Also see "Roof Drainage".
1. Building Codes: "1503.4.2 Scuppers" and "(P) 1503.4.1 Secondary (emergency overflow) drains or scuppers" in the 2015 International Building Code. Available on the UpCodes website.
2. Building Codes: "R903.4.1 Secondary (Emergency Overflow) Drains or Scuppers" from the 2015 International Residential Code. Also note exception 2 under subsection 1 under "Section R908 Reroofing". Available on the UpCodes website.
3. Technical: "Appendix B: Rates of Rainfall for Various Cities" on the ICC website provides the rainfall data for major US cities required for the sizing calculations mandated in the building codes. If your town is not listed, visit NOAA's Precipitation Frequency Data Server (PFDS) to look up the 100-year one hour rainfall data for your area.
4. Technical: For guidelines on proper sizing and placement of roof scuppers, see FM Global's DS 1-54 Roof Loads for New Construction (Data Sheet). You'll have to sign up in order to view data sheets on the FM Global website, but it's free and definitely worth your time.
5. Technical: An excellent technical article on scuppers is available here at the website of the Copper Development Association. Provides construction details for various scupper types. They talk about scuppers being made out of copper in that article, but that doesn't mean that scuppers have to be made out of copper or that the information provided is only applicable to copper scuppers. (Copper is a great scupper material, though).