Dry wells (stormwater infiltration wells) are an option for managing clean roof and other uncontaminated stormwater on-site; many state stormwater manuals explicitly allow dry wells for roof runoff and note that roof runoff generally does not require pretreatment, though an overflow is required for large storms[3].
Because dry wells used for infiltration are regulated federally as Class V injection wells under the Safe Drinking Water Act, there are baseline federal protections intended to prevent harm to underground sources of drinking water; follow EPA guidance and expect state or local rules on top of the federal program[1][2].
Check permits and local rules first
States may be delegated authority under the EPA underground injection control (UIC) program and frequently impose additional permit, registration, or design requirements for dry wells; homeowners should verify state and local rules before designing or installing a system[1][2].
State stormwater manuals differ on allowable uses, setback distances, and minimum vertical separation from seasonal high groundwater or an impermeable layer; some local guides use about 2 feet as a common minimum vertical separation but required separations vary and may be larger—always confirm the applicable local standard and any setbacks from wells, foundations, or septic systems[3][8][1].
Basic design principle: match storage + infiltration to runoff and drawdown time
Design starts by estimating runoff volume from the contributing area and the chosen design storm. A common sizing principle is:
and then size the dry well (stone-filled pit or manufactured chamber units) so that the stored runoff can infiltrate within the selected drawdown (dewater) time for the jurisdiction[9][10][4][3].
Practical conversion: about 0.623 gallons fall per square foot for each inch of rainfall (so 1 inch on 1,000 ft² ≈ 623 gallons), which makes quick back‑of‑envelope runoff estimates easy to compute: roof area × design rainfall (in inches) × runoff coefficient × 0.623 = gallons to capture[9][10].
Site-specific infiltration capacity is determined with a field infiltration/percolation test: pre-soak a test hole, measure the stabilized drawdown or infiltration rate, and use that stabilized rate in design calculations (rates are reported in inches/hour or mm/hour)[7].
Many local guides require applying a conservative reduction factor to the measured infiltration rate to account for long‑term clogging, seasonal changes, and construction disturbance—Los Angeles County guidance specifically calls for reducing the field rate when selecting the design infiltration rate[8][7].
Also follow the local drawdown-time expectation: for example, Connecticut recommends dry wells dewater completely between storms and uses a 6–12 hour minimum draining time to help pollutant removal, while Minnesota’s guidance may use longer drawdown windows (24–48 hours) for certain crediting purposes—use the timing your local manual requires[3][4].
Sizing a stone pit vs manufactured chambers
Two common approaches are a stone‑filled pit or modular manufactured chambers or units. Manufactured products list per‑unit void storage so you can combine units to reach required storage; for stone pits include both the product void storage (if using a modular element) and the stone void space when calculating total available storage[9][10].
After you determine storage volume needed, confirm the combination of void storage and the site infiltration rate will empty the system within the required drawdown time specified by local guidance[9][10][4].
Pretreatment, overflow, and prohibited flows
State manuals often allow roof runoff directly to dry wells without pretreatment but consistently require an overflow or bypass for storms that exceed device capacity—include a protected overflow path to a storm sewer or stable surface outlet in your design[3].
Do not route pollutant‑laden flows—vehicle‑wash water, petroleum or oily runoff, septic effluent, or other contaminated liquids—into infiltration dry wells; the EPA warns that such flows can carry petroleum and organics that threaten groundwater if accepted into stormwater drainage wells[1][3].
Setbacks and vertical separation
Jurisdictions set different setback distances from potable wells, buildings, and septic systems—do not assume a single universal separation distance; consult your local building or health department and the applicable state manual for required setbacks[1][3][5].
Minimum vertical separation from the bottom of the infiltration structure to seasonal high groundwater or an impermeable layer is set by jurisdictions and commonly used minima in some local manuals are about 2 feet, but confirm the exact local requirement because it may be larger and may include additional constraints[1][3][8].
Materials and practical notes
Manufactured chamber systems and pre‑engineered dry‑well kits provide predictable per‑unit storage volumes and can simplify sizing and installation; follow the manufacturer’s installation guidance and combine units as needed to meet the calculated storage requirement[9][10].
For homeowner budgeting, consult current consumer‑facing cost guides; national consumer estimate sites such as Angi, HomeGuide, and HomeAdvisor provide installed cost ranges to use when preparing a local budget—use the figures on those sites directly for up‑to‑date local estimates[11][12][13].
Checklist: steps before you dig
Confirm local/state rules and permits for Class V/stormwater drainage wells and any delegated UIC program requirements[1][2].
Locate utilities, potable wells, foundations, and septic systems; verify required setbacks in the applicable manual[1][3].
Run a pre‑soak infiltration test and use the stabilized rate for design, then apply the required conservative reduction factor per local guidance[7][8].
Calculate runoff volume (area × rainfall × runoff coefficient × 0.623) and size storage so it empties in the local required drawdown time; verify overflow routing[9][10][3].
Choose stone pit or manufactured chamber, confirm combined void storage, and follow manufacturer and local installation requirements[9][10].
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Checklist: steps before you dig
If none of these checklist items apply to your situation, consult your local permitting authority and EPA guidance before proceeding.
Confirm plans with local permitting authorities or a licensed professional before proceeding.