Drain Rock in Maryland: Washed Stone for Runs That Have to Stay Open

Two products get mixed up here, and the mix-up is expensive in different directions. Drain rock and river rock can look similar in a pile, because drain rock comes in rounded as well as angular varieties, but one is sold for how it works buried and the other for how it looks in a bed. Drain rock at 3/4 inch to 1-1/2 inch is washed to take the fines and dust out, and that washing is the product. On slow-draining Piedmont clay or in flat ground near the Chesapeake, the open space between the pieces is what carries water away from a wall, a footing or a septic bed.

• Size: 3/4 inch to 1-1/2 inch.
• Grading: washed to remove fines and dust.
• Type: rounded or angular varieties.
• Approximate weight: no figure is published for this product specifically. The nearest family on this site's own calculator pages, the 3/4 inch and #57 crushed stones, runs about 2,800 lb per cubic yard, or roughly 1.40 tons per cubic yard. Treat it as an estimate.

Choose it where water has to keep moving after the soil closes in

• A French drain or yard drain where standing water has to be collected and taken somewhere else.
• Backfill in the drainage zone behind a block, timber or stone retaining wall, where water building up behind the wall is a structural problem rather than a nuisance.
• A perimeter or foundation run carrying groundwater away from footings and basement walls.
• A septic infiltration or leach area, where the bed has to keep taking water through it.
• A dry well or infiltration pit collecting roof and downspout water that has nowhere else to go.

The common thread is that the stone is doing a job after it has been buried and soil has settled around it. A washed gradation is the only way the voids stay empty once that happens.

Where it is the wrong load: pads, drive surfaces and walkway bedding

Three jobs come up often enough to name.

• Example, a pole barn or equipment pad. Washed stone will not compact, so a machine or a slab lands on whichever pieces happen to sit proud. A pad wants CR-6 built in lifts and rolled. Drain rock belongs in a perimeter trench outside the pad, not under it.
• Example, a single-car gravel driveway. As the running surface it ruts under a tire and throws stone into the lawn, because nothing binds the top together. The drive surface is a job for Driveway Gravel over a compacted base. Drain rock belongs in the interceptor across the top of a sloping drive.
• Example, a flagstone walkway. Stone this coarse cannot be screeded and cannot be swept into a joint. Flags bed on Screenings. Drain rock belongs underneath a walk only where it crosses a wet strip and needs a drain running beside it.

The pattern is the same in all three: the pad, the drive and the walk are load and surface problems. Drain rock solves the water problem sitting next to them.

French drains, septic beds and the back of a wall on Piedmont clay

Central Maryland soils are silty and hold water, and rolling ground delivers a thunderstorm to the low point quickly. A drain here intercepts and carries. Line the trench with a geotextile, bed the perforated pipe on stone rather than on soil, fill around and over the pipe, then lap the fabric closed before the soil goes back. Without that wrap, fines migrate in from the trench walls and the run silts up from the outside in while the stone inside still looks clean.

Near the Bay and on the Eastern Shore the water table is the constraint. A run cut below it fills from beneath and delivers nothing to the outlet, so the design question is where the water is going, not how big the trench is. On limestone ground in the Hagerstown and Frederick valleys there is a different caution: water funneled to a single point can find its way down through cracks in the rock, so spread the discharge rather than concentrating it.

Behind a wall, the drainage zone and the soil backfill come up together in step, with the stone kept clear of the soil so the two never mix. A septic infiltration bed is built to whatever the county health department requires, and those requirements, not this page, set the dimensions.

Rounded or angular drain rock, buried or visible

Both varieties are washed, and the choice comes down to load and sightlines.

Neither variety is a base and neither is a surface. This is a layer that sits between things: under a walk, behind a wall, beside a footing, beneath a bed of soil. Anywhere it will be seen and walked on instead, River Rock is the product sold for that job, and it is not washed to a drainage grading.

Tons around an equipment pad, across a drive and under a wet walk

Length times width times depth in feet, divided by 27, gives cubic yards. Multiply by roughly 1.40 for an estimated tonnage. All three are worked examples, not real projects.

• Example, a perimeter drain around a 24 foot by 32 foot equipment pad. The perimeter is 2 x (24 + 32) = 112 feet. A trench 12 inches wide and 24 inches deep gives 112 x 1 x 2 = 224 cubic feet, divided by 27 = 8.30 cubic yards, about 11.6 tons.
• Example, a sloping single-car drive. An interceptor across it, 12 feet long, 18 inches wide and 24 inches deep, is 12 x 1.5 x 2 = 36 cubic feet, divided by 27 = 1.33 cubic yards, about 1.9 tons. An edge drain down the uphill side for 55 feet at 12 inches wide and 18 inches deep is 55 x 1 x 1.5 = 82.5 cubic feet, divided by 27 = 3.06 cubic yards, about 4.3 tons.
• Example, a flagstone walk crossing a wet strip of yard. A drain alongside it 26 feet long, 12 inches wide and 18 inches deep is 26 x 1 x 1.5 = 39 cubic feet, divided by 27 = 1.44 cubic yards, about 2.0 tons. The flags themselves bed on a finer material and are a separate order.

Run your trench dimensions through the river rock calculator, which the site uses for this material, then set a quantity and a delivery date on this page. Access matters more than usual on drainage work, since an open trench and a parked truck want to be near each other. The delivery page covers how that is arranged by ZIP code at checkout.