Roof Ponding
Roof Ponding
(OP)
Can someone help to clarify the "Primary Drainage System" as described for roof ponding requirements?
I have a roof with 1/4" per foot slope to the interior of a building, where roof drains are located (The interior is 1'-3" lower than the permeter).
My question is that if we provide a secondary, or overflow scupper, and this is elevated like 2" above the primary drain, can this drain be tied into the main drain stack below the roof? If so, can I only design the roof for 2" of water, vs almost 1'-3". Or do we need to provide a completely separate drain system (ie 2-full runs of pipe) to be able to do this?
I have a roof with 1/4" per foot slope to the interior of a building, where roof drains are located (The interior is 1'-3" lower than the permeter).
My question is that if we provide a secondary, or overflow scupper, and this is elevated like 2" above the primary drain, can this drain be tied into the main drain stack below the roof? If so, can I only design the roof for 2" of water, vs almost 1'-3". Or do we need to provide a completely separate drain system (ie 2-full runs of pipe) to be able to do this?






RE: Roof Ponding
RE: Roof Ponding
RE: Roof Ponding
Don Phillips
http://worthingtonengineering.com
RE: Roof Ponding
RE: Roof Ponding
RE: Roof Ponding
If a roof slopes less than 1/4" per foot, ponding should be checked, even if the drains can never get blocked up.
DaveAtkins
RE: Roof Ponding
Dave, not sure about that. In most cases it is true (and in fact AISC K2 mentions "flat roofs") but in some cases you can have 1/4" foot slope into discrete and closed-off valleys which have only one primary drain....say with a 4" ridge height, with scuppers way off in the distance.
The primary plugs up and this bath tub fills to the brim.
You'd design for the weight of the water, yes, but even with the 1/4"/ft slope I'd be very concerned about ponding in the discrete valley areas since the water there would flow to the lower areas first, pond there, and if your framing system wasn't stable per AISC K2, then you could possibly have a real issue with second-order load application of the water.
RE: Roof Ponding
In the situation you describe, there needs to be a secondary roof drain there. I often try to make sure that the architect and the plumbing designer follow thru on their design properly so that it does not come back on the structural guy.
RE: Roof Ponding