Roof Drainage

Roof leaders (downspouts) at primary thru-wall scuppers. Note how the secondary (overflow) scuppers are higher than the primary scuppers.

Here poor drainage leads to ponding water along the gutter edge of this low-slope roof. Ponding water can speed up the deterioration of the roofing material.

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Useful Links:

1. General: "Roof Drainage - Not My Problem...Maybe" by John Lawson is a very readable although sometimes highly technical article on the necessity of adequate roof drainage and the potential consequences of inadequate drainage. It's available at the Digital Commons section of the California Polytechnic State University website.

2. Building Codes: The "Roof Drainage" section of the 2015 International Building Code. Available at UpCodes.

3. Building Codes: The "Roof Drainage" section of the 2015 International Residential Code. Available at UpCodes.

4. Building CodesThe roof-relevant sections of the 2015 International Plumbing Code. Available at UpCodes.

5. Building Codes: “Secondary Drainage and Ponding Requirements in the IBC and IEBC” is a good place to start if you’re trying to understand the code requirements regarding regarding roof drainage as dictated by the 2015 International Building Code (including major changes from previous versions of the IBC). Article is from 2017 and is on the IIBEC (formerly RCI) website.

6. Downspouts: “Closed Faced vs Open Face Downspouts in Colder Climate Regions” is short but informative. From the Metal Era website.

7. Drains: See Roof Online’s Roof Drains page.

8. Gutters: See Roof Online’s Gutters and Downspouts page.

9. Inspection of Roof Drainage Systems: "Mastering Roof Inspections: Roof Drainage Systems, Part 1" at the International Association of Certified Home Inspectors' website is worth taking a look at. Part two of this inspection guide can be accessed from this page.

10. Scuppers: See Roof Online’s Roof Scupper Basics page.

11. 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.

12. Technical: For guidelines on designing roof drainage systems, see FM Global's DS 1-54 Roof Loads for New Construction (Data Sheet). You have to sign up in order to view data sheets on the FM Global website, but it's free and definitely worth your time.

13. TechnicalSiphonic Drains: "Fundamentals of Siphonic Roof Drainage" is a good introduction to this space-and-material-saving drainage innovation. Available to read on the website of Jay R. Smith Mfg. Co

14. ManufacturersSee here for drainage component manufacturers.