How big should a dry well be for a residential downspout?

A typical residential dry well is 4 ft diameter by 4 ft deep — about 1.86 cubic yards or 50 cubic feet of storage when surrounded by 1–1.5 inch drainage rock. That sizes well for a single downspout serving 600–1,000 sq ft of roof in average soil. For a whole-roof tie-in (1,500–2,500 sq ft) or for tight clay soils, scale up to 6 ft × 6 ft (about 6.3 cubic yards) or install a second well in series. Use the Drainage Rock Calculator to size rock by chamber dimensions.

How far from the house should a dry well be?

A dry well should sit at least 10 feet from any foundation, 25 feet from a septic field, and 50 feet from a private well. The whole point of the well is to dump water back into the soil — if it is too close to the foundation, that water finds its way back to the basement instead of percolating away. Site it at the lowest reachable point downhill of the inlet so gravity does the moving.

Do I need a permit to install a dry well?

Most municipalities allow homeowners to install a dry well for downspout or yard runoff without a permit, but a few require one if the well exceeds a certain volume, sits within a setback of a property line or septic system, or ties into a roof drain larger than a single downspout. Stormwater rules in coastal and watershed districts can also be stricter. Call your local building department before digging, and always call 811 for utility locates.

What is the biggest mistake when installing a dry well?

Skipping the non-woven filter fabric, or using woven landscape fabric instead. Soil migrates into the rock voids and packs the well tight within 5–10 years. The fabric is the cheapest piece of the system and the one that decides whether the well lasts a decade or four. Sizing the well for clay soil as if it were sand is a close second — clay percolates so slowly that an undersized well backs up the inlet and floods upstream.

How to Install a Dry Well — Step-by-Step Guide

An eight-step install guide for a residential dry well: locate away from the foundation, excavate, line with fabric, drop the chamber, surround with rock, cap, backfill, and verify outflow.

A dry well is the “where does the water go” answer when daylighting a french drain or downspout is not possible. It is a buried, fabric-wrapped pit of clean drainage rock around a perforated chamber that lets stormwater seep back into the soil instead of running across the lawn or pooling against the foundation. The work is mostly digging and rock; the parts that matter are siting, fabric, and the right size of stone. Use the Drainage Rock Calculator to size the stone before you order, and pair this guide with How to Install a French Drain if the dry well is the outlet for a longer drain run.

Tools & Materials

Tools:round-point shovel, post-hole digger or a rented mini-excavator (worth it — a 4-ft × 5-ft pit is a long afternoon by hand), mattock for clay or roots, 4-foot level, tape measure, utility knife, cordless drill with a 1/2-inch bit if you are converting a 55-gallon drum into the chamber, garden hose for the outflow test, and a wheelbarrow. A spotter at the surface while you work the bottom of a 5-ft pit is a sensible safety call.

Materials (size with the calculators below):

Precast dry-well chamber (concrete or HDPE plastic) sized for your runoff load — or a perforated 55-gallon HDPE drum drilled with 1/2-inch holes on a 4-inch grid as a budget alternative
Washed 1–1.5 inch drainage rock (#4 drainage stone, or coarse #57 if a #4 supply is unavailable) — size with the Drainage Rock Calculator using your pit dimensions
Non-woven filter fabric (4–8 oz/yd² geotextile) — not woven landscape fabric
4-inch solid PVC inlet pipe from the downspout or drain outlet to the chamber, sloped at least 1%
Inspection-port riser (typically a 4- or 6-inch PVC pipe with a screened cap) tied to the chamber and brought to finished grade
Topsoil for the top 4–6 inches — size with the Fill Dirt Calculator for the cap volume, or use the Gravel Calculator if you are capping with decorative stone instead of soil

Step 1: Locate the Well Away from the Foundation

Site the dry well at least 10 feet from any foundation, 25 feet from a septic field, and 50 feetfrom a private well. The whole point is to dump water back into the soil — placing the well too close to the foundation just routes that water back to the basement. Pick a low spot with naturally well-draining soil (sandy loam is ideal; heavy clay either needs a much wider well or a different drainage approach). Call 811 before you dig— utility locates are free and take 2–3 business days. Confirm local rules with the building department; some jurisdictions require a permit when the well ties into roof drains larger than a single downspout.

Step 2: Dig the Well 4–6 ft Deep by 4 ft Wide

For a typical residential dry well serving one downspout, excavate a 4-foot diameter pit, 4–6 feet deep depending on the chamber height and the local frost line. The well bottom needs to reach native, free-draining soil; if you hit a clay layer or groundwater before depth, stop and rethink the approach — a clay-bottomed well is just a buried puddle. For larger roof loads or whole-house downspout tie-ins, scale to 6×6 ft or install a second well in series. Pile spoils on a tarp.

Step 3: Line with Landscape Fabric

Roll non-woven filter fabric(4–8 oz/yd² geotextile) into the pit so that it covers the bottom and walls with at least 18 inches of overhang left at the top to fold over the cap in Step 6. Use one continuous piece if your pit is small enough, or overlap two pieces by at least 12 inches and pin the seam with landscape staples. Woven landscape fabric is not a substitute — its weave is too tight to pass the volumes of water a dry well handles, and it clogs almost immediately.

