Heat-welded seam

A heat-welded seam joins the overlaps of a thermoplastic sheet membrane, such as TPO or PVC, with hot air, so the laps melt and fuse into a single homogeneous joint. It is more durable than an adhesive-bonded seam, which relies on the bond holding up over time.

The mechanism is the key difference. Thermoplastic membranes can be re-melted, so a hot-air welder (hand tool or automatic robot) heats the two overlapping sheets to melting point as a roller presses them together, and they fuse into one continuous piece of material. The seam is then effectively as strong and as watertight as the sheet itself.

Compare that with the alternatives:

• Adhesive or tape seams (used on some membranes like EPDM) rely on a bond between two separate surfaces, which can degrade, peel, or be defeated by contamination or poor application over the years.
• A heat-welded seam has no bond layer to fail, it is the same polymer welded to itself.

That is why thermoplastic single-ply roofs over occupied space are specified with welded seams, and why seam quality control (probe-testing the welds, checking weld width and consistency) is a standard part of the installation.

For a builder the practical points are that heat welding is a skilled operation with a real defect rate if done poorly: the welder temperature and speed have to suit the membrane and the weather, the lap surfaces must be clean and dry, and cold welds or skips are leaks waiting to happen. Probe-test the seams, keep the welder settings dialled in for conditions, and treat seam welding as the critical waterproofing step it is, not a quick run with a heat gun.

Also known as: Hot-air welded seam, fused seam.

Category: Roofing / Membranes.

Related

• Roofing membranes
• TPO membrane

See also

• Fully adhered

References

• Roofing membranes (Chalkline) (verified 2026-06-01)

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Last updated: 2026-06-01. Verified: 2026-06-01. Quarterly review for currency.