Residential Large Multi Plane Flat Roof Drainage


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Hi All

 

I am working on a (2:12) residential standing seam roof design that has a single roof section around 6000 sq ft, being made up of a roof plane of approx 4400 sq ft with a second central raised void/clerestory that has approx 1600 sq ft of roof.  The raised section falls onto the lower section as shown in the attached image.

 

I have a restriction in that I cannot create a cricket across the front of the raised structure (red line) due to the span across (very large roof) and any flashing cannot go above the line with the orange arrow (approx 2 inches).

 

I had intended to do a box gutter (I know) as I have had plenty of experience with them but the drainage depths and the height vs pitch is a problem (messy but does work).

 

I'm looking at using a commercial method of siphonic drains (as I have plenty of internal space across this area) with a shallower box gutter and internal drains....

 

Does anyone have experience in designing / detailing this or can suggest a better way?

 

Thanks

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A siphonic system needs a certain amount of head; I believe even the smallest drains will need at least 2" of head before siphonic action can be induced - before the head is reached, all drainage is via gravity. Roof drains are designed to be integrated into a membrane, so if you don't have a membrane, I'd be nervous about the flashing detail.

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I'm following this thread as I find the challenge very interesting. Don't have a solution but am also wondering about keeping the area/drain clean and clear moving forward. Is there any way to redesign the roof? Is it visible from anywhere other than google maps overhead view? Or does the clerestory need the clearance shown?

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1. This will fail. 

2. Don't do this. 

 

Are you following what happened in Miami? If it turns out the drawings for this building did not show any positive drainage for the decks that were built dead flat, the architects will fry. What you are designing/drafting is a problem area no matter what someone sells you on for a drain system. It will require maintenance and whoever is in charge of that will not show up one day and then the rain will destroy the ceiling. Or worse, weaken the system slowly, grow mold, etc..

 

If you can not design it to shed the water and someone is pushing you to draw it in this dangerous fashion, walk away from the project. 

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Not withstanding Alan's maintenance comments, you might have a look at sawtooth roof drainage systems and consider using a lot of rainwater leaders for extra insurance

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