Roof Drain Restrictors
Roof Drain Restrictors
(OP)
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 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?






RE: Roof Drain Restrictors
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
RE: Roof Drain Restrictors
This whole thing seems weird to intentionally retain water on a roof.
RE: Roof Drain Restrictors
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.
RE: Roof Drain Restrictors
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.
RE: Roof Drain Restrictors
A beehive grate would be my recommendation.
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RE: Roof Drain Restrictors
RE: Roof Drain Restrictors
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
RE: Roof Drain Restrictors
RE: Roof Drain Restrictors
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?
RE: Roof Drain Restrictors
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.
RE: Roof Drain Restrictors
RE: Roof Drain Restrictors
Whether you have the flow retention device or no device, if you have 35 psf of snow you are going to have more than 6" of snow on the roof. Since that is the case, I believe it doesn't matter if you have the flow retention device because the rain load will be on top of the snow and not be able to get to the drain. Because of this, I do not believe the 35 psf snow and 31+ psf rain load are additive.
RE: Roof Drain Restrictors
RE: Roof Drain Restrictors
That is what jike was referring to. It is called ponding.
RE: Roof Drain Restrictors
As I stated before, I do not like roof ponding. Just think about the sole benefit though...when the fire burns through the roof, it will be put out, so the fire insurance will be less. This should also negate any need for sprinklers.
Seriously though, you probably should consider adding at least a portion of the water weight to the seismic calculations. The limit for including the snow is 30 psf or around 5.5" of water. Although I have not personally found this to be a code requirement, I believe it would be the intent of the code, since the ponding can happen any time of the year and will generally be around more than snow depending on your location, I would seriously consider adding at least a portion of the total weight of the water. Just a thought.
Mike McCann
MMC Engineering
RE: Roof Drain Restrictors
I generally provide scuppers around the perimeter and design the roof for the full amount of contained rainfall, including provision for ponding caused by deflection of the OWSJ as well as the structural steel frame.
Dik
RE: Roof Drain Restrictors
RE: Roof Drain Restrictors
The way that it looks right now, I have 7" of pitch from the edge of the building to the drain. No parapets or gravel stops are shown at the edge of the roof... So this means the maximum amount of water the system can hold is 7". I will have to increase the roof members to take this load and then check the ponding serviceability criteria. If required I will increase the members stiffness to accommodate this additional criteria. I will just end up being a little conservative in my loading criteria. From what I can see in the code there it is not required to consider both rain loads and snow loads acting together.
Interestingly enough, in talking to the civil engineer (who recommended this type of system) he says that they have designed several systems similar to this one. However, every single time the project switches back to a conventional detention pond (on the ground) system.
RE: Roof Drain Restrictors
RE: Roof Drain Restrictors
RE: Roof Drain Restrictors
RE: Roof Drain Restrictors
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