Step 4: Drop the Dry-Well Chamber or Perforated Drum

Lower the precast dry-well chamber into the center of the pit. Residential chambers run 30–50 cubic feet of internal storage; larger commercial sizes go higher. If you are using a perforated 55-gallon HDPE drumas a budget alternative, drill 1/2-inch holes on a 4-inch grid across the sides and bottom before setting it — the drum gives the well a void to fill rapidly during a storm while the surrounding rock continues to percolate. Connect a 4-inch solid PVC inlet pipe from the downspout or drain outlet to the upstream side of the chamber, sloped at 1% (1 inch per 8 feet) toward the well.

Step 5: Surround with 1–1.5″ Drainage Rock to Grade

Backfill the annular space between the chamber and the fabric-lined wall with washed 1–1.5 inch drainage rock— #4 drainage stone is the textbook choice; coarse #57 works when #4 is not stocked locally. Work in 6-inch lifts to keep the chamber centered and prevent voids. Stop filling about 6 inches below finished grade so there is room for the fabric cap and topsoil. Set a 4- or 6-inch inspection-port risertied to the chamber and extending to grade so the well can be inspected and flushed with a hose every few years — without an inspection port there is no way to maintain the system.

Step 6: Cap with Fabric

Fold the 18-inch fabric overhang from Step 3 across the top of the rock with at least 12 inches of overlap on each side. Trim excess with a utility knife. The fabric cap is what stops surface soil from raining down into the rock voids during the years of settlement and worm-mixing that follow the install. Without a fabric cap, the top 12–18 inches of rock will silt over within a decade and cut effective storage roughly in half.

Step 7: Backfill with Topsoil

Cap with 4–6 inches of topsoil— enough to support turf, perennials, or shallow plantings. Leave the inspection-port riser flush with finished grade and fitted with a screened cap. Tamp the topsoil lightly so it settles without leaving a hollow over the well. Crown the surface slightly above grade so the natural settlement that happens in the first season does not leave a depression that pools surface water (which then bypasses the inlet pipe and floods the cap from above instead of feeding the chamber).

Step 8: Verify Outflow with a Hose Test

Run a garden hose at full flow into the upstream inlet for 15–30 minutes and watch the inspection port. The water level inside the chamber should rise during the run and drop within an hourafter the hose stops — that is the percolation rate confirming the well is working. Persistent standing water past an hour means the well is undersized for the soil, the soil is too tight (heavy clay), or the inlet is partially blocked. Diagnose now while the cap is still soft: widen the well, add a second in series, or convert the system to a daylighted french drain instead. Note the post-test level so you have a baseline for future inspections.

Common Mistakes

Undersized well.A 3×3 ft pit handling a whole roof in clay soil backs up the inlet during the first heavy rain and stays flooded for days. Size the chamber to the runoff load and the percolation rate of the actual soil — not a generic 4×4 default.
No filter fabric.Soil migrates into the rock voids and packs the well tight within 5–10 years. Non-woven geotextile is the cheapest, most important piece of the system; woven landscape fabric is not a substitute.
Fines-laden gravel.Crusher run, dirty pit run, or unwashed driveway base carry fines that wash into the chamber and choke the perforations. Use only washed, angular drainage stone — #4 ideally, coarse #57 as a substitute.
Well too close to the foundation. Inside the 10-ft setback, the well becomes a basement-flooding machine: it dumps every storm event right back where the water came from. Walk the site before digging and confirm 10 ft minimum to the foundation, 25 ft to a septic field, 50 ft to a private well.
No inspection port. Without a riser, there is no way to verify the well is working, no way to flush it, and no way to diagnose a clog before a flooded basement forces the issue. Add a 4- or 6-inch riser tied to the chamber and capped with a screened cover at grade.
Ignoring the frost line. A shallow well that bottoms above the local frost line freezes shut every winter and channels meltwater straight at the foundation during spring thaw. Bottom the well below frost depth (or extend the inlet pipe deep enough that ice never blocks the chamber inlet).

Cost: DIY vs. Contractor

DIY materialsfor a typical 4-ft × 4-ft residential dry well run roughly $300–500:

Precast chamber or perforated 55-gal drum — $80–180 (drum, drilled DIY) or $150–300 (precast HDPE)
Washed 1–1.5″ drainage rock, ~2 cubic yards (2.7 tons) — $80–130 delivered
Non-woven filter fabric, ~50 sq ft — $25–50
4-inch solid PVC inlet pipe + fittings — $25–50
4- or 6-inch inspection-port riser + screened cap — $20–35
Topsoil cap, ~0.5 cubic yards — $20–40

Add $150–250for an afternoon’s mini-excavator rental if you do not want to hand-dig the pit — very much worth it for any well over 4 ft deep.

Contractor-installed dry wells run $1,200–2,000for a residential single-downspout install on straightforward access, including machine excavation, materials, the inlet tie-in, and final grading. Whole-roof systems with multiple wells in series, deep installs in rocky soil, or wells tied into a longer french drain push past $3,000. Hire a pro when the depth exceeds 5 ft, when the soil is heavy clay (sizing requires a percolation test), or when the well needs to clear a buried utility — the cost of fixing an undersized well after backfill is far higher than the install itself.

Related Calculators

Drainage Rock Calculator— Size #4 or coarse #57 stone for the well’s annular envelope
Gravel Calculator— Size base or decorative gravel if you cap the well with stone instead of topsoil
Fill Dirt Calculator— Estimate topsoil for the 4–6 inch cap

Related Guides

How to Install a French Drain— Pair with a dry well as the outlet when daylighting the trench is not possible

Written by Daniel McCarney — AceCalc