Heavy spring snow falls (A) and begins to melt, forming a pond on top of the membrane (B). Temperatures drop and ice forms (C), adhereing to the roof at the edges of the pond (D) and trapping the water below. Ice continues to form, creating downward pressure against the membrane (E). Water pushes the membrane down around an overlooked screw, opening a hole and letting pressurized water flow into the ceiling (F).

I have been in the roofing business since 1973 and often get calls to help find leaks that other companies have been unable to locate. Recently I got an email from an associate about an ongoing and elusive leak in a “butterfly” roof at a home in Boulder County, Colo.I began my investigation as I usually do: at the home talking with the homeowner. I learned that the 80 mil TPO membrane roof leaked only in the springtime and only after a snowfall of 9 to 12 inches. If the snowfall was less than that, the leak didn’t show up....