How to Build a Drainage Swale: Depth, Width, Slope and Where the Water Goes

To build a drainage swale, start at the outlet: pick a spot on your own property where water can spread out or soak in, then shape a shallow, wide channel to it with a steady fall of about 1 to 2 percent, gentle side slopes and a grass or stone lining. The exact dimensions matter less than the two things failed swales usually get wrong: a real place for the water to go, and a fall with no flat spots or dips.

Swale depth, width and slope at a glance

Three public design guides give real numbers. They differ because each is written for a different job: moving water, soaking it in, or both.

Dimension	DOE Building America	Massachusetts CZM	Minnesota Stormwater Manual
Fall along the length	2 to 4%	1 to 3%	0.5 to 2%
Bottom width	Not set	2 to 10 ft	At least 3 ft
Side slopes	Not set (whole channel 3 times as wide as it is deep)	About 25 to 30% (4 to 1 to about 3.3 to 1)	No steeper than 3 to 1
Depth	Sized for a 6-month, 24-hour storm	About 18 in (12 in of water plus 4 to 6 in of freeboard)	Sized to the flow, 6 in freeboard minimum
Flow speed on grass	Not set	Not set	Under 4 ft per second

Why the fall numbers don’t match: DOE’s 2 to 4 percent is for a swale whose job is to move water away from the house, while the Massachusetts (1 to 3 percent) and Minnesota (0.5 to 2 percent) ranges are for planted swales meant to slow the water down and let it soak in, so they run flatter. For a lawn swale, aim where they overlap: 1 to 2 percent fall, a flat bottom 3 feet or wider and sides of about 4 to 1 (4 feet out for every foot down). Massachusetts calls a 30 percent side slope about the steepest you can mow safely.

Below 1 percent, small bumps in the bottom can hold water, so the grading has to be tighter. Where the fall has to be steeper than about 3 percent, Massachusetts suggests check dams to slow the water.

Before you dig: outlet, setbacks and utilities

Pick the outlet first. Good endpoints are a lower, open part of your yard where water can spread over grass, a rain garden, or a dry well where the soil drains (see our dry well vs French drain comparison). The code’s drainage section, IRC R401.3, sends surface water to a storm sewer or another “approved point of collection that does not create a hazard,” so check with your town before tying into a storm sewer. Massachusetts adds: don’t aim a swale at a neighbor’s yard, the road or municipal drains, and keep every part of it on your property.

Check the setbacks. Massachusetts keeps a swale that soaks water in at least 10 feet from the foundation, 50 feet from a septic system and 100 feet from a private well. Near the house, a swale should carry water past, not hold it. R401.3 wants 6 inches of fall in the first 10 feet from the foundation and calls for drains or swales where lot lines or slopes make that impossible.

Check for groundwater. In spring, dig a test hole 2 feet below the planned bottom. Massachusetts treats blotchy red, orange or yellow soil, sticky blue-gray soil, or water seeping in within an hour as too wet for a swale that soaks water in.

Call 811 a few business days ahead and wait until every utility has responded, as 811 explains. Some lines are only inches down. Notice times differ by state; look yours up in our 811 dig laws by state guide.

How to build a drainage swale, step by step

1. Shoot the grade. Pull a string with a line level, or use a laser, from the top to the outlet. Divide the drop by the length: 6 inches over 50 feet is 1 percent, 12 inches is 2 percent.
2. Mark it out. Paint both top edges and the centerline, and put silt fence or a compost filter sock below the work.
3. Strip the sod and dig. Cut the bottom, then shape the sides out to the top lines. The Minnesota construction spec digs from the sides and keeps rubber-tired machines out of the bottom, because packed soil soaks up less water.
4. Check the fall every few feet. Minnesota’s typical finish-grade tolerance is 0.1 foot either way, about 1 1/4 inches. Shave high spots rather than tossing loose soil into dips.
5. Prepare the bottom. Loosen it if machines packed it and spread topsoil where you’ll seed. Massachusetts wants a soaking swale to drain within 24 to 48 hours; heavy clay may need sand worked in.
6. Armor both ends. Lay crushed stone where water enters and at the outlet, so the water doesn’t dig a hole where it leaves.
7. Seed and blanket, or sod. Seed right away and staple a natural-fiber erosion blanket tight to the soil. Minnesota requires temporary erosion control on every swale type so grass can root before the channel runs.
8. Keep heavy flow out until it greens up. Minnesota diverts runoff around new swales with sandbags, fiber logs or silt fence, and doesn’t call a site stable until it has 70 percent even plant cover.

