How to Route a Downspout to a Dry Well (4 Steps That Last)

TL;DR: A dry well is a gravel-filled pit that absorbs roof runoff underground. Route your downspout using 4-inch solid PVC sloped 1/8 inch per foot, place the well 10–20 feet from the foundation in soil that drains, and fill the hole with 3/4-inch washed stone. Most homeowners complete the install in one weekend for under $300 in materials.

_Last reviewed: July 2026 · 6 min read_

When downspouts dump water at the foundation, basements flood and soil settles. A dry well moves that runoff away from the house and into the ground where it belongs. The project takes a weekend, requires no special permits in most counties, and solves recurring puddles for decades if you size and site it correctly.

Okoniq Property Hub logs the installation date, gravel type, and pipe run so you know where the system is buried when you dig five years later.

How big does a dry well need to be?

Size the well by calculating roof area that drains to the downspout. Measure the roof footprint in square feet, then multiply by 0.623 to convert to gallons per inch of rain. A 1,000-square-foot roof section yields 623 gallons per inch of rainfall. For a 2-inch storm — common in the Midwest and Southeast — that's 1,246 gallons. A 4-foot-diameter, 4-foot-deep dry well holds roughly 375 gallons of voids (assuming 40 percent porosity in washed stone), so you need multiple wells or a larger hole for roofs over 600 square feet per downspout. Most single-story ranch homes use a 3×3×3-foot well per downspout; two-story colonials often require 4×4×4 feet or two wells in series. Sandy loam absorbs water faster than clay, so heavy clay soils need 20–30 percent more volume to prevent backup during consecutive storms.

Dig a test hole 2 feet deep, fill it with water, and time how long it takes to drain. If the water disappears in under 24 hours, the site drains well enough for a dry well. If it takes 48+ hours, you're dealing with clay and should either add a second well or route the pipe farther into the yard where topsoil is deeper. The dry well must sit in permeable soil — placing it in compacted clay or hardpan turns it into a cistern that overflows. Avoid placing wells under driveways, within 10 feet of septic drain fields, or inside the drip line of large trees whose roots will clog the stone voids within five years.

What pipe should I use to connect the downspout to the well?

Use 4-inch Schedule 40 solid PVC — not corrugated flex pipe, which sags and traps sediment. Slope the pipe 1/8 inch per foot (1 percent grade) so water flows by gravity without ponding. A 15-foot run drops 1.875 inches from the downspout to the well inlet. Mark the trench path with spray paint, call 811 to locate buried utilities, then dig a trench 12–18 inches deep. Lay the pipe on undisturbed soil or 2 inches of gravel; backfill with clean fill dirt, not clay clumps that settle and create low spots. Glue all joints with purple primer and PVC cement — push-fit couplings leak after freeze-thaw cycles.

At the downspout, use a cast aluminum or PVC elbow adapter that slips over the existing leader. Secure it with two sheet-metal screws so wind doesn't pull it off. Run the pipe underground to the dry well, entering through the sidewall 6 inches below grade so the inlet doesn't clog with leaves. If the run exceeds 20 feet, install a cleanout tee every 15 feet — a vertical 4-inch riser capped with a screw-on plug you can remove to snake blockages. Roots from maples and willows can invade joints within 10 years, so wrap the first 5 feet of pipe near trees with landscape fabric before backfill. Some landlords paint the pipe route on a property map; closing a seasonal home for winter includes checking that downspout connections are still tight.

How do I install the dry well itself?

Dig a hole 10–20 feet from the foundation, sizing it per the calculation above. A 4×4×4-foot cube requires moving 1.9 cubic yards of soil — rent a mini excavator for $200/day if you're installing multiple wells. Line the hole with non-woven geotextile fabric (Mirafi 140N or equivalent) to prevent silt from migrating into the stone and clogging voids over time. Overlap seams by 12 inches and drape the fabric up the sidewalls, leaving 12 inches of excess at the top to fold over the final layer of gravel.

Fill the hole with 3/4-inch washed stone (also called #57 stone or clean gravel) to within 6 inches of grade. Some installers place a perforated 18-inch-diameter plastic dry well barrel in the center to add structure, but it's optional — the stone itself is the reservoir. Fold the fabric over the top layer of stone, then cap with 6 inches of topsoil and sod or mulch. Mark the well location with a flat landscape rock or small sign so you don't drive heavy equipment over it. A properly installed dry well lasts 20–30 years before silt reduces capacity; wells in sandy soil can function for 40+ years.

Slope the final grade away from the house at 2 percent for the first 10 feet so surface water also sheds away from the foundation. This complements the underground drainage and prevents water from re-entering at the basement wall. If your yard is flat or slopes toward the house, consider a basement waterproofing membrane on the exterior wall in addition to the dry well.

When should I hire a contractor instead of DIY?

Call a drainage contractor if the water table is within 4 feet of the surface, your property sits on a slope steeper than 10 percent, or local code requires an engineered system. Coastal areas with high groundwater and clay-heavy regions like Dallas-Fort Worth or the Piedmont often mandate perforated pipe trenches or French drains instead of simple dry wells, which need a professional to design and permit. If you've had basement water issues or ceiling water stains from roof leaks that trace back to poor surface drainage, a contractor can evaluate the entire grading plan and tie the dry well into a comprehensive fix.

Excavating near old cast iron drain pipes also warrants a pro — hitting a corroded sewer lateral with a shovel can break it and trigger a $5,000 repair. If you're unsure about the 1/8-inch-per-foot slope or the soil drains slowly in your test hole, spend $300–500 on a site visit from a landscape drainage specialist who can calculate volumes, recommend well spacing, and mark safe dig zones. Most counties don't require a permit for a residential dry well under 500 gallons, but call your building department to confirm before you dig.

FAQ

How far from the house should a dry well be located?

At least 10 feet from the foundation to prevent water from seeping back toward the basement, and 20+ feet if your soil has high clay content or poor drainage. Keep wells 10 feet away from septic drain fields and property lines to avoid saturating a neighbor's yard.

Can I use a dry well if I have clay soil?

Yes, but you'll need a larger well or multiple wells to handle slower infiltration rates. Dig the test hole first — if water takes more than 24 hours to drain, double the calculated volume or run the pipe farther into the yard where topsoil is deeper and less compacted.

What's the difference between a dry well and a French drain?

A dry well is a single underground reservoir filled with gravel; a French drain is a trench with perforated pipe that disperses water along its length. Use a dry well when you have a defined low spot and good soil; use a French drain when water needs to move across a slope or around a foundation.

How do I maintain a dry well after installation?

Clean gutters twice a year so leaf debris doesn't wash into the downspout pipe. Inspect the connection where the pipe enters the dry well every spring — if water backs up during rain, the stone voids may be silted and the well needs excavation and refill. Most wells require no maintenance for 15–20 years if the fabric liner was installed correctly.

Can I connect multiple downspouts to one dry well?

Yes, if you size the well for the combined roof area. Run separate 4-inch pipes from each downspout and converge them with a wye fitting before the well inlet, or install a larger 5×5×5-foot well to handle the extra volume. Two downspouts from a 2,000-square-foot roof can dump 2,500+ gallons in a 2-inch storm, so calculate capacity before digging.

This is educational information, not engineering or grading advice. Consult a licensed civil engineer or drainage contractor if your property has high groundwater, steep slopes, or a history of foundation settlement.