Secondary Roof Drainage Q & A
Firstly, there is an explanation in the above-referenced section for where and why a secondary roof drainage system must be provided.
Secondly, the requirement to have a separate system is spelled out.
Thirdly, the sizing criteria are provided in a plain and understandable manner.
However, there are no directions in the Code as to how to install such a system. Because the requirement to have a secondary drainage system is relatively new, some clarification may be necessary. Here are some common questions and answers on this subject.
The roof perimeter construction doesn't extend above the roof. Is there a need for secondary drains?
Not always. If a low point(s) of the roof is located far enough from the roof edge, there is a possibility to accumulate more water than a roof can hold. A typical roof (at least in our neck of the woods) is designed for a snow load which translates to 6" of water, the maximum depth allowed. Assume that the roof insulation will be pitched at 1/4" per foot, and the low point is 30 feet away from the roof edge. The depth of water at this low point will be 7 1/2", exceeding the maximum allowed depth. The secondary drainage will be required.
However, the roof bearing capacity may exceed the depth described above as "typical." In that case, the roof will be able to support more water, the maximum allowed depth will increase and the low points could be located farther away from the roof edge.
There are scuppers provided. Is there a need for secondary drains?
Not always. This is the same as the previous situation. A simple calculation will help to figure the answer. It is possible for the water build-up to exceed the maximum allowed depth before the water reaches the scuppers. Another way to answer this question is to locate the roof low points close enough to the scuppers and to always check the depth of water ponding.
How far should a primary roof drain be located from the secondary drain?
A secondary roof drain achieves its function by controlling the water build-up. A standpipe or dam built within the secondary drain dictates the anticipated water depth. Because of that, no certain distance exists. If, for example, the standpipe is 2" high, and the secondary drain is 6 feet away from the primary drain, the depth of water will be approximately 3 1/2" (if the 1/4" pitch is maintained) before it reaches the standpipe's top. If the primary and secondary drains are located at the same elevation (in a valley), the standpipe height alone will dictate the water depth.
It is very important not to place secondary drains in any kind of sumps. A secondary drain placed in a sump will start working as soon as water reaches the top of the standpipe. If the sump is 2" or deeper, the secondary drain shall serve as the primary drain, totally defeating its purpose. If the sump is shallower than 2", it will be full of standing water, which may freeze and damage the drain and the adjacent roofing.
Where should the secondary drainage be discharged?
Good question! Where exactly is this magic "above grade" location which should also be "normally" observed? Let's begin from the above grade height. Quite often emergency drain outlets are located right below the roof level of low-rise buildings. At that level, outlets are not subject to tampering. Also, the water stream coming from 20-30 feet above will be highly visible. On the other hand, the flow of 60-90 gpm (assume a 2,000- to 3,000-sq.-ft. roof area at 3"/hr. rainfall) falling from this height may wash out small children and will most likely cause some damage to the grass. It seems to me that locating such outlets 3-4 feet above grade, preferably near an exit door, would work. A downspout nozzle will add a nice finishing touch to an outlet. And a screen installed in the nozzle will prevent birds from building their nests in the pipe.
There is no such thing as a "one fits all" recipe for any plumbing installation. Read the Code, use common sense and experience will come with years.