Drainage
Dry Wells Explained: When You Need One and How to Size It
A dry well stores stormwater underground and lets it soak away. When a dry well beats daylight discharge, how to size one with a percolation test, installation steps, and why they fail in clay.
Every drainage guide says "route the water to daylight," and every flat-lot owner asks the same question: what if there is no daylight? The honest answer is a dry well, and the honest caveat is that a dry well is a soil test wearing a plastic shell. Get the sizing and the soil right and it quietly swallows storms for decades; skip the perc test and you have buried money.
When a dry well is the right call
Choose a dry well when all three are true: the lot has no downhill exit for a pipe, the soil percolates acceptably, and the water source is intermittent (roof water, a sump discharge, a French drain outlet). It is the wrong tool for constant groundwater flows and for soils that hold water like a bowl.
Step 1: Perc test (30 minutes, free, non-negotiable)
- Dig a hole 12 inches wide and as deep as the planned well bottom.
- Fill with water and let it drain once to saturate the soil.
- Refill, then measure the drop per hour.
| Drop per hour | Soil verdict | Dry well outlook |
|---|---|---|
| 4+ in | Sand / sandy loam | Excellent, size normally |
| 1-4 in | Loam | Good, add 25-50% capacity |
| 1/2-1 in | Silty / clay loam | Marginal: oversize, add overflow |
| < 1/2 in | Clay | Poor: reconsider, or engineer it |
Step 2: Size the storage
Use at least a 1-inch storm; 2 inches where cloudbursts are common. A 600 sq ft roof section at 1.5 inches needs about 560 gallons. Commercial units hold 50-100 gallons each; the surrounding washed-stone envelope adds roughly 30-40% of its own volume in usable storage. Compute the pit volume with the cubic yard calculator, and remember stone displaces water: a gravel-only well stores only about a third of its dug volume.
Step 3: Build it
- Excavate 10+ feet from the foundation, bottom 2 feet above seasonal high water. A 4×4×4 ft pit is about 2.4 yards of spoil (haul-away math in the excavation cost guide).
- Line the pit sides with non-woven fabric; leave the bottom open in sand, fabric it in silt.
- Bed 6 inches of washed stone, set the unit(s), and connect the inlet pipe arriving at 1-2% fall.
- Add an overflow: a stub to a lower lawn area or a second well, so back-to-back storms have somewhere to go.
- Envelope with stone, wrap fabric over the top, cap with 12+ inches of soil, and photograph before backfilling.
Why dry wells fail
No perc test. The failure isn't the well, it's the clay around it. Test first, always.
Sized to the product, not the storm. One 50-gallon unit under four downspouts is a puddle generator with extra steps. Do the gallons math.
Silt intrusion. Roof grit and soil fines seal the stone envelope over years. A leaf strainer at the gutter and a cleanout on the inlet line are cheap life extension.
Too close to the house. A soaked zone beside a basement wall is a self-inflicted leak. Distance and downhill placement are the whole game.
Sources & standards: quantities and factors in this guide follow the references documented in our methodology, including ACI 302/318 (concrete practice), OSHA 29 CFR 1926 Subpart P (excavation safety), ASTM aggregate designations, and the International Residential Code. Local codes, climate, and site conditions govern.
Frequently Asked Questions
What is a dry well?
A buried storage chamber, either a perforated plastic tank or a gravel-filled pit, that accepts stormwater from a pipe and lets it percolate into surrounding soil. It is the terminus of choice when no downhill discharge point exists.
How big should a dry well be?
Size it to hold the design storm from its drainage area: roof area (sq ft) x rainfall (in) x 0.62 = gallons. A 500 sq ft roof section in a 1-inch storm needs ~310 gallons, roughly one 50-gallon unit plus its gravel envelope in sandy soil, or several units in slower soil.
Do dry wells work in clay soil?
Poorly. A dry well is only as good as the soil's percolation rate; in tight clay, water sits for days and the well surcharges in back-to-back storms. Run a perc test first, and if water drops less than an inch per hour, plan for a bigger well, an overflow, or a different solution.
How deep should a dry well be?
Deep enough that the inlet pipe arrives with 1-2% fall and the top of the well sits 12+ inches below grade, typically 4-6 feet total. Keep the bottom at least 2 feet above the seasonal water table, or the well fills from below.
How far from the house should a dry well be?
10 feet minimum, 15-20 preferred, and never uphill of the foundation. You are deliberately soaking the ground; do it where the water cannot find its way back to the basement.