Roof Drainage

Roof Drainage
You'd be very shocked to learn how little attention is paid to the "positive drainage" on a low sloped roof in Arizona.  To be blunt, unless a good consultant is helping with the original design, you've got water on your roof. 
A Safeway had standing water, and then it rained again. More water added to the load The roof caved in and broke a gas line, and the building burned down. Had the roof been regularly inspected and maintained, the fire could have been avoided.  
Do you see why positive drainage is essential?
It's the law! Let me say more about that bold statement. The Arizona Workmanship Standards say, " No standing water," and the building code means, "no standing water," 
Roof decks need to be designed so that water will flow to the drains and run off the roof. Generally, slope to the drain is achieved by; 
  • Build slopes into the framing of the roof. 
  • Installing tapered installed.
  • Building crickets.
  • Combining all the above. 
  • Whatever it takes to move the water to the drain, or leading edge. 
This assumes the drains are clear and functional. The best design sloped roof system is pointless if the roof drains are not working. Regular maintenance is essential.  Just ask the owners of that Safeway building.
What is Standing water?
It's normal to find small puddles of water on a low sloped roof.  It's not considered "standing water" or problematic until its been there for more than 48 hours after the rain under normal conditions. The conditions would normally allow the puddles to dry up.
The roofer's role
While the original design is fundamental, the roofer has some responsibilities as well.  Lets read the "Workmanship Standards of Arizona."
Roofs-flat built-up 2.
PD Roof holds water. At minor ponding (up to 1/2" deep in small areas equivalent to no more than 1/3 of the span is acceptable providing roof is dry within 48 hours after rainfall.  Verifiable documentation that the potential problem was submitted to the responsible party may relieve the roofer of responsibility. 
CR, if the drainage problem is due to the improper framing, the roofer should be responsible for repairing or replacing the effected roofing. 
I am not a lawyer,  so this is not legal advice. What I read says that the roofer is on the hook, even if the framing is wrong. They may be able to get off the hook if they properly document and notify the right person.  However, the word "may" scares me too much to take the chance.  
So roofers make sure the roof drains.  And the owners keep the roof clean and your drains clear and clean.

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