Screenings in Pennsylvania: Stone Dust for Bedding and Joints

Most orders for screenings start with a patio that has gone wrong. A flagstone terrace on the southeastern Piedmont, set three summers ago onto loose builder's sand over schist loam, now has two stones rocking under a heel and a low corner that holds a puddle for a day after rain. Loose sand under stone does not stay level, because there is nothing in it to lock. That is the mistake this material is bought to correct, and correcting it properly means dealing with the layer underneath as well as the layer you can see.

Quarter inch minus: the stone dust a Piedmont paver job runs on

Screenings are angular crushed stone at a quarter of an inch and down, which is a blend of small chips and rock dust with nothing larger in it. The dust is the working part. Under a plate compactor, the fine particles fill the spaces between the chips and the layer sets up firm and level, and because the pieces are crushed rather than rounded, they stay wedged against each other once they are in place.

Locally the material is called screenings, stone screenings or simply stone dust, and all three names land on the same product. It is a setting and finishing material, not a structural one, which is the distinction the rest of this page turns on.

Screenings on a rock bed, a shed pad and a downspout corner

Three jobs on the same suburban lot, three different answers.

The pattern is worth stating outright. Where something is being set level, screenings belong. Where water has to run through, they do not.

Red shale clay, thirty inches of frost and where the water goes

Philadelphia and the lowland municipalities around it work to a frost depth of about 30 inches, shallower than most of the state, and that lulls people into building thin. It should not. Frost here still cycles repeatedly through a winter, freezing and thawing the top few inches over and over, and the limestone valleys and red shale lowlands of the region weather down to soils that hold water rather than shed it.

A compacted screenings bed sitting on clay with nowhere to drain is a sponge waiting for December. Underneath it belongs an open, free-draining base that has been compacted and pitched so water leaves, with the ground shaped to a slight fall before anything goes down. Get that sequence right and a quarter-inch-minus bedding course will hold a flagstone terrace flat for years. Get it wrong and even a perfectly screeded bed will still heave.

Wetting and tamping before the flagstone gets set

Screed the screenings over the compacted base to an even thickness, dampen the material, then compact it. Damp fines pack far tighter than dry ones, and a plate compactor over a wetted layer will leave you a surface you can set stone onto directly. For a bedding course under pavers or flagstone, roughly an inch of screed material over the base is the usual working figure. For a compacted path or a pad surface where the screenings are the finish, two to three inches over the base is closer.

The same material brushed dry into the joints afterwards locks the units together. Sweep it in, tamp, wet lightly, then sweep in another pass once it has settled, because the first fill always drops.

Yards for a 14 by 18 shed pad and a 60 foot path

Worked examples, all hypothetical. Length times width gives square feet, times depth in feet, divided by 27 for cubic yards. This site publishes screenings at about 2,900 lb per cubic yard, which is 1.45 tons per yard; 2A modified runs heavier at about 3,000 lb, or 1.50 tons per yard.

Read it as coverage and a yard of screenings spreads over roughly 324 square feet at an inch, 162 at two inches and 108 at three. Joint filling is impossible to calculate properly, so add a modest allowance to the bedding figure rather than trying to measure it. The stone calculator holds the same densities and will take up to five separate areas if the patio is an odd shape.

Getting a load of fines onto a southeastern lot in one piece

Fines behave differently from stone on delivery day. A pile of quarter-inch minus sheds dust in a dry wind and turns heavy and sticky in rain, so pick a drop spot on hard standing close to the work, and plan to move it the same day or cover it. Wheelbarrow runs across a lawn will mark the grass, so lay a plank run if the route crosses turf.

Choose the amount and the delivery date on this page, with delivery worked out at checkout from your ZIP code and the truck needing a clear spot it can reverse out of. Excavation, base and edge restraints all want to be finished before the material arrives. Anything awkward about the access goes to Pennsylvania Rock Supply through Contact ahead of the date.