huangyhg

roof drain restrictors
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?

i would be worried about icing and blockage of the drain. how is this scenario to be handled?
i am not a big fan of rooftop detention, but if it must be - another thought. i would be nice, that if the works got plugged, the top of the proposed drain would be large enough, with enough air, to lift the restrictor out of the pipe, allowing the water to escape - unless the architect is providing overflow scuppers at the outside walls.
mike mccann
mmc engineering
they had mentioned over flow scuppers at the edge of the roof in case the drain was plugged. however, due to roof pitch, i believe they would have to install an auxiliary drainage system.
this whole thing seems weird to intentionally retain water on a roof.

if you are going to retain water on the roof then you need to go thru a ponding analysis.
if the roof pitch is significant then you will need separate interior overflow drains as you stated. we always ran those thru the outside building wall so hopefully someone will notice the "plugged condition".
we used to use undersized drains years ago with flat roofs but got away from them because the ponded water would cause the roof to wear out prematurely.
asce 7 calls for roof live load, snow load or rain load.
rain on snow surcharge is addressed under snow loads but will not apply in your situation, pg<20 psf.
controlled drainage systems are addressed under rain loads in asce 7.
most of this information is copied in ibc without much change.
the first design will easily plug with leaves. the second design may actually be less likely to plug up. however i believe intentionally ponding water on the roof is foolhardy.
a beehive grate would be my recommendation.
don't forget that it's 6" of standing water (static head) plus the hydraulic head of the water over the drain inlet.
ucfse:
from the sketch, i think the dead storage is only a couple of inches locally at the inlet, with the max live storage of around 4" for a head to the orifice of 3 to 4", plus, as you mentioned, any hydraulic charging or backwater effect to attain the orifice design flow.
also like like this could be a multiple orifice condition.
if this is an interior drain, don't forget to install multiple cleanouts at the bends to mitigate future headaches.
mike mccann
mmc engineering
from my experience, these small grates clog very easily with leaves, pine needles, rocks and other debris. this can reduce the flow down to a fraction of the unclogged capacity and can even block the drain entirely. the head you will need to drain the water from the roof is dependant on the capacity of the "partially-blocked" drains and secondarily on the level of any overflow scuppers that may be provided. this could easily exceed your 6 inches. a significant safety factor should be used to account for at least 50% blockage of the grates.
structuraly you will have to design for the ponding water up to the auxilliary drains plus a little for flow.
realistically, you are going to clog everytime it rains and have no problem locating all of the leaks in the roof.
why not use a standard roof drain?
i built a lab addition that had rain retention in the parking lot. the chgo metropolitan sanitary district obliged us to do this.
this leads to the idea of remote retention off the roof; free flow off the roof into a retention structure like a tank rupture retention structure. let that flow down thru a restrictor. favor the roof against collapse.

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