How Big Should a Dry Well Be for a Residential Roof?
Two houses with the same square footage can need very different dry wells. The footprint of the roof, the storm you plan for, and the way your downspouts split the load decide the answer.
On this page15 sections
- Dry Well Volume by Roof Footprint
- Why Getting the Size Right Matters
- Roof Measurements That Get Confused
- The Roof Dry Well Formula
- How to Find Your Input Values
- Worked Examples
- Choosing a Design Storm
- Sizing Per Downspout vs Whole Roof
- Common Mistakes
- One Well or Several?
- Quick Reference: Chambers by Roof Footprint
- Factors That Can Push the Size Up or Down
- Summary
- Frequently Asked Questions
- Related Calculators
You can skip the arithmetic with the Dry Well Sizing Calculator, which returns runoff volume and chamber count for your own dimensions. If you want to know what the system does before you size it, the guide on what a dry well is and how it works covers the parts and the infiltration process. This article focuses on one question: how many gallons your roof sends to the well, and how big the well must be to hold them.
Dry Well Volume by Roof Footprint
This table uses a 95 percent runoff coefficient, which suits asphalt shingles, metal, and tile roofs. The two columns show a moderate storm and a heavy one.
| Roof footprint | Runoff at 1 in storm | Runoff at 2 in storm |
|---|---|---|
| 800 sq ft | 473 gal | 947 gal |
| 1,000 sq ft | 592 gal | 1,184 gal |
| 1,200 sq ft | 710 gal | 1,420 gal |
| 1,500 sq ft | 888 gal | 1,776 gal |
| 2,000 sq ft | 1,184 gal | 2,367 gal |
| 2,500 sq ft | 1,480 gal | 2,959 gal |
A quick mental rule follows from these numbers. Every square foot of roof sheds about 0.59 gallons per inch of rain, so a 1,000 sq ft roof delivers roughly 600 gallons in a 1 in storm.
Why Getting the Size Right Matters
A dry well that is too small fills partway through a storm and overflows next to the house, which is the exact problem it was meant to solve. A well that is far too large wastes money on chambers, stone, and excavation, and a bigger hole means more risk of hitting utilities, tree roots, or the water table.
The size of the well also affects whether you can build the system at all. Many jurisdictions review stormwater infiltration against a stated design storm, and an undersized well can fail inspection. Check what your building or stormwater department requires before you buy chambers.
Roof Measurements That Get Confused
The most common sizing error is using the wrong roof area. These four numbers sound similar but mean different things.
| Term | What it measures | Use it for dry well sizing? |
|---|---|---|
| Roof footprint | The flat area the roof covers when viewed from above, including the eave overhang | Yes |
| Sloped roof area | The true surface area of the roof planes, larger than the footprint on a pitched roof | No, because it overstates rainfall |
| Floor area | Interior living space across all levels | No, a two-storey house has half the roof of its floor area |
| Catchment area | Everything that drains to one well, roof plus any added paved surface | Yes, this is the number the formula needs |
Rain falls straight down, so a steep roof catches the same rain as a flat roof with the same footprint. That is why pitch does not increase the volume of water, even though it increases the number of shingles on the roof. Roofers use sloped area for materials, which is covered in the roofing calculators, but a dry well needs the horizontal footprint.
The Roof Dry Well Formula
The calculation is the standard runoff formula. The 0.623 factor converts square feet and inches of rain into gallons.
Divide the answer by the capacity of one chamber or gravel unit and round up. When the roof drains through several downspouts, run the formula once for each downspout’s share, or once for the whole roof if every downspout feeds the same well.
How to Find Your Input Values
- Footprint: Measure the outside of the house at the roof edge, including the eave overhang, and add any attached garage, porch, or addition under the same roofline. Rain that lands on the overhang still reaches the gutter.
- Share per downspout: Count the downspouts and note which roof planes drain to each. A long ridge usually splits the roof roughly in half along each gutter run.
- Design storm: Look up local rainfall depth for the storm your area uses. The NOAA Atlas 14 precipitation frequency data gives depths for 1, 2, 5, 10, and 25-year storms in most US locations, and your county may name a required storm.
- Runoff coefficient: Use 90 to 95 percent for a standard roof. Use 95 percent if you want a conservative design.
Worked Examples
Example 1: Ranch house, one shared well
- Roof footprint: 1,500 sq ft
- Rainfall: 1 in
- Runoff coefficient: 95%
- Runoff volume: 887.8 gal
- Chamber: 100 gal
- Chambers needed: 9
The math is 1,500 × 1 × 0.623 × 0.95 = 887.8 gallons, which is 8.9 chambers rounded up to 9. At $75 per chamber, the chambers cost $675 before excavation and gravel.
Example 2: Same house, four separate wells
- Footprint per downspout: 375 sq ft
- Rainfall: 1 in
- Runoff coefficient: 95%
- Runoff per well: 221.9 gal
- Chamber: 100 gal
- Chambers per well: 3
The math is 375 × 1 × 0.623 × 0.95 = 221.9 gallons, or 2.2 chambers rounded up to 3 for each well. Four wells use 12 chambers in total, three more than the single shared well in Example 1. Each well rounds up on its own, which adds extra capacity, and separate wells also avoid long buried pipe runs and keep each infiltration area small.
Example 3: Two-storey house in a wetter region
- Floor area: 2,000 sq ft (two levels)
- Roof footprint: 1,000 sq ft
- Rainfall: 2 in
- Runoff coefficient: 95%
- Runoff volume: 1,183.7 gal
- Chambers needed: 12
The math is 1,000 × 2 × 0.623 × 0.95 = 1,183.7 gallons, or 11.8 chambers of 100 gal rounded up to 12. The two-storey layout halves the roof footprint compared with the floor area, but the heavier design storm doubles the volume, so the final well is larger than a 1 in design would suggest.
