Low-Slope Roofing and Rain

Sept. 12, 2011

Heavy rains can be a game-changer for low-slope roofs.

Aug. 14, 2011 was a record-breaker. No, it was not extremely large hail or extremely high temperatures. This time, it was extremely heavy rains.  Hurricane Irene was arriving on the East Coast with heavy rain just as this column was being drafted. Rain totaled 8 inches at New York’s JFK Airport, 5 inches in Philadelphia, and 11 inches in Seabrook Farms, NJ.

That shouldn’t mean much to the roofing industry. We construct our residential-style roofs at a minimum slope of several inches to the foot, pitched to external gutters that have been sized to meet building codes. Sure, sometimes the gutters fill up with leaves and other debris, but the gutters hang outside the building walls so in the worst case we get some water stains on the outside walls or drown our landscaping.

However, when we deal with low-slope roofs, where roof pitch ranges from negligible to perhaps half an inch to the foot (4%), things change. While smaller roof areas may be pitched to outside gutters, the majority of commercial roofs use internal drains. Building and plumbing codes require one primary drain in each roof bay, plus a secondary means of drainage in case the primary drain or downspout becomes blocked. For bays on the perimeter of a building, these may be overflow scuppers, generally at least 2 but not more than 4 inches above the low point of the roof.

If a roof floods, either due to drain blockage or some other unforeseen circumstance, several possibilities exist, none of them good:

  • The roof deflects. Most designs limit deck deflection to a quarter of an inch under a 300-pound load at the center of the span. You will find the term “L/240” used as a design limit. The L is distance between supports in inches. If, for example, the bar joists were spaced 5 feet apart (60 inches) and if the maximum deflection in the center of the span is specified at one-quarter inch, 60/240 is exactly one-quarter inch deflection, and this just meets code. A 6-foot span would be 72/240 = one third of an inch deflection and would not meet code.
  • The depth of ponded water exceeds flashing height. This is especially troublesome at pitch pockets that are rarely more than a couple of inches above membrane elevation, and are usually deficient in filler material.
  • The drain was never properly installed to begin with. See above for an example in which the EPDM single ply membrane just has a pilot hole over the drain, greatly reducing the drain’s capacity. Sealant should have been placed between the drain bowl and membrane, the drain clamping ring replaced, and the membrane trimmed back to a size comparable to the drainpipe.
  • The drain screen or scupper opening is blocked by debris.
  • The problem lies in the plumbing beneath the deck.
  • The roof is ballasted, and stones could have blocked the drain screen. Note that a perforated metal screen keeps the stones from migrating into the drain.
  • A mechanic ran conduit into a place where it is impossible to flash it properly.
  • On this roof, the insulation was correctly tapered to form a sump – see above. The secondary drain is an overflow drain, about 4 inches above the membrane. Note: A 4-inch-deep pond weighs more than 20 pounds per square foot.
  • A mother killdeer placed her eggs next to the roof drain, knowing that the drain is high and water will never reach it.

We are not capable of changing our weather, but we should be able to anticipate trouble. With improved weather forecasts, we generally will have a couple of days to prepare for the worst. Add these items to your to-do list and check them off sooner rather than later:

  • Cleaning the drains is something we need to do every spring and fall.
  • Get up on the roof while it is still wet. Puddles? Ponds? Are the drains in sumps as they should be? If not, next time some major maintenance is scheduled, have the drains recessed so the water can reach them.
  • Planning on re-cover or adding thermal insulation? Great time to add slope using tapered thermal insulation. It’s also a great time to add drains.
  • Are drain inserts in your thoughts as part of the re-cover? Not a good idea at all if they reduce the effective diameter of the drainpipes. 
  • If you are going to recover, what better time to remove any equipment that is on the roof but no longer used?

The Bottom Line: Roofing is all about water management. Position the drains where the water is, not where it should have been. Make sure you have a back-up system, whether it’s overflow drains or scuppers. As renowned architect Justin Henshell once suggested: Place the overflow drain right over the main entrance to the building, where it is certain to get some attention!

Richard (Dick) L. Fricklas was technical director emeritus of the Roofing Industry Educational Institute prior to his retirement. He is co-author of The Manual of Low Slope Roofing Systems and continues to participate in seminars for the University of Wisconsin and RCI Inc. - The Institute of Roofing, Waterproofing, and Building Envelope Professionals. His honors include the William C. Cullen Award and Walter C. Voss Award from ASTM, the J. A. Piper Award from NRCA, and the James Q. McCawley Award from the MRCA. Dick holds honorary memberships in both ASTM and RCI Inc. 

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