Hail Damage on a Flat Roof: TPO, EPDM and Modified Bitumen

TPO is a thermoplastic sheet, usually 45 or 60 mil, heat-welded at the seams. Hail rarely punches through it outright. What it does is deform the sheet into the insulation, and if the board underneath is polyiso, the foam cell structure crushes and does not come back. You get a dish. Water sits in the dish, the sheet flexes there every time the temperature swings, and eventually the weld line nearest the depression starts to work. EPDM is rubber and it stretches, which sounds like an advantage and mostly isn't. A rubber membrane over a soft substrate absorbs the strike and hides it completely. Over a hard substrate, like a concrete deck or a gypsum cover board, the same stone will split it. Modified bitumen behaves more like a shingle: the granules come off at the impact, the cap sheet fractures, and once the asphalt is open UV starts on it immediately.

A core is a two-inch plug cut clean through the membrane, the insulation and down to the deck, then patched the same day. It shows three things a photograph can't. Whether the foam under the strike is crushed. Whether there's trapped moisture sitting in the layers. And what the assembly actually is, because plenty of low-slope roofs in the Midwest are a second or third layer over something older and nobody knows that until a plug comes out. Carriers take cores seriously because they are physical evidence rather than interpretation. We pull them from the areas that felt soft underfoot during the grid walk, not at random, and each plug gets photographed with a tape next to it and a mark showing where on the roof it came from.

Steep-slope asphalt generally needs around an inch of stone before mat fracture starts showing up. Low slope is different, and it cuts both ways. A 60-mil sheet over a hard deck can shrug off inch-and-a-quarter hail with nothing worse than scuffing. The same storm over saturated, twelve-year-old polyiso will leave depressions from three-quarter-inch stone, because the substrate has no fight left in it. Age of the assembly matters more than the number on the hail report. That's also why two buildings in the same office park can get very different answers out of the same adjuster, and why the different answer isn't automatically wrong.

Low slope means a quarter-inch of fall per foot if you're lucky, and a lot of older Midwest buildings have less than that. Every crushed spot is now a birdbath. Standing water does three separate things to a damaged membrane. It adds weight and deepens the dish. It drives freeze-thaw cycling straight into the weakest point of the assembly through a Missouri or Iowa winter. And it grows enough algae to make the next inspection harder to read. If the roof already had ponding before the storm, get the pre-loss condition documented early, because otherwise a carrier will attribute the whole depression to drainage and close the file.

Not a test square. That's a steep-slope method and it doesn't transfer. The useful documentation on a membrane roof is a marked field grid with soft spots flagged and numbered, core photos tied to those numbers, and dented soft metals around the perimeter: coping cap, counterflashing, gutter and scupper faces, condenser fins, and the housing on any rooftop unit. Moisture readings help when the assembly holds enough water to justify them. Infrared scanning helps on some roofs and proves nothing on others, since it reads temperature differential and needs a clear night after a warm day. It will not see through wet ballast.

A silicone or acrylic coating over a hail-bruised membrane is the most common bad decision made on a commercial roof, and it's usually sold by somebody who never cut a core. Coating locks moisture into insulation that's already wet and buys about eighteen months. If the foam is crushed and saturated, the honest fix is a tear-off to deck with new insulation and new membrane. If the strikes are isolated to one elevation and the cores come back dry, targeted welded patching is real work and we'll quote it that way. What we won't do is coat a wet roof, and we'll say so plainly if that's what somebody else put in front of you.

Plenty of Plains and upper-Midwest commercial policies carry a percentage wind and hail deductible rather than a flat dollar figure. On a building insured for two million, a two percent hail deductible is forty thousand out of pocket before the policy pays anything, which changes whether a partial-scope claim is worth filing at all. Read the declarations page before anyone climbs a ladder. Be careful with a contractor who offers to make that deductible disappear, too. Waiving a deductible is insurance fraud in most Midwest states, and it lands on the building owner every bit as hard as on the contractor who suggested it.