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Roof Drain Sizing and Stormwater Sewer Sizing

Roof Drain Sizing and Stormwater Sewer Sizing

Roof Drain Sizing and Stormwater Sewer Sizing

I have a question regarding how to size the roof drain discharge sewer to nearby manhole.
For building, when we size the roof drain, the national plumbing code has form to select the roof drain size based on 25mm/15min storm for most area in Ontario.
When we size the storm sewer, water flow is from Rational Method Formula Q=CIA, I(mm/hr) rainfall intensity is from City's IDF curve. If I=190mm/hr(5-year), the water flow is bigger, we need much big pipe than using plumbing code.
So How to size the building discharge sewer, follow the plumbing code or Q=CIA?
Appreciate your help!

RE: Roof Drain Sizing and Stormwater Sewer Sizing

the question you need to answer is - what happens when the storm sewer is undersized and water backs up in the downdrain and on the roof?  Does the roof leak?  I would err on the side of conservatism.  Go with the bigger pipe.

RE: Roof Drain Sizing and Stormwater Sewer Sizing

The answer is: However the municipality tells you to.

The Building Code is a set of MINIMUM standards and the municipality may have higher standards that they wish you to use. A perfect example of that is the municipality telling you which design storm to use for sizing the sewers. I've had some municipalities tell me to use a 2 Year storm and others a 5 Year storm. Not to mention some municipalities require a minimum 0.6m/s full flow velocity in storm sewers while others require 0.76m/s and that can also effects your sewer design.

Of course, I'm guessing you're designing a larger site so there is also the possiblity you may be needing to take stormwater management into account which can also have an impact on storm sewer sizes.

RE: Roof Drain Sizing and Stormwater Sewer Sizing

I see the same thing on a regular basis, including a project I'm working on now.  The architect has sized the roof drains and downspouts according to the building code.  I am sizing the storm sewer system according to the city's storm water ordinance.  My storm sewers have a much higher design requirement than the roof drains.  When rainfall exceeds the roof drain capacity, it will overflow the roof and still make its way to the storm sewers, so I have to design accordingly.

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