PVC Mechanically-Fastened Seam-Weld Inspection
Why this matters
A mechanically-fastened PVC roof lives or dies at the seam. The membrane sheet itself is rated for 20-plus years; the field weld is what fails first. Sika Sarnafil, Carlisle Sure-Flex, Versico VersiWeld, and GAF EverGuard all publish identical weld-quality requirements because the chemistry of hot-air-welded plasticized PVC does not vary by brand. A 1.5 in nominal weld with a documented 1.5 in minimum pull test is the warranty floor across the industry. Skip the daily test cuts and the manufacturer voids the warranty on the entire roof, not just the bad seam.
Equipment and conditions
Weld with a hot-air welder rated for the membrane thickness:
- Hand welder: Leister Triac AT or equivalent, 20mm or 40mm nozzle. Used at penetrations, corners, T-joints.
- Robot welder: Leister Varimat V2 or Sievert SiWa for production seams. Minimum 5 in pressure roller.
Ambient temperature minimum is 25 F per most manufacturer manuals (Sika Sarnafil, Carlisle). Below 40 F, increase preheat dwell and reduce robot speed. Wind above 25 mph degrades weld quality because the hot-air stream cools before contact. Do not weld over wet, frost-covered, or dust-contaminated membrane.
Weld parameters
Production weld dimensions per manufacturer specifications:
- Minimum weld width: 1.5 in (38 mm) for mechanically-fastened systems
- Minimum weld width: 2 in (50 mm) for fully-adhered systems
- Robot speed: typically 6 to 12 ft per minute depending on membrane thickness and ambient
- Hot-air temperature: 1000 to 1100 F at the nozzle tip on most 60 mil membrane
Each robot, on each shift, on each project, requires calibration via test welds before production seams. Test parameters until a 2 ft test strip yields a clean film tear (not seam separation) under peel.
Step-by-step seam inspection sequence
Visual sweep of every production seam. Look for skips (membrane laid down with no weld), edge burns (overheated edge that has scorched and brittled), fishmouths (lifted membrane edge), and cold welds (incomplete fusion, identifiable by gray rather than glossy seam edge).
Probe every seam end and every T-joint with a rounded screwdriver or seam probe. The probe should not penetrate any portion of the weld. Any catch, gap, or lifted edge gets marked with crayon for repair.
Repair protocol: re-weld with a hand welder, sweeping in the direction of the existing seam. If the membrane is damaged, cut and patch with a target patch sized 4 in beyond the defect on all sides, hot-air welded.
Daily destructive test cuts. Cut two 1 in wide coupons across the seam in each work area, peel test in the field by hand. Per Sika Sarnafil and Carlisle, the membrane should tear before the seam separates. Document each test with location, time, ambient temperature, robot ID, and operator initials on the daily QC log.
T-joint reinforcement. Every T-joint (where three sheets converge) requires a target patch on the side where the cut sheet runs. Patch minimum dimension 4 in radius, hot-air welded with rounded corners (no 90 degree corners on patches; they crack at the bend point).
Probe technique
The seam probe is the single most important QC tool. A blunt probe (the round end of a screwdriver or a purpose-built Sika probe) is drawn along the weld edge with light to moderate pressure. The probe rides on top of the seam if it is well-welded; it dives into any unfused or cold-welded section. Probe both seam edges; the back side (the side hidden under the lap) is often where cold welds hide because the operator can see the front edge but not the back.
A weld that probes clean today and probes open six months later was always a cold weld; aging just made the bond fully release. Probe pressure must be consistent across the entire roof. Train and re-train every QC inspector on probe technique with a known-bad test panel.
Documentation requirements
Manufacturer warranties (Sika Sarnafil, Carlisle Syntec, GAF EverGuard) require a daily seam log naming the inspector, ambient conditions, robot calibration record, peel test results, and a roof plan marking the location of test cuts. Most also require photographic documentation of every T-joint patch.
FM Global FM 1-29 mechanically-fastened single-ply ratings require fastener pattern verification at the same time. The fastener pattern at the perimeter (typically 6 in on center) is tighter than the field (typically 12 in on center). Verify the spec sheet against the as-built pattern by lifting one perimeter sheet edge per 5,000 sq ft.
References
- Sika Sarnafil S327 Installation Manual, current edition
- Carlisle Sure-Flex PVC Installation Specifications
- GAF EverGuard PVC Application Specifications
- FM Global Property Loss Prevention Data Sheet 1-29
- ASTM D4434, Standard Specification for Poly(Vinyl Chloride) Sheet Roofing
- NRCA Roofing Manual: Membrane Roof Systems, single-ply chapter