Where to send roof runoff and how to size a dry well
Keep roof water out of the septic system — New York excludes roof, footing, garage and cellar drainage outright [1]. Send downspouts away from the foundation or into a dry well, rain garden or storm drain. Size a dry well for a design storm from NOAA Atlas 14 [2]: in our example calculation, a 1,500 sq ft roof in New York City needs about 184 sq ft (17 m²) of 3-ft-deep stone trench.
Updated 2026-09-29 · every regulation and price links to its source at the bottom of the page. A guide, not a construction design.
Can roof water go into a septic tank or the sewer?
Not into a septic system. New York’s rule reads: “Roof, footing, garage, cellar and surface water drainage must be excluded from the system.” [1] EPA advises pointing roof drains and sump pumps away from the drain field [3], and Washington makes that an owner’s duty [4].
The model building code sends surface water to a storm sewer or another approved place [5]. Whether a downspout may tie into a city sewer is up to your city; we found no national rule.
Where can you discharge downspouts?
The International Residential Code says: “Lots shall be graded to drain surface water away from foundation walls.” The ground must fall at least 6 in (152 mm) within the first 10 ft (3 m), and paving within 10 ft of the house must slope at least 2% away from it [5]. On expansive or collapsible soils, roof water must be discharged at least 5 ft (1.5 m) from the foundation or into an approved drainage system; for other soils the code sets no distance [6].
Wisconsin’s rain garden manual recommends placing a rain garden at least 10 ft (3 m) from the house, 8 ft (2.4 m) from a well and 5 ft (1.5 m) from a septic system [7].
A federal detail many owners miss: a dry well deeper than it is wide, or any underground system with perforated pipe, is a Class V injection well. It needs no individual permit, but the owner must send basic inventory information to the state’s injection-well program — for a new well, before building it. Rain gardens and shallow, wide trenches without perforated pipe are not Class V wells [8].
How far should a dry well be from the house, septic system and well?
| Distance to | Connecticut (recommended) [9] | Clark County, WA [10] | Washington septic rule [11] |
|---|---|---|---|
| House foundation | 10 ft (3 m) if downslope; 50 ft (15 m) if upslope | 10 ft (3 m) from any structure | — |
| Septic system | 50 ft (15 m); less under conditions | 10 ft (3 m) from the drain field | 30 ft (9 m) from the drain field, 10 ft (3 m) from the tank |
| Private well | 100 ft (30 m); not applied to clean roof runoff | — | — |
| Another dry well | 25 ft (7.6 m) | 10 ft (3 m) [12] | — |
How do you size a dry well for a heavy storm?
There is no national rule on which storm to design for: Connecticut uses the “water quality volume” of its own manual, Clark County, Washington, a minimum volume of gravel per roof area. Your local manual decides. Connecticut’s manual gives two methods:
- Static method. The dry well holds the whole design volume, with no credit for water soaking away during the storm. Void ratio 0.4 for stone, 0.3 for sand. It must empty within 48 hours: Td = 12·V / (K·A), bottom area only [13].
- Dynamic method. Bottom area A = V / (Dp + n·Ds + K·T/12): V is runoff (cu ft), Dp the ponding depth and Ds the stone depth (ft), n = 0.4, T = 2 hours of filling, K = 50% of the field-measured infiltration rate (in/hr). Tabulated soil rates are not allowed [14].
- Infiltration test. Measured in the field at the planned bottom, within 10 ft (3 m) of the dry well. A septic perc test is not accepted because it overestimates the rate [15].
- Rule of thumb (Clark County, WA). At least 60 cu ft of gravel per 1,000 sq ft of roof in coarse sand or gravel, 90 cu ft in medium sand; bottom at least 1 ft (0.3 m) above the seasonal high water table [12].
How heavy is a design storm in major US cities?
US design rainfall comes from NOAA Atlas 14, served by the Precipitation Frequency Data Server of NOAA’s Hydrometeorological Design Studies Center (hdsc.nws.noaa.gov; public domain data) for any point and return periods of 1–1,000 years [2]. Values below are for one point in each city center, in inches (mm); query your own coordinates for a real design. The last column is our example calculation from the box above.
