ROOF PONDING/DRAINAGE

[strguy11]

In reviewing a roof plan with a relatively flat roof (typical office building structure) the roof slope is created by lowering the columns at interior bays. This results in about an 8" fall from the exterior wall.

However, there is only one drain shown at each location, and no overflow drain. Therefore, it seems this is very likely to have ponding problems, since the only overflow drain is a scupper on the exterior wall.

If a secondary drain was installed next to the primary drain, would this suffice to eliminate the ponding problems? The code says to assume the primary drain is blocked, however, I dont see any items regarding placement of the secodary drain. My thought was to install the secondary drain about 2" higher than the other, therefore, only needing me to check for 2" + localized pond, instead of an 8" pond + resulting beam deflections. Any thoughts?

 

[jike]

You are on the right track with overflow drains. This is what we typically did.

The height of the water that can build up obviously needs to be less than the LL capacity of the roof.

 

[Ron]

strguy11...not sure where you are, but most codes require that overflow scuppers be not less than 2 inches above the drain outlet nor more than 4 inches above the outlet. You condition does not meet that requirement.

It is not a good idea to use parapet overflows for centrally located internal drains. I've seen roof collapses related to such design. Provide an overflow for each drain location, separately piped to the outside. The depth of ponding prior to overflow should obviously exert less load than the live load capacity of the structure, as Jike noted.

 

[cvg]

also do what you can to prevent blockage of the interior drains (if you must go this way). use domed "beehive" grates if you can to prevent floatable debris from covering the drain.

 

[SteelPE]

I’m not sure where it is listed (I believe it’s in the International Plumbing Code) but overflow drains are required to drain to daylight and should not be attached to the pipes of the primary drains.

 

[civilperson]

Change the slope direction of the roof to use gravity to carry water to the exterior or build sufficient strength in the roof members for full depth, (to scuppers), ponding. Any drains, primary or secondary, can be blocked by the same cause, i.e. ice, wind blown debris, leaves, etc.