Downspout used for air conditionre refrigerant line (C) Daniel FriedmanRoof Drainage Requirements & Specifications
How to calculate or look up roof drainage requirements for flat or sloped roofs

  • ROOF DRAINAGE CALCULATIONS - CONTENTS: how to determine the drainage requirements for a building roof; flat roof, low slope roof, or pitched roof drain requirements can be expressed in the number and size of roof drains needed.
  • POST a QUESTION or READ FAQs about figuring or specifying roof drainage systems
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Roof drainage requirements & specifications: this article describes several methods for determining the drainage capacity required to safely drain water off of building roofs whether from rainfall, melting snow, or a combination of those water sources. We describe online roof drainage requirements calculators, pertinent plumbing standards, a table-approach to looking up roof drainage specifications, and we provide a simple formula for calculating the water volume that must be drained from a roof.

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How to Calculate the Drainage Requirements for Flat Roofs

Reader Question: is there a rule of thumb for number of roof drains needed on flat roofs

6/24/2014 David Howard, AIA said:

What is the general area of a flat (1/4"/1'-0" slope) single-ply roof for each roof drain (ratio for the number of roof drains / roof area).


Excellent question, David.

We have had the analogous question arise in form of how many downspouts are needed per linear foot of roof gutter - which after all is draining the same horizontal footprint area of rainfall collection even though the sloped roof will have more actual surface area

I think a simple answer would, unfortunately, be nonsense. To design a sloped or flat roof drainage system we need to know several things:

The Roof Drainage Requirements: Calculations

I reviewed several roofing company specifications that discuss drains looking in vain for a good rule of thumb. A calculator (or table as we give below) gives the number of roof drains required to drain the particular roof under design, based on the parameters listed above.

Watch out: keep in mind that a design without adequate safety margins risks dangerous structural collapse of the roof or other catastrophes such as a building flood. Just consider what happens in "real life" when roof drains become clogged because no one remembered to inspect and clear the drains.

Watch out: to prevent dangerous building loading or even structural collapse, roof drainage systems using an internal or piped roof drain system should provide for overflow drains to handle excess water volume and weight loading should the roof drainage system become blocked or prove inadequate in unusual conditions.

The general approach to calculating roof drainage requirements includes

The design point of anticipated maximum rainfall intensity. E.g. for the UK BS EN 12056-3:2000 gives rainfall intensity in liters per second per square meter for a 2 minute storm event. For the UK, the maps in the standard show the intensity for various return periods from 1 year to 500 years.

An assumption about the degree to which the drain piping can be filled (U.K. est. = 1/3)
Assumed maximum rainfall rate or snowmelt + rainfall rate
Consideration of building roof structure and its ability to carry weight of ponded water or wet snow vs drain rate
For sloped roofs the roof catchment total area is multiplied by an factor that considers pitch
For nominally "flat" roofs the catchment area = the roof area

Continuing with citations from the U.K. standards who have published the most clear information I can find on the question

Gutters and down pipes should be adequately sized to deal with storm conditions in accordance with BS EN 12056-3. Flat roofs should be designed to drain the roof to one or two edges towards gutters and outlets. -

A roof drainage calculator for the UK complying with BS EN 12056 is provided by Harmer Drainage Systems and can be found at

I downloaded, installed, and tried out Harmer Drainage's free roof drainage calculator for a flat roof, choosing a roof drain product line (which has implications of drain pipe diameter and thus capacity), geographic area (the tool can plug in rainfall rates for the U.K.) and I used an assumed a roof area of 10x10m (100 M2). The calculator gives the required flow rate in liters per second for the drainage system as well as the maximum capacity for the drain system selected.

My example, using Ross on Wye (where we were recently visiting Herefordshire in the U.K. in part to compare North American roofs to UK roofs) found that to drain 1000 M3 using a high capacity plastic drain system I'd need 3 drains handling a total of 5.6 liters per second.

Specifications for Roof Drainage Requirements - Table Approach

The following table, adapted from data from several manufacturers and guided by information posted by J.R. Smith Manufacturing also presumes that you have properly specified the roof drainage design parameters we listed earlier in this article. Smith is basically using U.S. plumbing standards and references to obtain a maximum flow rate through a drain system.

Flat Roof Drainage Capacity of a Single Drain Pipe or Leader

Drain Pipe Diameter

Pipe Cross-Section Drainage Area

Rainfall Rate Inches Per Hour

5" 19.6 sq .in. 1 inch / hr 2 inch / hr 3 inch / hr 4 inch / hr 5 inch / hr 6 inch / hr

Maximum horizontal (projected) roof area that can be drained - in Square Feet

25,000 ft2 12,500 ft2 8,300 ft2 6,200 ft2 5,000 ft2 4,100 ft2

Notes: adapted from Smith (2014) cited below. We have rounded Smith's data and comment further below.

Roof drainage capacity for other roof drain pipe or leader diameters:

  • Dropping from the 5" drain in the table to a 4" drain cuts the serviced or drainable roof area to about 0.55 x the roof areas shown in the table
  • Dropping from the 5" drain in the table to a 3" drain cuts the serviced or drainable roof area to about 0.26 x the roof areas shown in the table
  • Increasing from the 5" drain shown to a 6" drain increases the roof area that can be served by about 1.6 x the table data
  • Increasing from a 5" drain shown to an 8" drain increases the roof area that can be drained by about 3.5 x the table data

Watch out: this table excerpts only a single example for a 4-inch diameter roof drain with a number of design assumptions that may not fit your project. The original source provides additional design details and includes drain diameters from 2-inches to 8-inches.

Watch out: there are further assumptions in this approach that may not fit your circumstance, building, environment, or structure, such as

  • Drains are assumed to be 7/24 full through their vertical run and completely full through their horizontal run; the impact of "head water" over the drain opening was not considered and would also impact the actual flow rate through the drainage system
  • Smith warns that the roof drains should be emptying into larger-diameter storm drains to avoid backpressure or hydraulic "jump" that could seriously impair the drain flow rates in the system.
  • Smith used a multiplier of 0.0104 in these calculations to convert inches of rain fall per hour into gallons of water per minute per square foot, resulting in the following formula for calculating roof drain flow rate

Required Roof Drainage Flow rate in Gallons per Minute = 0.0104 x Rainfall-Rate x Roof Area

Where Rainfall Rate is expressed in Inches per Hour
Roof Area is given in Square Feet

1 inch of rainfall (in an hour) will cover 1 sq .ft. to a depth of 1" in 1 hour. Converting this to gallons per minute per square foot (GPM /ft2):

144 sq .in. of water / 231 sq .in. of water in a gallon = 0.623 gallons per hour

1 US Gallon = 231 cubic inches. At 1-inch of depth that's to say a U.S. gallon will cover 231 square inches to a depth of 1 inch.

0.623 gph / 60 = 0.0104 gallons per minute or GPM


References for roof drainage system calculations and calculators

Flat & Low Slope Roof Articles


Continue reading at FLAT ROOF MOISTURE & CONDENSATION or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Also see FLAT ROOF DRAINAGE SYSTEMS if you have not already been there.

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ROOF DRAINAGE CALCULATIONS at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


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