When roof runoff has nowhere safe to go — no slope to daylight, no storm sewer, and a small yard that ponds after heavy rain — a dry well is one of the most practical fixes a homeowner can install. A dry well is an underground chamber that collects downspout water and releases it slowly into the surrounding soil. The most common reason dry wells fail is simple: they were sized wrong. Too small and the well overflows back toward the foundation; too big and you waste money and yard space.
Direct answer on dry well sizing: start with the roof area — one inch of rain on 100 square feet of roof produces about 62 gallons of runoff. Size the well to hold the runoff from your design storm (a 1-inch storm is the common residential target), then adjust for the storage medium: a gravel-filled pit stores water in only about 40 percent of its excavated volume, so the hole must be roughly 2.5 times the water volume, while prefabricated chambers store nearly 100 percent. Finally, confirm the soil drains at least half an inch per hour with a percolation test, and keep the well at least 10 feet from the foundation.
What a Dry Well Is and How It Handles Roof Runoff
A dry well is an underground infiltration structure — a pit of clean stone, a prefabricated plastic chamber, or a concrete ring — connected to one or more downspouts by buried pipe. During rain it does two jobs at once: it temporarily stores the surge of roof runoff, then infiltrates that water into the surrounding soil through its open sides and bottom. Once the storm passes, the well empties itself by percolation and is ready for the next rain.
Storage Plus Infiltration, Explained Simply
Think of a dry well as a buffer between the cloudburst and the soil. Roofs deliver water fast — a single downspout can discharge dozens of gallons per minute in a heavy storm — while most soils absorb water slowly, in fractions of an inch per hour. The well’s storage absorbs the peak; the soil handles the slow release. Sizing is the act of balancing those two rates so the well neither overflows in a design storm nor stays full for days.
When a Dry Well Is the Right Solution for a Small Lot
Dry wells suit properties with limited drainage space: downspouts discharging too close to the foundation, a flat yard that cannot be graded away from the house, no storm sewer to tie into, or local rules encouraging on-site retention. Because the system is buried, it preserves usable yard space.
When a Dry Well Is the Wrong Choice
Skip the dry well when the soil is heavy clay draining far below half an inch per hour, when the seasonal high water table sits less than two to three feet below the proposed well bottom, when the lot is too small to meet setbacks from the foundation, property lines, and septic components, or when runoff may carry oil or chemicals. Dry wells infiltrate water into the ground and must receive clean roof runoff only — never wastewater or septic effluent.
The 5-Step Dry Well Sizing Calculation
Dry well sizing is plain arithmetic. Work these five steps in order — each feeds the next, and skipping one is how undersized wells happen.
Step 1: Measure the Roof Area That Drains to the Well
Count only the roof surface actually connected to the well: measure each contributing section’s length and width and multiply, adding sections together for shared wells. A typical single downspout on a two-story home drains roughly 300 to 600 square feet.
Step 2: Choose a Design Storm
The design storm is the rainfall event the well must handle without overflowing. A 1-inch storm is the widely used residential starting point — most storms drop an inch or less, and it also captures the dirtiest “first flush” of larger storms. If your municipality requires a larger storm, use the larger value.
Step 3: Calculate the Runoff Volume
Runoff volume (gallons) = roof area (sq ft) x rainfall depth (inches) x 0.623. The 0.623 factor reflects that one inch of rain over one square foot equals 0.623 gallons (one cubic foot holds 7.48 gallons). Quick reference for a 1-inch storm: 500 sq ft = ~312 gallons; 1,000 sq ft = ~623 gallons; 1,500 sq ft = ~935 gallons.
Step 4: Adjust for the Void Space of Your Storage Medium
This is where most sizing mistakes happen. Washed stone occupies about 60 percent of a gravel pit, leaving roughly 40 percent void space — so divide the target water volume by 0.40, meaning the excavation must be about 2.5 times the water volume. Prefabricated chambers are open inside and store nearly 100 percent of their volume, needing far less digging for the same capacity.
- Gravel pit: excavation = runoff volume divided by 0.40
- Chamber: storage = chamber volume (a common 50-gallon chamber plus stone bed handles ~135-165 gallons total)
- Concrete ring: storage = interior volume, but heavy — usually a pro job
Step 5: Confirm the Soil Can Infiltrate the Volume
Storage only buys time; the soil must drain the water between storms. As a planning rule, soil at the well’s depth should infiltrate at least half an inch per hour. Slower soils need a larger well with more sidewall area — or a different drainage solution entirely.
Worked Example: Sizing a Dry Well for a 1,000-Square-Foot Roof
One downspout drains 1,000 sq ft; design for a 1-inch storm. Runoff = 1,000 x 1 x 0.623 = 623 gallons (~83 cu ft). A gravel pit needs 83 / 0.40 = ~208 cu ft — e.g., 6 x 6 ft by 5.8 ft deep. With 50-gallon chambers: 623 / 50 = ~13 chambers, or split the roof across two smaller wells. If the perc test lands at the 0.5 in/hr minimum, upsize by roughly a third as a safety margin.
Testing Your Soil: The Simple Percolation Test
Test at the exact spot and depth of the proposed well — topsoil results say nothing about the subsoil the well will drain into.
1. Dig a 6-inch-wide hole as deep as the planned well bottom.
2. Fill it with water and let it soak overnight to saturate the soil.
3. Refill to a marked level the next day and start a timer.
4. Measure the drop over one hour; repeat once for consistency.
5. At least 0.5 inch per hour is the commonly cited minimum; 1 inch per hour or more is comfortable; below 0.5 means clay or compaction — relocate or choose another method.
