Rainwater accumulation on a roof
Rainwater accumulation on a roof
(OP)
We are planning a commercial building in which the roof will slope to the center from the parapet walls on all four sides. There will be overflow scuppers through the parapet wall in case the roof drains become blocked. With blocked roof drains the max accumulation of water on the roof would be 6" depth before the scuppers would discharge. Our design snow load will be 25 psf. Opinions: what would be a logical design roof load?






RE: Rainwater accumulation on a roof
I'd check he snow saturated with rainwater. That's not my department (I work in southern areas), but 50 psf seems right.
RE: Rainwater accumulation on a roof
The engineer designing the system might have some input. We design roofs as temporary retention areas fairly often to prevent overloading the sewer system, typically we get input on the the depth of rainwater from the mechanical guys.
RE: Rainwater accumulation on a roof
This design is not a good idea.
Mike McCann
MMC Engineering
RE: Rainwater accumulation on a roof
Designing for deflection of the anticipated water depth is a problem in and of itself. Ponding creates progressive and cumulative deflections...the more it deflects, the more it ponds, then the more it deflects...and so on until collapse.
Next, though I'm not sure of your location, but building codes in the US require that overflow scuppers in parapet walls and interior drains must be no less than 2" above the top of the primary drain, but no more than 4" above the top of the primary drain. Your 6" variation would exceed this.
As JC noted, an overflow drain near the primary would be better.
I would design this by dividing the roof into 4 quadrants and place a primary drain and an overflow drain in each quadrant. This provides redundancy in the event of a clogged drain, and it changes your slope elevation difference to 1-1/2".
RE: Rainwater accumulation on a roof
RE: Rainwater accumulation on a roof
Is this anythng like befuddled?
Mike McCann
MMC Engineering
RE: Rainwater accumulation on a roof
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: Rainwater accumulation on a roof
Sure, befuddled is one synonym. Thanks...I'll use that one next time.
woodman88,
Stormwater retention is a common requirement in lots of places, including where I am in Brisbane, Australia. But we would never consider storing it on the roof.
RE: Rainwater accumulation on a roof
I live in Everett, 20 miles north of Seattle, and have seen some situations where it is done, and the results. I would never do it.
Mike McCann
MMC Engineering
RE: Rainwater accumulation on a roof