Rooftop Detention

[CivilNJEP]

Can detention of 2.8" of rain be detained on top of a warehouse roof without adding any additional cost?

We are trying to obtain city approval for development but there isn't a structural engineer on the team yet.

As part of standard structural calculations, the typical rain/snow loads (40psf as I am told in northeast) that are accounted for in the roof exceed the amount of water (15psf) that I need to detain. So, I would like to think that it is possible to include this detention in my stormwater calculations without adding cost to the roof.

Has anyone ever done this?

 

[psmart]

We see a lot of situations where engineers are modeling roof-top detention. Sometimes the detention is intentional, and in other cases they're just trying to estimate the maximum depth and load that will result from undersized roof drains.

In your case, the roof would need to be "flat" to within a couple of inches to avoid excessive loads in the low spots. Based on your average depth of 3" of water, a 6" low-spot would have 9" of standing water, producing a localized load of about 45psf.

This might introduce additional construction requirements for the flatness of the roof. Of course, you'll have to balance the flatness with whatever pitch you need in order to drain the roof completely.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[CivilNJEP]

What is the flattest a typical warehouse roof can be? Given the large size of the building (300k sf) we are assuming bays 50 ft by 50 ft.

 

[hydrae]

I would not want to do it without evaluating earthquake sloshing which will increase the loads enormously.
Hydrae

 

[civilperson]

Standing water of any depth can travel up and horizontally between layers of building materials by capillary action causing unfindable leaks!