Drywell

Short definition

A drywell is a subsurface gravel-filled pit that receives roof runoff from downspouts and lets it infiltrate into the surrounding soil. It’s an alternative to surface discharge or a sewer connection, useful where the lot is too small or steep for natural-ground discharge. WA jurisdictions regulate sizing, soil-perc testing, and setbacks.

What it is

A drywell — sometimes called a “soakaway” in older British references — is a hole in the ground sized to a specific catchment area (the roof area feeding it), lined with permeable geotextile fabric, filled with clean drainage gravel, and connected to a downspout or yard drain via inlet pipe. Water enters at the top, percolates through the gravel, and infiltrates into the soil through the geotextile.

What goes into a properly built drywell:

1. Sized to the catchment. The pit volume needs to handle the design-storm runoff from the roof area feeding it. WA jurisdictions typically use NOAA Atlas 14 IDF curves and a 25-year design storm for residential.
2. Soil-percolation tested. The native soil has to actually accept water at a predictable rate. Clay soils may not perc fast enough for a drywell to work.
3. Setbacks. Typically at least 5 feet from the foundation, 10 feet from a septic drain field, and 100 feet from a private water well. Local rules vary.
4. Geotextile fabric lining the pit — keeps surrounding soil out of the gravel and maintains long-term infiltration.
5. Clean drainage gravel — typically 1–1.5 inch washed rock; never fines.
6. Inlet pipe with a debris filter or first-flush diverter at the downspout so leaves and grit don’t clog the gravel.
7. Permeable cap — soil and turf or gravel; never concrete.

A common rule-of-thumb sizing reference (from UK soakaway design, useful for residential WA scale): a 1 m³ pit with gravel, located at least 5 m from the building. WA stormwater design uses local rainfall intensity data and is more rigorous for permitted installations.

Why it matters to a homeowner

Drywells are often the answer when a lot is too small or too sloped for the standard “extend the downspout 4 feet from the foundation and let the yard handle it” approach. Urban Seattle lots, hillside properties in Issaquah and Bellevue, and any home with a paved-over yard are typical candidates.

For Seattle homeowners in CSO basins, a properly sized drywell is one of the qualifying improvements that disconnects a downspout from the combined sewer, which can be RainWise-rebate eligible. Verify the current rebate rate directly at the program source before assuming a specific amount.

When a contractor proposes a drywell, ask three questions: (1) Has soil percolation been tested at the proposed location? (2) Has the sizing been calculated for your roof catchment area and local design storm? (3) What permit, if any, does this require in your jurisdiction? A drywell built without those checks often fails within a few years.

Common failure modes

• Undersized for catchment — overflows in heavy rain.
• Soil percolation declines over time — sediment accumulation, biofilm, root incursion.
• Geotextile fabric clogs — typical replacement is 10–20 years.
• Tree-root invasion — grows into the gravel; reduces capacity.
• Built too close to foundation — moisture migration into the basement.
• No first-flush diverter — leaves and grit clog the gravel quickly.

Washington note

WA stormwater regulation is jurisdiction-specific:

Seattle — Per the 2021 Stormwater Code, drywells and other infiltration BMPs are sized using the NOAA Atlas 14 design storm and require soil-perc testing. Seattle Department of Construction and Inspections and Seattle Public Utilities jointly enforce.

King County unincorporated — The King County Surface Water Design Manual is the standard, and many WA cities adopt it by reference. The manual includes sizing tables and BMP design specs.

Statewide — The Department of Ecology’s Stormwater Management Manual for Western Washington (SWMMWW) is the design reference adopted by many Western WA jurisdictions. Verify the current edition before citing specific design figures.

In Eastern WA (Spokane, Tri-Cities), high-permeability soils make drywells one of the easier and more effective stormwater solutions. Western WA lots with glacial till or heavy clay often perc poorly, and a drywell that looked good on paper can fail to drain after the first wet winter — soil-perc testing matters.

For pre-purchase scoping in WA, if a sewer scope reveals an existing drywell on the property, ask the seller for any service or maintenance record. A 20-year-old drywell with no maintenance is likely near the end of its functional life.

Common variants

• Drywell (subsurface infiltration pit for stormwater) vs. dry well (irrigation/water-source term, different).
• Drywell vs. French drain — French drain is a linear surface or subsurface drain; drywell is a single point destination.
• Drywell vs. rain garden — rain garden is a planted depression with partial infiltration plus plant uptake; aesthetically and ecologically richer.
• Soakaway (UK term) — same concept under a different name.