Thermoplastic membrane
TPO Roofing Contractors in Dallas, TX
The sheet is the inexpensive part. What decides whether this roof reaches twenty years is the quality of every weld and how the membrane is held to the deck.
Roll width sets the spacing of every joint across a field like this, whatever the sheet is made of.
At a glance
- Service life
- 15 to 25 years
- Installation
- Mechanically attached, fully adhered, or induction welded to plates.
- Typical fit
- Wide open fields on steel deck with light rooftop traffic.
What a single-ply roof is made of
A thermoplastic roof arrives on the building as rolls of sheet, commonly forty-five to eighty mil thick, with a polyester scrim buried between two layers of compound. The scrim carries the tensile strength. The compound above it is the sacrificial part that takes ultraviolet light, heat, and whatever gets dropped on it, which is why the amount of material sitting above the scrim tells you more about how long the sheet will last than the total thickness does.
The sheet is only the top of the assembly. Beneath it sits a coverboard, then insulation board, then the deck, and each of those decides more about performance than the color of the membrane does. A thin sheet over soft insulation dents under a boot. The same sheet over a coverboard shrugs off that boot, because a firm substrate spreads a slow concentrated load instead of letting the membrane stretch into a dent.
PVC is the chemically resistant relative of TPO. It welds the same way and the detailing looks similar from a distance, but the compound tolerates grease, animal fat and industrial exhaust that will soften a thermoplastic polyolefin sheet within a few seasons. On a building with kitchen exhaust discharging onto the roof, that difference settles the choice by itself. PVC costs more per square foot, and across a long service life its plasticizer slowly migrates out, which is what leaves an old sheet stiff and prone to cracking at the folds.
Three ways the sheet gets held to the building
Mechanical attachment sets rows of screws and plates through the membrane along one edge of each sheet, and the following sheet laps over and covers them. It is the quickest method and the least expensive, and it needs a deck that will hold a fastener. The tradeoff appears in wind: the field between rows lifts and flutters during a gust, and every cycle works the plates against the underside of the sheet.
A fully adhered roof is bonded to the coverboard across its whole face. Nothing flutters, the surface stays flat, and load transfers evenly into the substrate rather than concentrating at fastener rows. Adhesive is temperature sensitive and slower to place, and it is the usual answer on gypsum, cementitious and lightweight concrete decks that cannot be trusted to hold a screw.
Induction welding splits the difference. Coated plates are screwed through the coverboard and the insulation into the structural deck on a set grid, the membrane is rolled out loose over them, and a hand-held tool bonds the sheet to each plate from above. No fastener passes through a lap, and the sheet lies flat rather than billowing. A weighted magnet drops onto each point the moment it is welded and stays there for a stated dwell while the bond sets, so the tool carries straight on down the grid with a trail of magnets cooling behind it.
The ways a thermoplastic roof fails
Nearly every early failure on this system is a weld. A hot-air welder run too fast, run too cold, or run across a sheet with dust on it produces a lap that looks correct and holds for two or three seasons before thermal cycling opens it. That is why the machine is set against a test weld pulled on the actual roof in the actual conditions, at the start of the day and again whenever the weather shifts, and why a probe is drawn along the finished edge of every lap.
The sheet also pulls. Over years the membrane contracts slightly and puts the whole field under tension, and tension finds the places where the roof changes plane. At the base of a curb the membrane bridges instead of settling into the corner. At the perimeter it tugs against the termination. At an inside corner it can draw the flashing off the vertical face entirely. Bridging is visible from a standing position, because the sheet stretches straight across the angle rather than following it, and that unsupported span is where the split begins.
Then there is everything that lands on it. A dropped self-tapping screw, a cut zip tie, a dragged condenser, grit tracked over from an adjacent roof: all of it works on a sheet a couple of millimeters thick. Hail is a different kind of load entirely. A boot is slow and concentrated, and the board under the sheet spreads it. A stone delivers its energy in milliseconds, and what matters then is how much the whole build-up can deflect before the reinforcement reaches its limit. Where a membrane sits over something with no give in it, a bare concrete deck being the clearest case, a strike can stretch the compound and fracture the scrim underneath while the top face still reads as sound, and the roof stays watertight for a season or two on a broken reinforcement layer that only shows itself from below. Walk pads set along the route from the hatch to each unit remove most of the traffic half of that problem for the cost of an afternoon.
