How Do Dry Wells Work?

Thanks to a new garage build, Roger Cook, landscape contractor, and Kevin O’Connor of This Old House, need to install a 36′ long, 8′ wide, and 7′ deep engineered trench to manage stormwater and additional runoff at their renovation project.

How Dry Wells Work

Stormwater can cause costly flooding and damaging erosion to properties. To prevent this, downspouts and drains redirect rainfall and runoff from non-absorbent surfaces (like a roof and driveway) into a concrete galley or dry well that has been buried underground. The perforated unit(s) allow water to dissipate slowly and drain out of the holes, into the ground.

Each of the galleys featured in this video hold 300 gallons of water—giving the homeowner a whopping total of 2400 gallons worth of water storage. Additionally, thanks to the stone foundation, this system will be able to process and drain even more water than that.

A job this scale is expensive—about $25,000—but it wasn’t a surprise to the homeowners. Because the new garage build reduced the amount of available ground that typically helps absorb rainfall, the need for a dry well had been part of the original design.

Steps for Installing a Dry Well

1. After removing 100 yards of material to prep and dig the hole, landscape fabric is laid on top of the hole to prevent debris, soil, and subsequent fines (pieces of soil that are so small—like silt and clay—that easily pass through larger sieves) from filling in and clogging the dry well.
2. 12 inches of stone is added to the hole to act as the foundation.
3. Concrete galleys, also called dry wells, are placed on top of the stone. This particular project calls for 8 galleys—per the engineers’ recommendation.
4. Once the galleys are placed inside the hole, the hole is backfilled with gravel to set the dry wells. Eventually, the stone will cover the entire system—about 12 inches worth of gravel will be laid on top of the galleys.
5. Clean-outs—easy access points that allow the homeowner to clean out debris that may clog the drains—are added and connected to the dry wells.
6. At the foot of the driveway, a trench drain will be cut and tied into the tanks and the home’s downspout.

Considerations that Affect the Job:

Percolation Test: Also known as a “perc test,” this test determines how fast water drains into the soil. The faster the water drains into the soil within a certain amount of time, the better location for a dry well.

Calculating Impervious Surfaces: The more non-absorbent surfaces that a building site has, like driveways and roof surfaces, the larger the dry well system required to effectively manage all the additional runoff and avoid flooding and erosion.

Capability for a Worst Case Scenario: This particular system is large enough to handle a 100-year storm—a storm that can expect to drop 9″ of rain over 24 hours.

A project this size is better left to the professionals, but if there is an area of your home or yard that pools and floods during rainstorms, it is possible to build and install a simple dry well for your home.