TPO roofing is a single-ply thermoplastic polyolefin membrane, and what sets TPO roofing apart from rubber and asphalt flat roofs is the seam. Where two sheets overlap, a hot-air welder melts them into each other, so the finished lap is one piece of plastic rather than two pieces held together by tape or glue. The ASTM D6878 standard requires a reinforced sheet that can be welded or bonded to itself for watertight field splices and repairs, and TPO makers weld it.
That matters because seams are where flat roofs quit, and a welded seam has no adhesive to age out. What it does need is good workmanship on the day it is made: the right heat, the right travel speed, and a probe run along the finished weld to find any spot that did not fuse. On a roof that spends weeks under snow and meltwater, a missed spot does not stay hidden for long.
TPO membrane thickness: 45, 60, or 80 mil
A mil is a thousandth of an inch. Carlisle’s Sure-Weld line comes in 45, 60, and 80 mil, the 80 sold as Sure-Weld Extra, and Elevate’s UltraPly in 45 and 60. Thicker sheet puts more material over the reinforcing scrim at the center of the membrane, and that material is what takes the puncture from a dropped wrench or a hailstone.
| Thickness | Where it tends to fit |
|---|---|
| 45 mil | Small roofs with little foot traffic, or a building you expect to sell within a few years |
| 60 mil | Most commercial roofs in the area |
| 80 mil | Roofs with monthly HVAC service or real hail exposure |
Cold does not decide it: ASTM D6878 caps the brittleness point at 40 below, and Carlisle reports its sheet at 50 below. Thickness gets matched to what the roof has to carry and how long you plan to own the building.
Mechanically fastened or fully adhered TPO
A mechanically fastened TPO roof is screwed down through plates set inside each lap, and the next sheet is welded over the plates. It goes on faster, costs less, and tolerates a deck that is not perfectly clean. The tradeoff is that the sheet between rows can lift and flutter in a hard wind.
A fully adhered roof is glued to the insulation or cover board across its whole surface. It lays flatter, holds better against wind uplift, and costs more. Cold is the complication on adhered work, and the product sheets are specific about it: Elevate rates the low-VOC bonding adhesive it sells for both TPO and rubber roofs for application as low as 20 degrees, but its specification for adhered RubberGard EPDM roofs keeps the adhesive itself between 60 and 80 degrees while it is being used, and the same care applies on a TPO job. On a January job in Eau Claire, where the average high is 23 degrees, that means the adhesive lives in a heated box until the minute it goes down.
When the roof being replaced has only one layer and dry insulation, the new TPO can often go over it instead of after a tear-off; the recover versus replace guide covers when that works.
How TPO handles ponding and hail
Water standing on a flat roof 48 hours after a rain is ponding by NRCA’s definition, and NRCA notes that snow and rooftop equipment can sag a deck into low spots. When NRCA collected the makers’ ponding limits, the built-up, modified bitumen, and EPDM makers drew the line at 48 hours, while TPO makers ranged from 48 hours to no limit at all. Standing water is still a drainage problem to fix, but TPO is the membrane most likely to forgive it.
Hail is the other test. Carlisle’s TPO carries a UL 2218 Class 4 rating, and FM Global rates whole roof assemblies against severe hail, a two-inch steel ball dropped from about twelve feet, and very severe hail, two-inch ice balls fired at 152 to 160 feet per second. The rating belongs to the assembly, not the sheet, and the assemblies Carlisle lists for very severe hail put a high-density cover board under the membrane. For a building whose insurer writes to FM standards, ask about that rating before the spec is set.
Where TPO falls short
TPO gets harder to repair as it ages. Carlisle requires membrane that has been out in the weather to be prepared with its weathered membrane cleaner before any hot-air weld, and Elevate, which sells both membranes, notes in its rubber roofing literature that some thermoplastic sheets lose their initial weldability and flexibility. A patch on a 15-year-old TPO roof is still a weld, but it takes more prep and more care.
EPDM rubber is the better fit on roofs with lots of odd details worked in cold weather, on buildings where an owner wants one seamless sheet, and on many porch and addition roofs; the EPDM rubber roofing page covers where rubber wins. And if the roof already leaks, the flat roof repair page covers how each membrane gets patched.
TPO roofing cost in Eau Claire
Commercial TPO work runs about $8 to $18 per square foot installed, the same band as the rest of our commercial membrane work, and flat sections on homes about $7 to $16. Where a roof lands in that band depends on four things more than on the membrane: whether the old roof comes off, how much insulation the energy code requires once the deck is exposed, whether a cover board goes down for hail or foot traffic, and how many curbs, drains, and pipes have to be flashed. A roof with six rooftop units is a different job from an open deck of the same size.