Sixty-mil TPO is the membrane we reach for most often on flat and low-slope buildings around Evansville. It handles the swing between soaking-wet Ohio Valley summers and hard winter freezes without the seam problems we still see on older single-ply roofs, and it costs less to install than the thicker 80-mil sheet most owners don't actually need.
Evansville summers run hot and humid for months at a stretch. A dark membrane on a warehouse roof off US-41 or the Lloyd Expressway can sit well past 160 degrees on a July afternoon, and that thermal cycling is what breaks down cheaper membranes over time. The white or light-gray TPO sheet reflects most of that heat back instead of absorbing it, which keeps the roof deck cooler and cuts the daily expansion-contraction stress on the seams.
Winters here bring their own load. We get real freeze-thaw cycling, not the deep-freeze conditions of the upper Midwest, but enough swings through 32 degrees to matter. TPO stays flexible at low temperatures better than older PVC formulations did, and the heat-welded seams don't rely on adhesive that can go brittle when the mercury drops.
Southern Indiana sits in a corridor that sees serious spring and summer thunderstorm activity, with straight-line wind events that can hit a flat roof harder than most owners expect. A hot-air welded TPO seam forms a bond between sheets rather than relying on adhesive or tape, so the seam is often stronger than the membrane around it. That matters when wind gets under a roof edge or penetration flashing during a storm cell moving up from the Ohio River.
We test every seam with a probe on install day, not only a sample section. On a roof this size, a missed weld at one drain or curb is the kind of thing that shows up as a leak eighteen months later, usually during the next big storm.
Hail rides along with a lot of our severe weather here, and 60-mil TPO holds up to typical Vanderburgh County hail sizes without bruising the way modified bitumen granules can. It's not immune to a direct hit from golf-ball-plus hail, but for the pea-to-quarter-size hail that's far more common in our storm cells, the membrane comes through clean.
For office buildings, retail boxes, and light-industrial space where the roof only sees occasional maintenance foot traffic, 60 mil is the right call. It's the standard sheet we spec on most reroofing jobs across the metro, from the medical office parks near the hospital campuses to strip retail along Green River Road.
Where we push toward 80 mil instead is on buildings with a lot of rooftop equipment, frequent contractor traffic, or exposure to plant exhaust that can carry fine particulate and light chemical residue. That extra 20 mils of wear course buys real puncture resistance in those settings.
Most of the roofs we install 60-mil TPO on around here go down mechanically attached, with plates and fasteners securing the insulation and membrane through the deck. On buildings where wind uplift ratings need to be higher, or where fastener penetrations aren't ideal for the deck type, we go fully adhered instead. Either way, the insulation package underneath matters as much as the membrane itself for keeping energy costs down through the humid stretch of the year.
With proper installation and basic maintenance, most 60-mil TPO systems in the Evansville area run 20 to 25 years before recover or replacement becomes the better economic choice. Roofs that see heavy foot traffic or sit near plant exhaust vents tend to wear faster.
It depends on how much service traffic the units generate. Light residential-style rooftop units with infrequent maintenance visits are fine on 60 mil. Heavier commercial package units with regular technician access are often a better fit for 80 mil, or a walkway pad system layered over the 60-mil field.
TPO can tolerate short-term ponding after a heavy thunderstorm without damage, but ponding that lasts more than 48 hours points to a drainage problem, not a membrane problem. We look at slope and drain placement before we ever blame the membrane.
Sometimes, if the existing roof is dry, structurally sound, and local code and insurance requirements allow a recover. More often on older Evansville buildings we're tearing off built-up or aging EPDM first, since a wet substrate underneath will ruin a recover job regardless of the new membrane's quality.
Forty-five mil sheet is thinner and more prone to punctures and long-term wear, especially with our hail exposure. We don't install it on commercial buildings because the cost savings up front don't hold up against a shorter service life.