What Is a Dry Well and How Does It Work?
A single heavy storm can send hundreds of gallons off your roof in a few minutes. Where that water goes decides whether your yard drains cleanly or your foundation wall stays damp for days.
On this page16 sections
- How a Dry Well Works
- Common Dry Well Types and How Much They Store
- Why a Dry Well Matters
- Dry Well vs Similar Drainage Terms
- The Dry Well Sizing Formula
- How to Find Your Input Values
- Worked Examples
- How Much Extra Capacity to Add
- Checking Drain-Down Time
- Common Mistakes
- Is a Dry Well the Right Fix?
- Quick Reference: Chambers Needed by Roof Area
- Factors That Change How a Dry Well Performs
- Summary
- Frequently Asked Questions
- Related Calculators
The idea is simple, but the size is what makes or breaks a dry well. To see how many chambers your roof needs, run your numbers through the Dry Well Sizing Calculator. It turns catchment area, rainfall depth, and runoff coefficient into gallons and a chamber count. The rest of this guide explains what you are sizing, how the system behaves in a storm, and where it fits among other drainage options.
How a Dry Well Works
Rain lands on the roof, runs into the gutters, and drops through the downspout. Instead of spilling onto the lawn, the downspout connects to a buried solid pipe that carries the water to the dry well. The water fills the chamber or the voids between the stones, then soaks out through the sides and bottom into the surrounding soil. This process is called infiltration, and it is the reason a dry well can remove stormwater without any pump or outlet pipe.
The well does not need to be empty when it rains, but it does need to empty between storms. A well that is still full from the last event cannot take the next one, which is why soil drainage matters as much as storage volume.
| Part | What it does |
|---|---|
| Inlet pipe | Carries water from the downspout or catch point to the well, usually buried with a slight slope |
| Leaf and debris filter | Keeps sediment, shingle grit, and leaves out of the system so it does not clog |
| Chamber or gravel pit | Stores the water until the soil absorbs it |
| Filter fabric (geotextile) | Wraps the gravel or chamber to keep soil from migrating into the void space |
| Overflow outlet | Sends excess water to a safe discharge point when a storm exceeds the design volume |
| Access cover | Lets you inspect the inlet and clear debris |
Common Dry Well Types and How Much They Store
Most residential dry wells fall into three styles. Capacity depends on the product, so always confirm the figure on the manufacturer’s spec sheet before you size anything.
| Type | Typical storage | Best for |
|---|---|---|
| 50 gal plastic chamber | 50 gal per unit | One small downspout or a shed roof |
| 100 gal plastic chamber | 100 gal per unit | A standard house downspout |
| Gravel infiltration pit | About 40 gal of void space per unit in the calculator | Low-cost builds where excavation is easy |
| Precast concrete ring | Varies by diameter and depth | Large catchments and deeper installations |
A gravel pit stores water only in the gaps between stones, not in the stone itself. Clean crushed stone usually has roughly 30 to 40 percent void space, so a gravel pit needs far more excavated volume than a hollow chamber holding the same amount of water. If you plan to build one, the aggregates calculators help you estimate the stone you will need to haul.
Why a Dry Well Matters
Roofs and pavement are impervious surfaces. They shed almost all the rain they receive, and that concentrated flow has to go somewhere. When it pools beside the house, it can raise hydrostatic pressure against the foundation, wash out landscaping, erode the soil at the downspout, and push moisture into a basement or crawl space.
A dry well keeps that water on your own property and returns it to the ground, which is also what most stormwater programs want. In the US, the EPA treats most drywells that receive stormwater as Class V injection wells under its Underground Injection Control program. For a typical house, roof runoff is usually acceptable, but many states and counties still require registration or a permit, set minimum distances from other structures, and restrict runoff from areas that handle oil or chemicals. Call your local building or stormwater department before you dig.
Dry Well vs Similar Drainage Terms
People use several names for underground drainage, and some of them describe different systems. This table keeps them straight.
| Term | What it is | Key difference |
|---|---|---|
| Dry well | Buried chamber or gravel pit fed by a pipe | Stores runoff and infiltrates it at one location |
| Soakaway | UK name for the same idea | Same function, different regional term |
| Seepage pit | Larger, deeper, often stone-lined pit | Also used for septic effluent in some regions, which is a different system |
| French drain | Perforated pipe in a gravel trench | Moves water along a path while it infiltrates |
| Catch basin | Surface inlet box with a sump | Collects surface water and sends it to a pipe; does not infiltrate on its own |
| Rain barrel | Above-ground storage tank | Holds water for reuse; no infiltration |
If your problem is standing water on a lawn or paved area rather than roof runoff, a catch basin may be the better starting point. The Catch Basin Sizing Calculator covers that case, and the guide on catch basin sizing for impervious surfaces explains how driveways and patios change the numbers.
