Dry Wells Explained

A dry well is a buried pit or tank that catches water — most often from a downspout — and lets it soak slowly into the surrounding soil. It is essentially a temporary underground reservoir. The catch is simple and unforgiving: it only works where the soil can actually absorb water. In heavy clay or where the water table sits high, a dry well fills and backs up instead of draining.

What a dry well actually is

Picture a hole in the ground, several feet across and several feet deep, filled with coarse gravel or fitted with a hollow plastic chamber. A pipe carries water into it. The water fills the empty space between the stones (or inside the chamber), then seeps outward through the walls and floor into the soil around it. Over the hours after a storm, the well empties and is ready for the next one.

That is the whole idea. There are two common forms:

• Gravel-filled pit. The oldest version — a lined hole packed with washed stone. Cheap to build, but the stone takes up much of the volume, so it holds less water than a chamber of the same size.
• Manufactured chamber. A perforated plastic tank or stacked crate modules wrapped in filter fabric. It gives you far more storage volume per cubic foot dug, which is why it has largely replaced the gravel-only pit for roof runoff.

Either way, the well is usually wrapped in landscape fabric so soil does not migrate in and silt it up, and the top is capped with soil, sod, or an access lid.

Where a dry well fits

A dry well lives at the end of a run, not the middle. The classic use is the terminus of a buried downspout line: rain hits the roof, funnels into the gutter, drops down the downspout, travels underground away from the house, and finally reaches the dry well where it soaks away. If you are planning that underground run, our guide to underground downspout drainage covers the pipe part; the dry well is what you connect it to when there is no daylight outlet or storm sewer to send the water to.

That last point matters. A dry well is the answer when you have nowhere obvious to send water — no ditch, no lower ground, no municipal storm connection. Instead of moving the water somewhere, you make somewhere for it to disappear into the earth.

The one condition everything depends on

Soil drainage. If the ground around the well cannot absorb water at a reasonable rate, nothing else you do will save the design.

The rough field test is a percolation check: dig a hole roughly where the well will go, fill it with water, let it drain once to pre-soak, then refill and time how far the water drops. Fast-draining sandy or gravelly soil swallows water quickly and makes an excellent host. Heavy clay can take many hours to drop an inch — far too slow to keep up with a downpour draining off a roof.

A high water table causes the same failure for a different reason: if groundwater already sits near the depth of your well, there is nowhere for new water to go. The well stays full and backs up toward the inlet after every rain.

Sizing: it depends, and here is what it depends on

There is no universal “make it this big” number, and anyone who gives you one without asking about your soil is guessing. Two variables drive the size:

• How much roof drains into it. More square footage of roof means more gallons per storm.
• How fast your soil absorbs. Slow soil needs a bigger well to hold water while it waits its turn to soak in.

The interaction is the important part. The same roof over fast sand might need a modest well; over slow soil it might need one several times larger to avoid overflowing — assuming the soil drains at all. Local rainfall intensity matters too, which is one reason infiltration sizing is often set or checked against local stormwater guidance.

Factor	Pushes size up when…	Pushes size down when…
Roof area feeding it	Large roof / whole-house run	Small section of roof
Soil absorption rate	Clay or slow loam	Sand or gravel
Storm intensity	Heavy, short downpours common	Gentle, spread-out rain
Water table depth	Groundwater sits high	Deep, dry subsoil

Treat this as a starting frame, then confirm against a percolation test and your local code rather than a rule of thumb.

Maintenance

A dry well is low-effort, not no-effort. The enemy is sediment: roof grit, leaf debris, and fine silt gradually clog the soil pores around the well and the fabric wrapping it, and a clogged well drains slower every year. To slow that down:

• Keep leaves and grit out of the gutters feeding it (guards help).
• Put a silt-trapping catch basin or sediment sump upstream so debris settles before it reaches the well.
• Clean that upstream trap on a schedule.
• Watch for surface water lingering over the well after storms — the first sign it is losing capacity.

Even well-maintained, a dry well has a finite life measured in years to a couple of decades depending on how dirty its incoming water is.

Common mistakes

• Installing one without testing the soil. The single most common failure. If you have not done a percolation test, you do not know whether a dry well will work — you are hoping.
• Placing it too close to the house. A dry well releases water into the ground; do that near the foundation and you have just relocated the problem underground. Keep it out and downhill.
• Ignoring the water table. A well that sits in seasonally high groundwater has nowhere to drain and stays full.
• No sediment protection. Feeding raw roof and gutter debris straight into the well shortens its life dramatically.
• Undersizing for the roof. A well built for a corner of the roof will overflow when a whole downspout points at it.
• Assuming it is allowed. Some areas regulate stormwater infiltration and setbacks. Confirm locally before digging.
• Expecting it to never need attention. It will slow down as sediment accumulates; plan to maintain the upstream trap.

The bottom line

A dry well is an elegant solution to one specific problem: you have roof water and nowhere to send it, and your soil drains. When both of those are true, it quietly makes the water disappear. When the soil does not cooperate, no amount of digging changes the physics — and that is the first thing to check, not the last.