Rainwater accumulation on a roof

[rittz]

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?

 

[JedClampett]

In my opinion that's a pretty hinky design concept. Can't you add an overflow drain at the center?
I'd check he snow saturated with rainwater. That's not my department (I work in southern areas), but 50 psf seems right.

 

[bookowski]

Definitely need to check ponding instability. I would take your 6in. height and then add in the deflection under the 6in of water and iterate... and then throw a little extra on there. I'm guessing that you already thought of that though.

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.

 

[msquared48]

And what about the situation where the drain freezes with a heavy snow accumulation, and you get a sudden melt with rain? Ideal situation for a roof collapse, and it WILL happen.

This design is not a good idea.

Mike McCann
MMC Engineering

 

[Ron]

Agree with others....problematic design.

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".

 

[hokie66]

I know that rittz is primarily concerned that the roof doesn't collapse, but am always bemused as to why architects like to design roofs that will inevitably leak.

 

[msquared48]

Bemused?... Bemused?... Really, bemused?

Is this anythng like befuddled?

Mike McCann
MMC Engineering

 

[woodman88]

hokie66, There are places like Seattle, Washington where in some areas you are required to control the rate of drainage to the public stormdrain system. You either have to support the rain on the roof or provide a catch basin to hold it. Some times the owner would rather deal with the leaks than loss of floor area. In Arizona all the new home tracts have large catch basins for this reason.

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.

 

[hokie66]

Mike,
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.

 

[msquared48]

Hokie:

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

 

[bookowski]

We do stormwater retention on the roof all the time (NYC). It's definitely a bad detail but at the same time you can design for it structurally and rittz probably doesn't have the power to make them change that decision. I would say that given you don't have the ability to change the detail just make sure that you consider ponding and 2nd order effects and be conservative. From the responses you see here it's clear that no one will fault you for conservatism in this type of situation.