Stone Dust: The Setting Bed Under New Hampshire Flatwork

Sand or stone dust under the flagstone. That is the decision most people are actually making when they land here, and in New England the answer usually comes out the same way.

Stone dust is 1/4 inch minus angular crushed stone with the fines left in, sold here as Stone Dust and also called screenings or crusher dust. Wet it, pack it, and it sets up like a floor.

Sand does not do that. It stays loose, and loose material under flatwork moves.

Choose it when the layer has to hold its shape under something

Stone dust belongs where a thin course must stay flat and stay put under weight it did not carry itself.

• Under pavers, brick and flagstone, where a piece that rocks once rocks forever.
• Swept into the joints afterwards, packing the gaps tight.
• Leveling a subgrade before concrete, decking or a set unit.
• As the wearing surface on a low-traffic path, wetted and compacted.

The angular grain is the reason. Crushed faces bite into each other and stay bitten. Rounded particles roll past each other instead, which is why a sand bed under an irregular flagstone lets the edges tip after a season of freeze and thaw.

Where it fails: new lawn ground, deep lifts and open drainage

Three jobs it gets bought for by mistake.

New lawn ground is the common one. Screenings are crushed rock. Nothing roots into them, and a fine crushed layer under grass seed gives you a hard pan two inches down. Lawn work wants screened loam.

Deep fill is the second. Stone dust compacted in one thick lift stays soft in the middle. A section deeper than two or three inches wants a coarse base below it with the dust only as the finish.

Open drainage is the third. Fines are what make it pack, and the same fines mean water will not run through it. A French drain filled with screenings stops draining almost at once. Send that work to #67 Stone or drain rock.

One inch of stone dust is what stops flagstone rocking

Over a compacted base, one inch of stone dust screeded flat is the working figure for pavers and cut stone.

Irregular flagstone wants more, closer to two inches, because the underside of each piece is a different thickness and the bed has to absorb that.

Screed it damp, set the piece, tap it down, then check it with a level across two pieces rather than one. A stone that reads flat on its own can still sit proud of its neighbor.

Sweep the joints with the same material once the field is laid, wet it, and sweep again when it settles. Two passes beat one.

It is the setting course, never the layer carrying the load

The mistake that costs a patio is treating stone dust as the whole build.

Under flatwork in this state, the load path runs: firm subgrade, then a compacted base of crushed gravel at four to six inches for foot traffic, then the inch of dust, then the surface. Frost reaches about 48 inches in the southern towns and deeper up north, so nothing here sits below it. The base is there to spread movement evenly instead of letting it show up under one corner.

Where granite ledge sits close to grade, the base gets thinner because there is nowhere to dig. Grade the surrounding ground to carry water away instead, because a shallow section over rock with water trapped in it heaves worse than a deep one.

Yardage for a 9 by 9 spa pad, a 5 by 30 dog run and a new lawn

Three hypothetical jobs. Stone dust runs about 2,900 lb per cubic yard on this site's published figures, roughly 1.45 tons per yard.

Hot tub pad, 9 ft by 9 ft, 2 inches of dust over a compacted base

• 9 x 9 = 81 sq ft
• 81 x 2 / 12 = 13.5 cu ft
• 13.5 / 27 = 0.50 cu yd
• 0.50 x 1.45 = 0.73 tons

Small enough that rounding up to a full ton is the sensible call. The leftover fills joints.

Side-yard dog run, 5 ft by 30 ft, 3 inches as a packed surface

• 5 x 30 = 150 sq ft
• 150 x 3 / 12 = 37.5 cu ft
• 37.5 / 27 = 1.39 cu yd
• 1.39 x 1.45 = 2.0 tons

New lawn area, 1,200 sq ft, 4 inches deep

• 1,200 x 4 / 12 = 400 cu ft
• 400 / 27 = 14.8 cu yd

Not a stone dust job at all. At the 2,200 lb per cubic yard this site publishes for screened topsoil, that is about 16.3 tons of loam.

Run your own figures through the stone calculator, or the paver base calculator when you are sizing the layer underneath.

Getting a fine material onto a dry site

Set the quantity and pick a date on this page. Delivery is arranged at checkout by ZIP code, and the truck needs firm ground with room to back and tip.

Two things worth doing before the day. Have the drop spot on pavement or on plywood if you can, because screenings landing on grass leave a gray mark that outlasts the project. And have a tarp for the pile.

That last one matters more with this material than with any clean stone. Stone dust soaks up rain, and a saturated pile is heavy, sticky work that will not screed. Cover it the night before if the sky looks like anything.

Working out what sits underneath? The aggregates list shows the base materials, or ask New Hampshire Rock Supply through the contact page.