- Seattle and the rest of the Pacific Northwest are outside Atlas 14: the server answers “not within a project area”, and older NOAA Atlas 2 values still apply [2].
- Illinois: many agencies require the State Water Survey’s Bulletin 70 (updated as Bulletin 75), which gives higher totals than Atlas 14 for most of the state [17].
- NOAA Atlas 15 will replace Atlas 14 once published: preliminary estimates in September 2026, publication in 2027 [18].
| City | 1 hr, 10-yr | 1 hr, 100-yr | 24 hr, 10-yr | 24 hr, 100-yr | Trench bottom, 1,500 sq ft roof |
|---|---|---|---|---|---|
| New York, NY [16] | 1.89 (48) | 2.87 (73) | 5.59 (142) | 8.78 (223) | 184 sq ft (17.1 m²) |
| Los Angeles, CA [19] | 1.02 (26) | 1.61 (41) | 4.13 (105) | 6.42 (163) | 100 sq ft (9.3 m²) |
| Chicago, IL [20] | 2.09 (53) | 3.07 (78) | 4.45 (113) | 7.20 (183) | 203 sq ft (18.9 m²) |
| Houston, TX [21] | 3.23 (82) | 4.84 (123) | 8.74 (222) | 17.01 (432) | 314 sq ft (29.2 m²) |
| Phoenix, AZ [22] | 1.26 (32) | 2.01 (51) | 2.20 (56) | 3.43 (87) | 123 sq ft (11.4 m²) |
| Philadelphia, PA [23] | 2.05 (52) | 2.91 (74) | 4.84 (123) | 7.76 (197) | 199 sq ft (18.5 m²) |
| Atlanta, GA [24] | 2.13 (54) | 3.27 (83) | 5.04 (128) | 7.52 (191) | 207 sq ft (19.2 m²) |
| Miami, FL [25] | 3.39 (86) | 5.20 (132) | 8.27 (210) | 14.41 (366) | 330 sq ft (30.6 m²) |
| Denver, CO [26] | 1.30 (33) | 2.24 (57) | 2.83 (72) | 4.65 (118) | 127 sq ft (11.8 m²) |
What if your soil is clay or silt?
A dry well only works where water soaks away. Clark County reserves dry wells for coarse and medium sands and sends other soils to rain gardens or bioretention [12]. In the dynamic method, a slow measured rate shows up at once as a large area and a drain time over 48 hours.
Wisconsin sizes a rain garden as a share of the roof area: 0.13 in sandy, 0.30 in silty and 0.35 in clayey soil for a garden 6–7 in deep [27]. Example calculation: 1,500 sq ft of roof on silty soil — 0.30 × 1,500 = 450 sq ft (42 m²).
Costs, approximate, as of September 2026: a plastic dry well module $90–$350, a concrete dry well installed $300–$5,000 or more [28]; a guide last updated in 2023 put the national average for a finished installation at $3,030 [29].
Frequently asked questions
Can I connect my gutters to my septic tank?
No. New York excludes roof and footing drainage from septic systems [1], and EPA advises keeping downspouts away from the drain field [3].
How far from the house should a dry well be?
Connecticut recommends 10 ft (3 m) when the dry well is downslope of the foundation and 50 ft (15 m) when upslope [9]; Clark County, WA, requires 10 ft from any structure [10].
How big a dry well do I need for a 1,500 sq ft roof?
It depends on the storm and the soil. In our example calculation for New York City, about 184 sq ft (17 m²) of 3-ft-deep stone trench [14] [16].
Do I need a permit for a dry well?
If it is deeper than wide or uses perforated pipe, it is a federal Class V well: no individual permit, but inventory information goes to the state before you build [8]. Local permits vary.
Which rainfall data should I use?
NOAA Atlas 14 for your exact coordinates [2]; in Illinois, check whether Bulletin 70/75 is required [17].
