A vapor barrier (plastic sheeting at least 6 mils thick) addresses this, but where you place it relative to the sand or gravel layer depends on your project.
If the slab will be covered with moisture-sensitive flooring like hardwood, vinyl, or carpet, place the concrete directly on the vapor barrier with no sand or gravel between them. This gives you the tightest moisture seal. The same applies if the slab will be exposed to rain during curing.
If neither of those conditions applies, you can place a granular base layer on top of the vapor barrier before pouring. This gives you an easier surface to level and can help with concrete curing. There’s genuine debate among engineers and contractors about which approach is better in various situations, because the placement affects curing time, cracking risk, and moisture control differently. For most residential projects where you plan to install flooring, putting concrete directly on the barrier is the safer bet.
Recommended Base Layering
A typical slab-on-grade base, from bottom to top, looks like this:
- Compacted subgrade: The native soil, cleared of organic material and compacted firmly.
- Gravel base: 4 to 6 inches of compacted crushed stone (3/4-inch is standard) providing structural support and drainage.
- Sand leveling layer (optional): A thin layer of washed concrete sand, typically 1 to 2 inches, to create a perfectly smooth, level surface on top of the gravel.
- Vapor barrier: 6-mil or heavier polyethylene sheeting, positioned based on your flooring and curing conditions.
- Concrete slab: Poured to the specified thickness for your project.
This layered approach gives you the structural strength of gravel, the leveling benefits of sand, and proper moisture protection. For light-duty slabs like a small shed pad or a walkway with no vehicle traffic, you can simplify to compacted sand over compacted subgrade. But for garages, driveways, house slabs, or anything carrying real weight, the full gravel base with an optional sand leveling layer is the approach that holds up over time.