Designing roof structures for ponded water with weirs

[JAE]

Most flat(1/4" per foot slope) roofs I've designed have been part of a system where we have main roof drains and secondary roof drains 2" higher per the IBC. We design for the standard roof dead, live, snow, etc. loads.

Have any of you designed a similar roof but without the secondary roof drains and used instead a large weir at the ends of the building to serve as a secondary drain? (a weir being a wide gap in the parapet wall at one end) And then included some weight of water in the design of roof members.

In this case, the weir would be set at the "low" end of the roof slope such that the roof would have to fill up the 2" depth prior to running over the various roof crickets to get to the weir.

My question is centered around the structural design of the roof members. I can see the logic in sizing a weir large enough so that the water can flow out at a good rate. But with a very large roof, won't there be some sort of build-up across a wide roof where the water depth will be deeper due to the flow of the water? Sort of like a river being higher upstream?

And if so, would you design just for the actual calculated build-up of water weight or would you include an additional safety factor on the water?

Would the standard AISC ponding checks come into play?

 

[UcfSE]

The FBC includes rain loads in section 1611. An equation is provided for rain load based on the static and hydraulic head at the secondary drainage system at its design flow. Does the IBC have a similar provision, would that help you at all? The static head is easy enough but you may need the hydraulic head from the drainage engineer (if there is one).

I would still use the ponding stability checks.

 

[msquared48]

You have a small reservoir with a spillway and need to construct an area volume curve so you can do a backwater analysis for the maximum depth of retention.

Personally, I would never store water on the roof as drains do plug. It would merely compound the potential problem.

Mike McCann
McCann Engineering