Roof Drain Restrictors

[SteelPE]

I’ll try to keep this short.

I am designing a building where there are some drainage problems. To save money the civil engineer/contractor would like to reduce the amount of drainage discharged into the street drainage system by installing some “flow restrictors” on the main drains for the roof. It appears that the flow restrictors are a 6” high PVC with holes drilled in the side to restrict the amount of water flowing into the drainage system. On the top of the PVC pipe is a grate that would allow the water to freely flow into the drainage system (if the water ever got to be that high). Please see attached.

Due to the location, the building has already been designed for a roof snow load of 35psf (code requirements).

Question, should the roof design load be increased to reflect the potential of 6” of standing water on the roof together with the snow load? Or is it unrealistic to say that you would have the full 6” of water together with the full snow load?

 

[GarySki]

The city of Chicago requires that these restrictors be provided. There are some successful pre-fab ones on the market now. The skupper is the overflow provision and there are general enough drains to accommodate the 100% drainage in an overflow condition. Debris and ice are apparently not an issue if proper maintenance is done. I'll look for my supplier contact if you need it.

 

[jike]

It is generally better to pond the water in a retention basin then on a roof.

 

[dik]

They have controlled flow drains that intentionally retard the rate of roof runoff. These are commercially produced items.

You have to look out for ponding and added water containment and should have 'failproof' scuppers or some means of limiting the height of water.

Dik