Have your roof measurements? Enter the footprint, pick your design storm, and get the gallons and chamber count for each downspout.
Choosing a Design Storm
The design storm is the single biggest lever on well size. A higher number costs more but protects against heavier events. These are general ranges, so use your local data and any required storm first.
| Design rainfall | Typical use | Effect on well size |
|---|---|---|
| 1 in | Moderate climates, small yards, budget builds | Baseline |
| 1.5 in | Areas with regular heavy storms | 50% larger than baseline |
| 2 in | Wetter regions, or when local rules ask for a larger storm | Double the baseline |
| 3 in or more | Coastal or storm-prone areas, critical sites near a foundation | Triple the baseline or more |
Rainfall intensity, meaning how fast the rain falls, matters for surface inlets and gutters even when total depth stays the same. The guide on rainfall intensity and catch basin size explains why short bursts stress a drainage system differently than a long soaking rain.
Sizing Per Downspout vs Whole Roof
You have two ways to apply the formula. Neither is wrong, but they lead to different layouts.
The whole-roof method adds all downspouts into one catchment, then sends the total to a single well or a row of wells connected by pipe. It needs a buried collector line from each downspout and a well location that can take the combined flow. The per-downspout method gives each downspout its own well sized for that share of the roof. It uses short pipe runs and spreads infiltration across the yard, which works better when the soil is only moderately permeable.
Pick the per-downspout layout when your downspouts sit on opposite sides of the house, and pick one larger well when a single downspout location has plenty of room and good soil.
Common Mistakes
- Floor area instead of footprint. A two-storey house has a smaller roof than its total living space suggests.
- Sloped roof area. Rain falls vertically, so the horizontal footprint is the correct input, not the true plane surface.
- Missing overhang. Eaves catch rain that flows into the gutter, so measure to the roof edge.
- Missing paved areas. A patio, walkway, or driveway that drains to the same downspout adds to the catchment. The guide on sizing for impervious surfaces shows how to add them.
- Rounded-down results. A result of 4.2 chambers needs 5, not 4.
- Tall walls over a lower roof. Some drainage designers add part of the wall area that sheds onto a lower roof, so check how your local rules treat it.
One Well or Several?
| Situation | Better layout |
|---|---|
| Small house with one or two downspouts close together | One shared well |
| Downspouts on opposite sides of the house | One well per side |
| Moderately draining soil with limited room | Several smaller wells spread out |
| Fast-draining soil and a large open yard | One larger well, a longer trench of chambers, or both |
| Rain you also want to reuse for plants | Rain barrel first, dry well for the overflow |
Quick Reference: Chambers by Roof Footprint
These counts use a 1 in storm and a 95 percent runoff coefficient, so they match the first table. Change either input and the numbers change.
| Roof footprint | 50 gal chambers | 100 gal chambers |
|---|---|---|
| 800 sq ft | 10 | 5 |
| 1,000 sq ft | 12 | 6 |
| 1,200 sq ft | 15 | 8 |
| 1,500 sq ft | 18 | 9 |
| 2,000 sq ft | 24 | 12 |
| 2,500 sq ft | 30 | 15 |
Factors That Can Push the Size Up or Down
Soil is the main reason a calculated size is not the final answer. Sandy soil drains between storms, so the calculated volume can be enough. Clay drains slowly, so many owners add extra capacity or a shallow infiltration trench. A percolation test tells you which case you have.
Gutter capacity also limits how much water reaches the well. If the gutters overflow in a heavy burst, water leaves the roof before it ever enters the downspout, which means the dry well can look underused while the foundation still gets wet. Clean, adequately sized gutters and downspouts keep the real flow close to your calculation.
An addition, a new garage, or a future driveway all raise the catchment later, so build in room to add chambers if you expect changes. Setbacks from the foundation, septic system, drinking-water well, and property line often limit where the well can sit, and those distances vary by jurisdiction. The earthwork and drainage calculators and the irrigation and water calculators cover materials for related site work.
Summary
- Measure the roof footprint, including the eaves, not the sloped surface or the floor area.
- Add any paved area that drains to the same downspout.
- Choose a design storm from local rainfall data, with 1 in as a moderate baseline.
- Apply a runoff coefficient of 90 to 95 percent for a standard roof.
- Calculate gallons with the 0.623 factor and divide by the capacity of your chamber or gravel unit.
- Round up for every well, and decide whether to size per downspout or for the whole roof.
- Add margin for slow soil and confirm local permit rules before you dig.
Frequently Asked Questions
How many gallons does a typical house roof send to a dry well?
A 1,500 sq ft roof footprint sends about 888 gallons in a 1 in storm at 95 percent runoff. Heavier storms scale the volume in direct proportion.
Does roof pitch change the dry well size?
No. Rain falls vertically, so the flat footprint decides the volume. A steeper pitch adds roofing material but not extra water.
Should I size one dry well for the whole roof?
You can, if all downspouts connect to the same well. Many owners prefer a separate well per downspout because it avoids long collector pipes and spreads infiltration across the yard.
What design storm should I use?
Start with any storm your local building or stormwater department requires. Without a requirement, 1 in works as a moderate baseline, and 2 in is a common choice in wetter regions.
How do I size a dry well for a flat roof?
Use the same formula. A flat roof has the same footprint and runoff coefficient as a pitched roof, so the volume follows the same calculation. Check that the roof drains and scuppers carry the water to the well without ponding.
Is a bigger dry well always better?
Not always. Extra storage helps when soil drains slowly, but a very large well costs more, takes more excavation, and does not fix poor soil or a high water table.
Related Calculators
Size it before you buy. Get the runoff volume, chamber count, and cost for your roof in one step.