Roof 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.
InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.
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.
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 maximum rainfall rate anticipated in the area where the building is located - the flow in liters per minute per square meter for some x number of minutes. This data is available in excruciating detail for most countries in weather map form and as you'll read below has also been programmed into some roof drainage system calculators
The total area of roof to be drained
The roof pitch or slope (Factors for roof pitch are used to multiply or increase the rainfall rate if we are draining a sloped roof)
The roof drain or gutter or drain piping or downspout leader size-diameter & other size constraints (e.g. 6 5/8" I.D.). For example in the U.K. designers specify compliance with BS EN 12056-3:2000 that includes a calculation for the allowable capacity of the rainwater pipes. The standard recommends that the pipe should be designed at a maximum of 1/3rd full (Filling degree 0.33)
- see www.roofconsult.co.uk/calculations/drainage/notes.htm
Required roof drainage rate in liters per second - a consideration that may be based on type of roof covering, roof projections or penetrations, and the structural strength of the roof - its ability to carry weight of accumulated water or water + wet snow
Roof drain flow resistance factors such as roof drain slope, constrictions, elbows, bends (standard plumbing calculations on drain carrying capacity)
Local and model code requirements or standards that regulate roof drainage in your area
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. - www.buildingregs4plans.co.uk/guidance_flat_roof_drainage.php
A roof drainage calculator for the UK complying with BS EN 12056 is provided by Harmer Drainage Systems and can be found at www.harmerdrainage.co.uk/calculator/register/
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
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
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
Building Regs 4 Plans, Website: http://www.buildingregs4plans.co.uk/, [busy complex pages, helpful data if you can make it]
Tinsley Lane North
RH10 9FF - http://www.ribaproductselector.com/Docs/5/12915/external/COL796841.pdf
Harmer Drainage Systems, Alumasc Exterior Building Products Ltd.,
White House Works, Bold Road, Sutton, St Helens, Merseyside, WA9 4JG, UK, Telephone for technical support: 44 (0)1744 648400, Email: email@example.com
Harmer Drainage System Flat Roof Drainage Requirements Calculator (download) - see http://www.harmerdrainage.co.uk/calculators/
Jay R. Smith Manufacturing, 2781 Gunter Park Dr. E.
Montgomery, AL 36109-1405
Comments* (2000 character limit)
, Email: firstname.lastname@example.org , Telephone:
(800) 467-6484, Website: http://www.jrsmith.com/
Try the search box below or CONTACT US by email if you cannot find the answer you need at InspectApedia.
Ask a Question or Search InspectApedia
Use the "Click to Show or Hide FAQs" link just above to see recently-posted questions, comments, replies, try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.
Devitt, Gerald J., and Richard A. Ritter. "Flat roof auxiliary drain system." U.S. Patent 4,248,258, issued February 3, 1981.
"Flashing: the plain solution to leaky walls", Thomas E. Remmele, Manager, Technical Services, Sto Corporation, Building Standards, November/December 1999 p. 21-25.
Fu, Michael C., Caroline C. Hayes, and E. William East. "SEDAR: expert critiquing system for flat and low-slope roof design and review." Journal of Computing in Civil Engineering 11, no. 1 (1997): 60-68.
Gilgan, Dan P., Jerry F. Gilgan, and Bobby J. Lowery. "Flat style roof drainage system and low point column head." U.S. Patent 6,185,889, issued February 13, 2001.
Hellebrand, Otto. "Edge section for flat roofs with internal drainage." U.S. Patent 3,389,515, issued June 25, 1968.
"Weather-Resistive Barriers [copy on file as /interiors/Weather_Resistant_Barriers_DOE.pdf ] - ", how to select and install housewrap and other types of weather resistive barriers, U.S. DOE
Mosbaek, Johannessen, and Werner Nill. "Method for delaying run-off of flash-storm water or ordinary rainwater from roofs and other surfaces with water-retention capability." U.S. Patent 5,800,092, issued September 1, 1998.
Paquette, Jean-Paul, and Luc Trudeau. "Double-level drainage system for flat roofs." U.S. Patent 5,144,782, issued September 8, 1992.
Walter, Ana, Jorge de Brito, and Jorge Grandão Lopes. "Current flat roof bituminous membranes waterproofing systems–inspection, diagnosis and pathology classification." Construction and Building Materials 19, no. 3 (2005): 233-242.
Wright, G. B., L. B. Jack, and J. A. Swaffield. "Investigation and numerical modelling of roof drainage systems under extreme events." Building and Environment 41, no. 2 (2006): 126-135.
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. Tel: (416) 964-9415 1-800-268-7070 Email: email@example.com. The firm provides professional home inspection services & home inspection education & publications. Alan Carson is a past president of ASHI, the American Society of Home Inspectors. Thanks to Alan Carson and Bob Dunlop, for permission for InspectAPedia to use text excerpts from The Home Reference Book & illustrations from The Illustrated Home. Carson Dunlop Associates' provides extensive home inspection education and report writing material.
The Illustrated Home illustrates construction details and building components, a reference for owners & inspectors. Special Offer: For a 5% discount on any number of copies of the Illustrated Home purchased as a single order Enter INSPECTAILL in the order payment page "Promo/Redemption" space.
TECHNICAL REFERENCE GUIDE to manufacturer's model and serial number information for heating and cooling equipment, useful for determining the age of heating boilers, furnaces, water heaters is provided by Carson Dunlop, Associates, Toronto - Carson Dunlop Weldon & Associates Special Offer: Carson Dunlop Associates offers InspectAPedia readers in the U.S.A. a 5% discount on any number of copies of the Technical Reference Guide purchased as a single order. Just enter INSPECTATRG in the order payment page "Promo/Redemption" space.
The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 25th Ed., 2012, is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. Field inspection worksheets are included at the back of the volume.
Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Enter INSPECTAHRB in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
Special Offer: Carson Dunlop Associates offers InspectAPedia readers in the U.S.A. a 5% discount on these courses: Enter INSPECTAHITP in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
The Horizon Software System manages business operations,scheduling, & inspection report writing using Carson Dunlop's knowledge base & color images. The Horizon system runs on always-available cloud-based software for office computers, laptops, tablets, iPad, Android, & other smartphones