Flagstone Patio Base: Gravel, Sand, and Depth
A flagstone patio base for a dry-laid patio is usually 4 to 6 inches of compacted dense-graded crushed stone topped with about 1 inch of sand or stone dust for setting the stones. That combination drains well, resists frost heave, and lets you bed irregular flagstones level. Mortared flagstone patios are different: they need a concrete slab underneath. Choose the base based on the look you want and how much movement your climate causes.
Volume and tonnage calculator →Dry-laid base layers
| Layer | Material | Depth |
|---|---|---|
| Subgrade | Native soil, compacted | — |
| Fabric | Geotextile, optional | — |
| Base | Dense-graded crushed stone (3/4 in minus) | 4 to 6 in compacted |
| Bedding | Stone dust or coarse sand | About 1 in |
| Flagstone | Natural stone | 1 to 2 in thick |
Use the deeper base in cold climates, on clay soils, or where the patio gets heavy use. See how much paver base do I need.
Step by step
- Lay out and excavate to the total depth: base, bedding, and stone thickness.
- Slope about 1/8 to 1/4 inch per foot away from the house.
- Compact the subgrade.
- Add base in lifts of 2 to 3 inches, compacting each. See compacting gravel base in lifts.
- Spread bedding and screed it.
- Set flagstones, tapping them level with a rubber mallet and adding or removing bedding as needed.
- Fill joints.
Stone dust vs sand
Stone dust packs firmer and resists washing out, which suits irregular joints. Coarse sand drains better. Many installers use stone dust for bedding under flagstone. See stone dust driveway for more on the material.
Joint filling
| Joint fill | Look |
|---|---|
| Polymeric sand | Firm, fewer weeds |
| Stone dust | Natural, may need topping up |
| Pea gravel | Casual, drains well |
| Moss or creeping plants | Garden style; wider joints |
Mortared flagstone
Mortared patios set stone on a mortar bed over a reinforced concrete slab, usually 4 inches thick on a compacted gravel base. They look formal and are easy to sweep but can crack if the slab moves. Never mortar flagstone directly onto gravel.
Material estimate
A 12x14-foot patio, 168 square feet, at 5 inches of base needs 70 cubic feet, about 2.6 cubic yards or 3.6 tons, plus 10 to 15 percent for compaction. Bedding at 1 inch adds about 14 cubic feet, roughly half a yard. Use the asphalt base calculator for your size.
Flagstone thickness
Stones 1 1/2 to 2 inches thick work well dry-laid. Thinner flagstone breaks more easily and is better suited to mortared installations.
Bottom line
A dry-laid flagstone patio base is 4 to 6 inches of compacted crushed stone plus about 1 inch of stone dust or sand, sloped away from the house. Mortared flagstone needs a concrete slab. Fill joints with polymeric sand, stone dust, gravel, or plants.
Questions people ask
What is the best base for a flagstone patio?
4 to 6 inches of compacted dense-graded crushed stone with 1 inch of stone dust or sand.
Can I lay flagstone directly on dirt?
It will shift and settle; a compacted gravel base is better.
Should I use sand or stone dust under flagstone?
Stone dust packs firmer; coarse sand drains better.
Do mortared flagstone patios need concrete?
Yes, mortar should be set on a concrete slab.
Related guides
- How Much Paver Base Do I Need? Depth and Tons
- Stone Dust Driveway: Does Screenings Work as a Driveway Surface?
- Polymeric Sand Coverage: Bags per Square Foot
- Compacting a Driveway Gravel Base in Lifts
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