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Roof drain calculator

Enter your building's ZIP code and roof area to get the 100-year, 1-hour rainfall rate that roof drains are sized for, the flow your drains need to carry, and how many drains of a given rating that takes. It is a planning estimate: a licensed engineer or plumber sizes the final system to your local code.

Drain flow for your roof

Enter the building's 5-digit ZIP code.

Optional. If your building department or engineer gives you a design rainfall rate, enter it in inches per hour. It replaces the looked-up rate.

Enter the roof area, in square feet, that drains to these drains. Measure it flat, as length times width, not along the slope.

Optional. Enter the area, in square feet, of any wall that rises above this roof and sheds rain onto it: its length times its height above the roof. The calculator adds half of it, as the International Plumbing Code does; Uniform Plumbing Code states can count less, so check your local code.

Optional. Enter one drain's flow, in gallons per minute, from the manufacturer's published rating at the depth of water over the drain that your designer allows.

Design rainfall
– in/hr

Enter a ZIP code, or a design rate.

Flow the drains must carry
– gpm
Drains needed
– drains

Enter a ZIP code or a design rate, and the roof area, to see the flow the drains must carry.

Plan secondary (overflow) drainage too (see below), and have a licensed engineer or plumber size the final system to your local code.

How the calculator works

  • Area = roof area + 50% of the adjacent wall area (sq ft)
  • Flow (gpm) = 0.0104 × rainfall (in/hr) × area
  • Drains = flow ÷ the drain's rated flow, rounded up

Worked example. At 3.5 in/hr, 18,000 sq ft of roof needs drains that carry 655.2 gallons a minute: 3 drains rated 250 gpm. Try it

  • The design rainfall is the 100-year, 1-hour rate the International Plumbing Code sizes roof drainage on, unless you enter a rate from your building department; the code also accepts rates from approved local weather data.
  • Drainage area = roof area (measured flat) + half the area of any wall that sheds rain onto the roof, following IPC Section 1106.4. States that use the Uniform Plumbing Code add up to half, depending on how the walls are laid out, so check your local code.
  • Flow (gpm) = 0.0104 × rainfall rate (in/hr) × area (sq ft). One inch of rain on one square foot is 144 cubic inches; at 231 cubic inches to the gallon that is about 0.623 gallons an hour, or 0.0104 gallons a minute.
  • Drains needed = required flow ÷ one drain's published flow at the design water depth, rounded up to a whole drain.
  • The calculator doesn't size pipes, leaders or overflow drains. The code sizes piping from the flow through the drains, using tables the designer applies.
  • Rates by ZIP code are NOAA Atlas 14's 100-year, 60-minute estimates, read from the grids NOAA publishes, which are also the basis of its own Precipitation Frequency Data Server, at the internal point the Census Bureau gives for each ZIP code area. The result names the Atlas 14 volume and version it comes from.
  • Atlas 14 doesn't cover Washington or Oregon, where NOAA's current documents are older studies, so the calculator has no rate for ZIP codes there. Enter the rate your building department uses.
  • NOAA Atlas 15 will replace Atlas 14 once it is published, and the calculator will move to it then.

What the 100-year, 1-hour rainfall rate means

It is the amount of rain in one hour, at one location, that has a 1% chance of being exceeded in any given year. The name describes odds, not a schedule: NOAA notes that a 100-year amount has about a 26% chance of happening at a location over 30 years.

The International Plumbing Code sizes roof drainage on this rate, taken from the code's rainfall maps, which come from the National Weather Service, or from approved local weather data. An article in STRUCTURE, the structural engineering magazine, names NOAA's Precipitation Frequency Data Server as the best source for rainfall data. If your building department sets its own rate, enter it as the override.

Why standing water is a problem on a low-slope roof

NRCA counts short-lived ponding as normal, but water still standing 48 hours after a rain, in weather that would dry it, can harm the roof. NRCA's technical staff list the damage: surfacing and membranes that deteriorate early, debris, plant and fungus growth, ice, splitting in some membranes, harder repairs, and more water inside when there is a leak.

Water is heavy, too: each inch of depth weighs about 5.2 pounds per square foot. A roof that sags under it collects more water where it sags. That is why the codes require the structure to carry the water that would build up if the primary drains were blocked.

Primary drains, overflow drains and drain ratings

Primary drains carry the design storm. Where parapets or other construction would trap water if those drains backed up, the plumbing code also requires secondary (emergency overflow) drains or scuppers. They discharge separately, where occupants or maintenance staff will see the water, and they are sized without counting on the primary drains. This calculator sizes primary drains only, so check your local code for how overflow drains are sized.

A drain doesn't have one fixed capacity. Research for the American Society of Plumbing Engineers found that flow through a drain rises with the depth of water over it and varies widely from drain to drain, so the code sizes drainage on the manufacturer's published flow at the expected water depth. Enter that rating as the drain's rated flow.

When to bring in an engineer or plumber

Use the calculator to check an existing roof, compare bids or plan ahead. The drain layout, drain and pipe sizes, overflow design and the water depth the structure can carry are design decisions for the building's designer, usually a plumbing engineer working with the structural engineer under your local code. Drain makers say the same: Jay R. Smith's catalog says local code requirements take precedence over its catalog data.

Bring one in before you add rooftop equipment, change the slope or insulation, or add, move or replace drains, since drainage and structure affect each other.

Treat the drain count as a starting point. Zurn notes that its published flows come from lab tests, and that wind, debris, roof obstructions and slope can change a drain's real flow.

Sources

  1. State of Ohio, Ohio Administrative Code, Rule 4101:3-11-01 Storm drainage (Ohio Plumbing Code, effective 11/1/2017)
  2. NRCA, Professional Roofing, Designing roof drains (Kurt Fester, July/August 2026)
  3. Jay R. Smith Mfg. Co., Roof Drain Technical Data Section
  4. MIFAB, MIFAB Roof Drain Selection Guide: Sizing and Placement of MIFAB Roof Drains (Canada)
  5. National Institute of Standards and Technology (NIST), Handbook 44 (2026), Appendix C: General Tables of Units of Measurement
  6. Zurn, Roof Drain Sizing Methods and Graphs (Z100 flow performance data)
  7. NOAA National Weather Service, Hydrometeorological Design Studies Center, NOAA Atlas 14 Precipitation Frequency Estimates in GIS Compatible Format
  8. U.S. Census Bureau, Gazetteer Files (2026 ZIP Code Tabulation Areas)
  9. U.S. Census Bureau, Gazetteer File Record Layouts
  10. NOAA National Weather Service, Office of Water Prediction, HDSC Current Precipitation Frequency Documents
  11. NOAA National Water Prediction Service, NOAA Atlas 15 Informational Page
  12. NOAA National Weather Service, Hydrometeorological Design Studies Center, HDSC Frequently Asked Questions
  13. STRUCTURE magazine (NCSEA), Calculating Rain Loads per 2021 IBC (Sandra Hyde, P.E.)
  14. NRCA, Professional Roofing, Still water runs deep (Jason Wilen, July 2012)
  15. NRCA, Professional Roofing, Tech Today: Adequate drainage must be considered when designing roof slope (Mark S. Graham, March 2005)
  16. ASPE Research Foundation, via the Florida Building Commission, Storm Drainage System Research Project: Flow Rate Through Roof Drains (Julius Ballanco, PE)

Updated October 2026.