Test in the wet season if possible — a rate measured in dry August can look better than the same soil performs in wet March, and the well must work in the worst conditions.
Dry Well Types Compared
Gravel-filled pit: cheapest materials and fully DIY-friendly, but the largest excavation given the 40 percent void ratio — best for permeable soils and modest volumes. Prefabricated plastic chamber: open interior stores nearly its full volume, so the hole is far smaller and lightweight units set easily — best for tight lots, at higher material cost. Concrete ring: durable but heavy — usually a pro job. All three need a washed-stone bed, a full non-woven geotextile wrap, and at least a foot of soil cover.
Where to Place the Dry Well: Setbacks and Clearances
Treat these as minimums and verify against local code, since requirements vary:
- At least 10 feet from the foundation (15 feet with a basement).
- At least 10 feet from property lines in most residential guidance.
- Well away from septic tanks, drain fields, and private wells — commonly 50 to 100+ feet from drinking-water wells.
- Well bottom at least 2-3 feet above the seasonal high water table.
- Downhill from the downspout with ~1/8-1/4 inch of pipe slope per foot.
- Call 811 before digging to mark buried utilities.
Installation Steps at a Glance
1. Mark the well location and pipe trench; confirm setbacks and utility marks.
2. Dig the pit to size (typically 3-5 ft deep, slightly sloped sides); trench the inlet with steady downhill pitch.
3. Line the excavation with non-woven geotextile, leaving extra to fold over later.
4. Spread 6 inches of washed stone, set the chamber, connect the 4-inch downspout pipe.
5. Backfill with washed stone; fold the fabric over the top into a silt-blocking envelope.
6. Cover with 12+ inches of soil, grade away from the house, reseed.
7. Add an overflow path (grate, pop-up emitter, or daylighted pipe) for extreme storms.
Overflow Handling and Cold-Climate Considerations
Every dry well needs a planned overflow — a grate or overflow pipe near the top that releases water onto the ground away from the foundation when a storm exceeds the design event. In freezing climates, add a flow diverter or removable downspout connection so you can disconnect the well in winter; a frozen inlet turns the system into an ice plug, and freeze-thaw can heave shallow chambers. Burying the inlet below the frost line where practical helps further.
Maintenance That Keeps a Dry Well Working
Wells fail slowly, then all at once — almost always from silt clogging the stone voids. Clean gutters, add leaf guards, and fit a small catch basin or filter on the inlet to trap sediment. After major storms, check that the overflow path is clear. Ponding above the well years later signals clogging: the fabric envelope and clean stone are what deliver a 15- to 25-year service life.
Common Sizing and Installation Mistakes
- Forgetting the void ratio — the classic error leaving a gravel pit under half its intended capacity.
- Skipping the perc test and finding clay at 4 feet after digging.
- Placing the well too close to the foundation, so water migrates back to the basement.
- Using unwashed stone or skipping the geotextile wrap — both invite rapid clogging.
- No overflow provision for storms beyond the design event.
- Connecting anything but clean roof runoff — never wastewater or contaminated water.
Dry Well Costs: DIY vs. Professional
Treat all figures as estimates. A DIY gravel-pit well for one downspout typically runs a few hundred dollars in materials (washed stone, fabric, 4-inch PVC), with the main cost being a weekend of digging; one widely cited DIY guide puts a basic project near $200-$250 in materials plus 6-8 hours of labor. Chambers add roughly $50-$150 each but cut digging sharply. Professional installations commonly land in the low four figures. Check whether your municipality requires a permit for subsurface drainage.
When to Call a Professional
DIY suits a single-chamber well in sandy soil on a straightforward lot. Call a drainage contractor or engineer when the perc test fails, the water table is high, setbacks cannot be met, a large roof must be managed, or local code requires engineered calculations.
Final Thoughts
Dry well sizing is not mysterious: measure the roof, pick a design storm, compute the runoff, adjust for void space, and verify the soil. Most failures trace to skipping one of those steps. Do the math first, respect the setbacks, wrap everything in geotextile, and give extreme storms a safe overflow. Next step: run the percolation test this weekend at your proposed spot — if the soil passes, you can size and dig with confidence.
Frequently Asked Questions
How big a dry well do I need for a 1,000 sq ft roof?
For a 1-inch storm, a 1,000 sq ft roof yields about 623 gallons. A gravel pit needs roughly 208 cubic feet of excavation (about 6 x 6 x 5.8 feet). With 50-gallon chambers, plan about 12-13 chambers — or split the roof across two smaller wells.
How far should a dry well be from the house?
At least 10 feet from the foundation — 15 feet with a basement. Infiltrating water migrates outward, so a closer well can send water back toward the foundation. Also keep ~10 feet from property lines and much greater distances from septic systems and drinking-water wells.
What happens when a dry well fills up completely?
A properly built well has a planned overflow — a grate, pop-up emitter, or overflow pipe — that releases excess water away from the foundation during storms beyond the design event. Without one, a full well backs water up the inlet pipe to the downspout.
Dry well vs. French drain: which should I choose?
Choose a dry well for concentrated roof runoff with nowhere to send it — it stores and infiltrates a point-source volume. Choose a French drain for shallow groundwater or surface water crossing the yard — it intercepts a flow path. The two are often combined, with a French drain feeding a dry well.
How long does a dry well last?
A well-built dry well — washed stone, full geotextile envelope, sediment control on the inlet — commonly serves 15 to 25 years. Silt clogging the stone voids is the usual cause of death, which is why gutter guards and inlet filters matter most.