If downspouts feed the swale, carry them out in solid pipe and daylight them at its upper end; our guide on how to bury a downspout covers the pipe and fall.

How to size a swale for the water it gets

Massachusetts sizes a swale to hold the first inch of runoff from the hard surfaces that drain to it. Its worked example uses 2,500 square feet of roof, driveway and patio:

• Water volume: 2,500 sq ft × 0.08 ft (1 inch) = 200 cubic feet
• Cross-section: a 3-foot bottom with 4 to 1 sides and 12 inches of water is 11 feet wide at the water line, so the water area is (11 + 3) ÷ 2 × 1 = 7 square feet
• Length: 200 ÷ 7 = 28.6 feet
• Total depth: 12 inches of water plus 6 inches of freeboard is 18 inches, so the top is 3 + 6 + 6 = 15 feet wide

A gentle 18-inch swale takes a 15-foot strip of yard. Short on room, make it shallower and longer rather than steepening the sides. If the ground also stays soggy, see swale vs French drain for when to build both.

Grass swale or rock lined swale

Grass is the default: it’s cheaper, it mows, and it filters the water. Minnesota keeps flow in established grass under 4 feet per second and uses erosion matting or rock above that.

You can’t easily measure flow speed, so look after a hard rain. Bare streaks, exposed roots or a cut down the middle mean the water is too fast for grass there. Three fixes:

• Check dams. Low stone or log barriers across the swale. Massachusetts keeps them no taller than half the bank, and Minnesota keys them at least 3 feet into the sides with the middle lower than the ends.
• Rock lining. Mixed stone sizes over nonwoven geotextile, the way Minnesota beds its rock check dams. Stone that’s too small washes out.
• Turf reinforcement mat. A permanent mat the grass grows through, for stretches that erode.

Minnesota’s check dam guidance sets the top of each dam level with the base of the one above, so spacing equals dam height divided by slope. A 1-foot dam on a 2 percent swale goes every 50 feet (1 ÷ 0.02). Swales under 3 percent and 200 feet usually don’t need dams if they’re stabilized right away, and dams rarely work above 10 percent.

In cold climates like central Pennsylvania, where we build, frozen ground soaks up almost nothing, so a winter swale is just a channel. Give it an overflow path, as Massachusetts suggests, and don’t pile plowed snow in it.

DIY or hire a pro

A short swale across open lawn, with obvious fall and an outlet on your own land, is a fair DIY job with a shovel, a rented sod cutter and a string line. Bring in help when:

• It runs within 10 feet of the foundation, or the ground there tips toward the house
• The run is long and nearly flat, or steep enough to need check dams or rock
• There’s no outlet on your property, or you need a storm sewer tie-in
• It’s near a wetland, stream or septic field
• The yard also has standing water after rain, which a swale alone may not fix

If you’d like the route, outlet and fall worked out on paper first, our backyard drainage plans lay out swales, downspout lines and drains from your photos and measurements.

Not sure a swale is the right fix? Match your symptom in our yard drainage guide, and if the ground itself stays soggy, size a buried drain with the French drain calculator.

Frequently Asked Questions

How deep should a drainage swale be?

Deep enough for the design water plus at least 6 inches of freeboard. Massachusetts’ planted swales are about 18 inches deep for 12 inches of water; one carrying less water can be shallower.

What slope does a swale need?

About 1 to 2 percent along its length, or 1/8 to 1/4 inch per foot. Minnesota’s range starts at 0.5 percent, and DOE’s guide runs from 2 up to 4 percent.

Should a swale be lined with grass or rock?

Grass, unless the water moves too fast for it. Minnesota’s limit on established grass is 4 feet per second; above that, use check dams, rock over geotextile or a turf reinforcement mat.

Can a swale run right next to the house?

It can carry water past the house; the building code calls for drains or swales where there’s no room for 6 inches of fall in the first 10 feet. A swale that holds and soaks in water should stay 10 feet away.

Sources: U.S. DOE Building America Solution Center, Swales, Drains, and Site Grading for Stormwater Control; Massachusetts CZM, Stormwater Solutions for Homeowners Fact Sheet: Vegetated Swales; Minnesota Stormwater Manual (MPCA), Design criteria for dry swale (grass swale); Minnesota Stormwater Manual (MPCA), Construction specifications for dry swale (grass swale); Minnesota Stormwater Manual (MPCA), Check dams for stormwater swales; 2021 International Residential Code, R401.3 Drainage; 811, Before You Dig

Written and reviewed by Thayne Muthler, owner of Muthler Landscaping & Construction (PA HIC #PA212584). How we check facts: editorial policy.