TPO and PVC Roofing: How Heat-Welded Membranes Go On
8 step by step guides
TPO and PVC are thermoplastic single-ply membranes. They arrive in wide rolls, go down over insulation on a low-slope roof, and are joined by melting the overlaps together with hot air. Done properly the roof becomes one continuous waterproof sheet. Done badly, a seam that looks perfect from the ladder opens at the first hard freeze. The difference is almost entirely in the preparation, the test welds and the details.
Start with TPO and PVC roofing explained for what the membranes are and how thickness and type are chosen. Then look at the two attachment methods, mechanically fastened and fully adhered, and the skill that holds the whole system together, hot-air seam welding.
Most leaks on thermoplastic roofs are not in the open field. They are at walls, curbs and corners, penetrations and coated edge metal, so those guides get the same detail as the field work. When a roof is already leaking, how to repair a TPO roof covers cleaning, patching and when a weathered sheet will no longer weld.
TPO and PVC Roofing Guides
- Guide 1TPO and PVC Roofing ExplainedTPO and PVC are single-ply thermoplastic roofing membranes, sold in rolls 5 to 12 feet wide and 45 to 80 mil thick. Sheets are lapped about 6 inches for mechanically fastened systems and welded with hot air into one continuous waterproof sheet, with a finished weld at least 1-1/2 inches wide on most products. TPO is the common, lower cost choice. PVC costs more and resists grease, chemicals and fire better.ProfessionalReading guide, about 15 minutes
- Guide 2How to Install Mechanically Fastened TPOA mechanically fastened TPO roof is secured with fasteners and plates driven through the sheet edge into the deck, then covered by the next sheet, which is hot-air welded over them. Sheets are commonly 10 feet wide with about a 6 inch lap. Fasteners along the seam are commonly about 12 inches on centre in the field and 6 inches or closer in the perimeter and corner zones. Fastener rows and spacing come from the wind uplift design, not from habit.Professional1 to 3 days for a small commercial roof
- Guide 3How to Install Fully Adhered TPOFully adhered TPO is glued to the substrate across its entire area. Adhesive is applied to both the substrate and the back of the sheet, allowed to flash off until tacky and not stringy, then the sheet is rolled into it and pressed with a weighted roller. A 3 to 4 inch strip at each lap stays adhesive-free for welding. Substrate must be clean, dry, and within the adhesive's temperature limits.Professional1 to 3 days for a small commercial roof
- Guide 4How to Hot-Air Weld TPO SeamsTo weld a TPO seam, clean the lap, make a test weld and peel test, then pass a hot-air welder along the seam so the top and bottom sheets melt together under a silicone roller. The finished weld is commonly at least 1-1/2 inches wide. Temperature and speed are found by test weld, not by number. After cooling, probe the full edge of every seam and reweld any gap.ProfessionalVaries: a few minutes per test, then the length of the seams
- Guide 5How to Flash TPO Walls, Curbs and CornersFlash a TPO roof at walls and curbs by running the membrane up at least 8 inches above the roof surface, adhering it to the substrate, welding it to the field sheet with a lap of 1-1/2 inches or more, and terminating the top edge with a fastened bar and sealant under counter flashing or cladding. Corners use pre-molded pieces or formed patches, and every T-joint gets a round patch.ProfessionalHalf a day to 2 days depending on the number of details
- Guide 6How to Flash TPO PenetrationsFlash a pipe through a TPO roof with a pre-molded boot slipped over the pipe and welded to the field membrane, with a stainless clamp at the top edge and a bead of sealant. Where the pipe cannot be slipped over, use a split boot or a field-wrapped flashing with a round T-joint patch. A sealant pocket is a last resort. Weld flanges at least 1-1/2 inches wide, then probe.Professional20 to 45 minutes per penetration
- Guide 7How to Install TPO-Coated Edge MetalTPO-coated edge metal is steel laminated with a layer of TPO so the roof membrane can be hot-air welded straight to it. Fasten the flange to a sound, blocked substrate in a staggered pattern, commonly 3 to 4 inches on centre, leave a joint gap per the maker, cover every joint with a welded cover strip, then weld the membrane to the clad surface at least 1-1/2 inches wide and probe it.ProfessionalHalf a day to 2 days depending on the perimeter
- Guide 8How to Repair a TPO RoofTo repair a TPO roof, find the actual leak, scrub the area with TPO cleaner and let it dry, cut a patch of matching TPO with rounded corners that extends at least 2 inches past the damage, and hot-air weld it all the way around, with a finished weld at least 1-1/2 inches wide. Probe the edge. If weathered membrane will not weld, use a manufacturer-approved coating or recover.Professional1 to 3 hours per repair
Also on this site
TPO and PVC Roofing: Common Questions
What is TPO roofing?
TPO is thermoplastic polyolefin, a single-ply roofing membrane sold in rolls about 5 to 12 feet wide and 45 to 80 mil thick. It is laid over insulation on a low-slope roof, either mechanically fastened or adhered, and the seams are welded together with hot air so the roof becomes one continuous sheet.
Is TPO or PVC better for a flat roof?
Neither is better everywhere. TPO costs less and suits most general roofs. PVC resists grease, chemicals and fire better, which is why it is used over restaurants and industrial buildings, and it is more flexible for detailing. The two cannot be welded together, and PVC must be kept away from asphalt and polystyrene insulation.
Can I weld a TPO roof myself?
It is not a beginner task. Welding needs a calibrated hot-air welder, a test weld and peel test before each session, a clean dry membrane and a probe check of every seam. Mistakes hide until the seam opens in cold weather. A small repair patch is realistic for a skilled person, but full seams are professional work.
What slope does a TPO roof need?
Manufacturers typically require at least 1/4 inch per foot so water drains. Many flat roofs have less than that, which is why tapered insulation, crickets and correctly placed drains are designed first. Water that stays on a membrane for more than 48 hours after rain is generally treated as ponding.
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