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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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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)
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.
References for roof drainage system calculations and calculators
Continue reading at FLAT ROOF MOISTURE & CONDENSATION
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