Sources
- 10 NYCRR Appendix 75-A Wastewater Treatment Standards — Residential Onsite Systems (New York, eff. 2016-03-16), 75-A.3(a) (US) · prohibition · checked 2026-09-29 · source
- NOAA Atlas 14 Precipitation-Frequency Atlas of the United States (NWS HDSC), Precipitation Frequency Data Server (PFDS) (US) · regulation · checked 2026-09-29 · source
- US EPA SepticSmart — How to Care for Your Septic System, Inspect and Pump Frequently; Maintain Your Drainfield (US) · recommendation · checked 2026-09-29 · source
- WAC 246-272A-0270 Operation, monitoring, and maintenance — Owner responsibilities (Washington On-site Sewage Systems rule), (1)(e), (f), (h), (k) (US) · regulation · checked 2026-09-29 · source
- International Residential Code 2024 (ICC), Chapter 4 Foundations — R401.3 (via UpCodes, GSA adoption), R401.3 Drainage (US) · regulation · checked 2026-09-29 · source
- International Residential Code 2024 (ICC), Chapter 8 Roof-Ceiling Construction — R801.3 (via UpCodes, GSA adoption), R801.3 Roof Drainage (US) · regulation · checked 2026-09-29 · source
- Wisconsin DNR «Rain Gardens: A how-to manual for homeowners», siting (US) · recommendation · checked 2026-09-29 · source
- US EPA Underground Injection Control — Stormwater Drainage Wells (Class V), Definition; Minimum federal requirements (US) · regulation · checked 2026-09-29 · source
- Connecticut Stormwater Quality Manual (CT DEEP, 2023), Chapter 10.3 Siting, Table 10-3 Recommended Minimum Horizontal Setback Distances (US) · recommendation · checked 2026-09-29 · source
- Clark County (WA) Stormwater Manual 2015, BMP T5.10A, Setbacks (US) · regulation · checked 2026-09-29 · source
- WAC 246-272A-0210 Location (Washington On-site Sewage Systems rule), (1) Table IV Minimum Horizontal Separations; (3), (4) (US) · regulation · checked 2026-09-29 · source
- Clark County (WA) Stormwater Manual 2015, BMP T5.10A Downspout Full Infiltration – Drywells, Design Criteria (US) · regulation · checked 2026-09-29 · source
- Connecticut Stormwater Quality Manual (CT DEEP, 2023), Chapter 10.5 Sizing of infiltration systems, Sizing Methods — Static Method; Table 10-5 (US) · recommendation · checked 2026-09-29 · source
- Connecticut Stormwater Quality Manual (CT DEEP, 2023), Chapter 10.5 Sizing of infiltration systems, Dynamic Method; Table 10-6 (Infiltration Trench) (US) · recommendation · checked 2026-09-29 · source
- Connecticut Stormwater Quality Manual (CT DEEP, 2023), Chapter 10.2 Infiltration testing, Field infiltration testing (US) · recommendation · checked 2026-09-29 · source
- hdsc.nws.noaa.gov (US) · regulation · checked 2026-09-29 · ✓ · source
- isws.illinois.edu, Rainfall Frequency Information in Illinois; Comparison of ISWS Bulletin 70 and NOAA Atlas 14 (US) · common practice (not a rule) · checked 2026-09-29 · source
- NOAA Atlas 15 Informational Page (NWS Office of Water Prediction), Timeline (US) · regulation · checked 2026-09-29 · source
- hdsc.nws.noaa.gov (US) · regulation · checked 2026-09-29 · ✓ · source
- hdsc.nws.noaa.gov (US) · regulation · checked 2026-09-29 · ✓ · source
- hdsc.nws.noaa.gov (US) · regulation · checked 2026-09-29 · ✓ · source
- hdsc.nws.noaa.gov (US) · regulation · checked 2026-09-29 · ✓ · source
- hdsc.nws.noaa.gov (US) · regulation · checked 2026-09-29 · ✓ · source
- hdsc.nws.noaa.gov (US) · regulation · checked 2026-09-29 · ✓ · source
- hdsc.nws.noaa.gov (US) · regulation · checked 2026-09-29 · ✓ · source
- hdsc.nws.noaa.gov (US) · regulation · checked 2026-09-29 · ✓ · source
- Wisconsin DNR «Rain Gardens: A how-to manual for homeowners», Table 1 (US) · recommendation · checked 2026-09-29 · source
- CostHelper — 2026 Cost of a Dry Well, typical costs (US) · price · checked 2026-09-29 · ~ · source
- LawnLove — How Much Does Dry Well Installation Cost, cost table (US) · price · checked 2026-09-29 · ✓ · source
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