Where a white membrane earns its keep in North Texas
The reflective surface is not a brochure feature here. A dark low-slope roof in this climate runs far above ambient on a summer afternoon, and every degree of that lands on the insulation, the deck and the equipment sitting up there. A clean white sheet sends most of it back. The effect fades as the surface soils, which is one more reason to keep the field washed and the drainage clear.
The building types that suit this system are the ones with wide open field and not much bolted to the roof. Distribution buildings, big-box retail, newer office shells and light manufacturing all get a good result, because the cost advantage lives in the open field and evaporates in the details. A roof carrying forty penetrations, six curbs and a pair of expansion joints spends most of its labor budget on the parts where a welded sheet is fussiest.
So the honest summary is narrow. On a large, simple, well-drained roof over a deck that holds a fastener, thermoplastic single-ply usually returns the lowest cost per year of service of anything covered here. It is also the least tolerant of a rushed crew. A hundred thousand square foot roof carries something on the order of ten thousand linear feet of lap, and every foot of it was made by hand with a hot-air gun.
Fit
The size of the open field decides whether this system saves money
Good fit
- Wide open fields on steel deck, where welded sheet goes down fast and the price advantage is real.
- Buildings where summer cooling load matters and a reflective surface pays part of its own cost.
- New shells and re-roofs that can carry a coverboard under the membrane.
- Kitchen exhaust and industrial discharge, using the PVC version rather than TPO.
- Decks that will not hold a screw, once the attachment moves to fully adhered.
Watch for
- Cold welds. Ask how the machine was set, whether test welds were pulled, and whether every lap was probed.
- Bridging at curb and wall corners, where the sheet stretches across the angle instead of sitting in it.
- Punctures and abrasion from rooftop traffic, which this assembly resists least of the five.
- Storm bruising that fractures the scrim while the top surface still looks intact.
- Plates on a mechanically attached roof, which flutter, wear, and eventually cut the sheet from below.
- A bid that comes in cheaper because the coverboard quietly came out of the assembly.
Common questions
Should we use TPO or PVC?
Chemistry decides it. If anything greasy or solvent-bearing lands on the roof, which means restaurant exhaust, food processing, or certain industrial discharges, PVC is the sheet that survives it. Everywhere else TPO does the same job for less money. Both weld, both come in comparable thicknesses, and both live or die on the quality of the laps.
How thick should the membrane be?
Sixty mil is a sensible floor on a commercial building, and eighty is worth the difference on any roof that gets walked or that has taken hail before. The more useful question is how much compound sits above the scrim, because that is the layer weathering away. Two sheets quoted at the same total thickness can have meaningfully different amounts of material where it counts.
Can a weathered single-ply roof still be repaired?
Yes, with more preparation than a new one needs. Years of ultraviolet exposure leave a surface layer that a weld will not fully bite into, so the area gets cleaned and abraded before any patch is welded down, and the weld settings get proved on a test piece cut from the same roof. Patch material also has to match the chemistry already up there.
Why has our white roof turned grey?
Airborne dust, pollen, tire particulate and biological growth settle into the surface texture and stay there. The membrane is fine. What has dropped is reflectivity, and with it the cooling benefit the white surface was bought for. A low-pressure wash restores much of it, and the roofs that recover best are the ones where debris and standing water are not feeding the growth in the first place.
Does a mechanically attached roof make noise inside the building?
It can. The field between fastener rows lifts and settles in gusts, and in a building with an exposed steel deck and no ceiling below it, that movement is audible. Fully adhered and induction-welded assemblies hold the sheet flat and eliminate most of it. If the space underneath is occupied and quiet, that is worth raising before the attachment method is chosen.