The Dry Well Sizing Formula
Every dry well starts with one question: how much water reaches it in your design storm? The standard conversion uses 0.623, the number of gallons in one square foot of surface covered by one inch of rain.
Divide the result by the capacity of one chamber or gravel unit, then round up to the next whole unit. A rounded-down result leaves the well undersized for the storm you designed it for.
How to Find Your Input Values
- Catchment area: Measure the roof footprint that drains to the downspout, then add any patio or walkway that also feeds it. Measure the horizontal footprint, not the sloped surface.
- Rainfall depth: Use a design storm for your area. A 1 in storm is a common moderate choice, and local rainfall data from your county or the NOAA precipitation frequency maps will show what heavier storms look like.
- Runoff coefficient: Use about 90 to 95 percent for a hard roof, 85 to 95 percent for asphalt or concrete, and much lower for gravel or lawn.
- Chamber capacity: Read it from the product label. Do not assume that a “100 gal” chamber holds exactly that much once it is buried and wrapped.
Worked Examples
Example 1: Small house roof section
- Catchment area: 750 sq ft
- Rainfall: 1 in
- Runoff coefficient: 90%
- Runoff volume: 420.5 gal
- Chamber: 100 gal
- Chambers needed: 5
The math is 750 × 1 × 0.623 × 0.90 = 420.5 gallons. That divides to 4.2 chambers of 100 gal, which rounds up to 5. At $75 per chamber, the units cost $375 before excavation and gravel.
Example 2: Large roof in a wetter climate
- Catchment area: 1,200 sq ft
- Rainfall: 1.5 in
- Runoff coefficient: 95%
- Runoff volume: 1,065.3 gal
- Chamber: 100 gal
- Chambers needed: 11
The math is 1,200 × 1.5 × 0.623 × 0.95 = 1,065.3 gallons, or 10.65 chambers rounded up to 11. A bigger design storm changes the result quickly, which is why the rainfall input deserves real local data. Many owners in this range split the load between two wells on separate downspouts instead of building one very large pit.
Example 3: Driveway with a gravel pit
- Catchment area: 480 sq ft (20 ft × 24 ft)
- Rainfall: 1 in
- Runoff coefficient: 90%
- Runoff volume: 269.1 gal
- Unit: gravel pit, 40 gal void per unit
- Units needed: 7
The math is 480 × 1 × 0.623 × 0.90 = 269.1 gallons. That divides to 6.7 units of 40 gal, which rounds up to 7. Gravel pits are cheap per gallon but take up a lot of ground, so check the space you have before you commit to one.
Ready to size your own dry well? Enter your roof or paved area, pick a design storm, and get the runoff volume and chamber count in seconds.
How Much Extra Capacity to Add
The formula gives the volume for one design storm in ideal conditions. Real soil is rarely ideal, so most owners add a margin. These are practical rules of thumb, not code values, so defer to your local requirements if they are stricter.
| Site condition | Suggested extra capacity | Why |
|---|---|---|
| Sandy or gravelly soil | 0 to 10% | Water leaves the well quickly between storms |
| Loam | 10 to 20% | Moderate drainage leaves some water stored longer |
| Clay or compacted fill | 25 to 50%, or add an infiltration trench | Slow absorption can leave the well full for the next storm |
| High or seasonal water table | Do not rely on a dry well | The well cannot drain if the ground below it is saturated |
Checking Drain-Down Time
Storage volume tells you whether the well can hold a storm. Drain-down time tells you whether it will be ready for the next one. A percolation test measures how fast your soil absorbs water in inches per hour, and you can use that rate for a quick check.
For example, 24 in of stored water in soil that absorbs 1 in/hr takes about 24 hours to empty. Many local codes expect a dry well to empty within 24 to 72 hours, and many stormwater manuals treat soils slower than roughly 0.5 in/hr as poor candidates for infiltration. If your percolation test comes back slower than that, talk to a local drainage professional before you buy materials.
Common Mistakes
- Missing catchment area. A downspout that also picks up a patio or driveway carries more water than the roof footprint alone.
