You will have 100% runoff. The water will have vacate the roof in the same hour that it falls. Use a circular or beehive grate. Call your local supplier for the Q's for various grates they sell. calc/ the total water on the roof and make sure it can all get into the grate in time. Just remember to add the total water load to your live load for the roof.

Go to you local librairy and look up your local building code (Southern Standard Building Code?) - the Plumbing chapter, roof drains.  There are tables that will size the roof drain, vetical downspout, horizontal leader and overflow wier as a function of area and slope.  I can give you the values from the Florida Building Code, (if you give me the area) but this is not really applicable to VA.

On can analytically derive these tables, but why re-invent the wheel, especially when not following the local building code can be construed as negligence.

Grate flow rates are typically determined by the the minimum of the orifice flow equation and the wier flow equation with 3"-6" of head.
Pipe flow rates are typically determined by the manning equation.
Wier overflow rates by the wier equation.

Clifford H Laubstein
FL Certified PE #58662

You can get a hydraulic wheel that will help you with the
calculations.  A quick Google search came up with this web site. http://www.portalsplus.com/drain_calc.htm

Hope this helps.

http://www.kesco.sphosting.com