Drain Rock: The Washed Stone That Only Ever Carries Water

By the second week of April a camp lot has told you where its wet spots are. Water stands over ground that is still frozen underneath, the low corner turns to soup, and the same seam runs along the foundation every year.

Drain rock is the material that answers it. Washed, 3/4 inch to 1-1/2 inch, supplied in rounded or angular varieties, with the fines and dust taken out of it. It carries water and nothing else.

A handful of drain rock: washed, 3/4 to 1-1/2 inch, nothing binding it

Pick some up and your glove stays clean. No grit, no film, no powder in the bottom of the bucket. The pieces are larger than most of the stone on a landscape job and they sit loosely against each other.

Tip it into a bucket of water and it does not cloud. That washing is the whole specification, because anything fine enough to travel is also fine enough to sit in the gaps between stones and shut the layer down.

Angular is the one to pick when backfill will bear against it, since the flat faces hold their arrangement under pressure. Rounded suits a feature that stays visible and is meant to look like natural stone.

Mud season shows you the trench; June is when you dig it

The timing on this ground is not a preference, it is the calendar.

Frozen ground cannot be trenched and cannot be compacted, so from roughly December through the thaw the work is on hold. Through mud season the ground is soft enough to trench and too soft to get a machine across without tearing the lot to pieces, and the trench walls slump as fast as you cut them.

The window opens when the frost is out and the ground has drained, and it runs to the fall freeze. In the White Mountains and the North Country that window opens later and shuts earlier, so a late September dig up there is a gamble a southern town would take in October.

Mark the wet spots in April, when the evidence is on the ground. Dig in June, when you can stand on it.

Getting the trench, the fall and the fabric ready before the load lands

Order of operations, because this is the part that decides whether the drain lasts.

1. Find where the water is going to come out. A drain that daylights downhill works; one that ends in a wet corner just moves the puddle.
2. Cut the trench to a steady fall along its length. A consistent slope beats a steep one with a flat spot in it.
3. Line the trench, walls and floor, with geotextile fabric, with enough overhang to fold across the top later.
4. A few inches of stone, then the perforated pipe, holes down, then stone around and over it.
5. Fold the fabric closed across the top before any soil goes back.

Skip the fabric and the surrounding till will be in the stone within a couple of wet springs. There is no way to clean it out afterwards.

French drains, footing perimeters, leach fields and a channel that shows

The trench along a foundation or across a slope above a building, intercepting water before it reaches the wall.

The perimeter drain at footing level on a new build, carrying groundwater away from the footing and the basement wall.

The drainage layer in a septic drain field or leach field, where the stone holds the void space an infiltration system is built around. That work is designed and inspected to a specification, so the design governs the size and depth, not this page.

Behind a retaining wall, giving water a route down to the drain at the base rather than building pressure against the blocks.

And a dry channel that stays visible, where the rounded variety carries surface runoff along a chosen line and looks like stone rather than like aggregate.

Two jobs that will swallow a load of it: an entrance and a paver base

Both come up regularly. Neither is a drain rock job.

A construction entrance. The point of one is a coarse stone that shakes mud off a tire and does not get pumped down into the subgrade by a loaded truck. This size is too small for that and it will disappear into the mud in a week. Use this instead: #4 crushed stone, or railroad ballast where the traffic is heavy.

A paver patio base. A base has to compact into something that will not move under the pavers, and a washed stone with no fines cannot do that. Use this instead: crushed gravel compacted in lifts, with stone dust as the setting course on top. Drain rock still has a place on that job, in a drain line below the base if the patio sits on ground that holds water.

Tons for 60 feet of foundation drain and a 28-foot dry channel

Two hypothetical jobs. This size falls in the 3/4 inch crushed stone family on this site's published weights, about 2,800 lb per cubic yard, near enough to 1.40 tons per cubic yard, and approximate for a washed stone at the larger end of that range.

Foundation perimeter drain, 60 ft long, 18 inches wide, 18 inches of stone

• 60 x 1.5 = 90 sq ft in plan
• 90 x 1.5 ft deep = 135 cu ft
• 135 / 27 = 5.0 cu yd
• 5.0 x 1.40 = 7.0 tons

Dry channel, 28 ft long, 3 ft average width, 4 inches

• 28 x 3 = 84 sq ft
• 84 x 4 / 12 = 28 cu ft
• 28 / 27 = 1.04 cu yd
• 1.04 x 1.40 = 1.46 tons, so order 1.5

The pipe itself displaces a little, so those figures run slightly generous, which is the right direction. Convert other shapes in the river rock calculator, which is the one this material is set up in.

What the layer does after the first winter

Nothing visible, which is the idea. The stone does not break down, does not rot and does not settle much once it has been placed.

What changes is what arrives from outside. A daylight outlet fills with leaves in the fall and with a mouse nest over the winter, and a blocked outlet backs the whole line up. Check it in April and again in November.

A surface channel collects silt and leaf litter between the stones. Blow it out rather than raking it, since a rake lifts as much stone as debris.

Once the trench is open and the weather is settled, pick a quantity and a delivery date here. Delivery is arranged at checkout by ZIP code, and the truck needs firm ground with room to back and tip.