- Too little foundation setback. Many codes require about 10 ft of setback so the saturated soil does not sit next to the footing.
- No percolation test. Sandy soil and heavy clay need very different well sizes for the same runoff.
- Overlooked water table. A well that sits in wet ground cannot drain.
- No overflow path. Storms larger than the design event will overtop any dry well, so grade a safe route for the excess.
- No inlet filter. Shingle grit and leaves clog the voids over time and shorten the life of the system.
Is a Dry Well the Right Fix?
A dry well suits roof downspouts and small paved areas when the soil drains and the water table is deep. Other tools do better in different situations.
| Situation | Better option |
|---|---|
| Well-draining soil, deep water table, downspout runoff | Dry well |
| Long wet strip along a wall or slope | French drain |
| Standing water on a yard or paved area | Catch basin with an outlet pipe |
| You want to reuse the water for plants | Rain barrel or water collection system |
| Heavy clay or high water table | Grading, a drain to daylight, or professional design |
Many drainage plans combine two of these. A rain barrel can take the first few gallons off a downspout for the garden, and a dry well can take the overflow.
Quick Reference: Chambers Needed by Roof Area
These figures assume a 1 in design storm and a 90 percent runoff coefficient. Change either input and the numbers move, so use the calculator for your own project.
| Catchment area | Runoff volume | 50 gal chambers | 100 gal chambers |
|---|---|---|---|
| 500 sq ft | 280.4 gal | 6 | 3 |
| 1,000 sq ft | 560.7 gal | 12 | 6 |
| 1,500 sq ft | 841.1 gal | 17 | 9 |
| 2,000 sq ft | 1,121.4 gal | 23 | 12 |
| 2,500 sq ft | 1,401.8 gal | 29 | 15 |
Factors That Change How a Dry Well Performs
Soil permeability is the biggest factor. Sand and gravel drain fast, while clay holds water and can make a well useless. The depth to the seasonal high water table and the bedrock comes next, because the bottom of the well needs unsaturated soil beneath it.
Slope and layout also matter. A well on a steep slope can send water out the downhill side and toward a neighbor or foundation, so place it where the ground drains away from buildings. Tree roots, buried utilities, a septic system, and a drinking-water well all limit where you can dig, and most jurisdictions set minimum distances from each. In cold climates, the frost line affects how deep the inlet pipe must sit so it does not freeze and block flow. Roof material changes the runoff coefficient slightly, and gutter condition decides how much debris reaches the filter. For wider drainage projects, the earthwork and drainage calculators and the irrigation and water calculators cover related materials.
An overflow outlet also needs a stable landing point. If it discharges onto bare soil, erosion follows, and a small apron of stone slows the flow. The Riprap Rock Calculator estimates the stone for that apron.
Summary
- Understand that a dry well stores runoff underground and lets it infiltrate into the soil.
- Confirm that your soil drains and that the water table sits well below the planned bottom of the well.
- Measure the full catchment area, including any patio or driveway that feeds the downspout.
- Pick a realistic design storm and a runoff coefficient that matches the surface.
- Calculate the runoff volume and divide by the capacity of your chamber or gravel unit, rounding up.
- Add a margin for slow soils, and plan an overflow path for larger storms.
- Check local permit rules and setbacks before you dig.
Frequently Asked Questions
What is a dry well used for?
A dry well collects roof or paved-surface runoff and releases it slowly into the soil. Homeowners use it to keep downspout water away from foundations and to reduce erosion and standing water in the yard.
How deep is a typical dry well?
Most residential dry wells sit 2 to 4 ft deep. The right depth depends on the frost line, the water table, and the chamber or pit design.
Does a dry well work in clay soil?
It can, but not as a simple stand-alone pit. Clay absorbs water slowly, so the well usually needs extra capacity, an infiltration trench, or a different drainage solution such as a drain to daylight.
Do I need a permit to install a dry well?
Many jurisdictions require one, especially near property lines, septic systems, or private wells. Because stormwater dry wells fall under underground injection rules in the US, check with your building or stormwater department before you dig.
How far should a dry well be from the house?
Many codes require at least 10 ft from a foundation wall, and some require more. Septic systems, wells, and property lines often have their own minimum distances.
How long does a dry well last?
A well with a good inlet filter and regular debris cleanout can last decades. Clogged voids and silted-up fabric are the usual causes of early failure.
Related Calculators
Know the size before you dig. Get the runoff volume, chamber count, and cost for your roof in one step.