Solder penetration into the seam’s third leg was the decisive workmanship variable in Dunn, Ceruti, and Grey’s cyclic-testing study. Specimens that reached Leg 3 generally survived the study’s full 75-year simulated-equivalent benchmark, while specimens stopped at Leg 1 or Leg 2 failed much earlier. The benchmark represents a modeled load history, not a universal 75-year roof-life guarantee.
What the 2021 study tested
The article by Kelsey Dunn, Jeffry Ceruti, and Christopher Grey, published in Building Enclosure on July 12, 2021, examined 1/2-inch-wide, fully soldered flat-lock copper seams. The work was conducted under contract by the International Training Institute and addressed a practical gap: at the time, there was no industry consensus defining acceptable flat-lock solder workmanship from in-service performance data. The authors’ central question was how different levels of soldering quality respond to the repeated expansion and contraction imposed by weather.
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A flat-lock roof panel moves as its temperature changes. Because the copper is attached over plywood, the panel and substrate do not move identically in every direction. Those differential movements impose tension and compression on the folded seam. Repeated movement can concentrate strain in the copper legs or in the soldered joint, eventually producing fracture, seam separation or buckling.
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The specimens were workmanship variations, not factory-controlled commercial products. Their averages show how these prepared groups behaved under the selected test program; they are not precise predictions for every roof made by a particular installer.
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How thermal cycles were converted into laboratory loads
Modeling the roof assembly
The modeled assembly contained plywood sheathing, self-adhered underlayment and copper panels joined with 1/2-inch soldered seams. WUFI thermal and moisture modeling generated annual roof-temperature profiles for Boston, Boulder, Miami, Phoenix and San Diego. The authors modeled both new and aged copper. Aged copper produced the higher temperatures in all five locations, so those results were used for the subsequent load calculations.
The model used the temperature difference between copper and plywood to calculate the panel’s thermal load. A finite-element model then estimated seam forces while allowing the panel to deform. That detail matters: buckling can relieve part of the thermal stress, so a calculation that treated the panel as perfectly flat would not represent the tested behavior.
Why Boulder supplied the test history
The authors converted the modeled temperature histories into annual stress-range cycle counts at different force levels. Boulder, Colorado, had the highest cycle counts at several stress levels, so its annual history was selected as the laboratory loading sequence. Each sample was subjected repeatedly to tension and compression cycles representing those annual movements.
The apparatus continued until a specimen failed or reached the study’s 75-“year” endpoint. In this context, a year is a simulated equivalent based on the selected Boulder cycle history. Reaching 75 years means that the specimen completed the defined test benchmark without an earlier failure; it does not demonstrate performance beyond 75 years or establish a field service-life warranty.
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How the five workmanship groups differed
The International Training Institute prepared five specimen groups by varying pre-tinning, solder travel speed, solder quantity and the depth to which solder was intended to flow into the folded seam. Before and after cycling, the investigators documented seam dressing, gaps between hooked copper edges, solder voids, solder penetration into the seam legs and the geometry of solder at the edge of the seam.
| Group | Preparation and intended penetration | Result in the study | Observed pattern |
|---|---|---|---|
| 1 | Front- and backside pre-tinning; highest-quality practice; solder sweated into Leg 3 | 10 of 10 reached the 75-year benchmark; one failed at the benchmark | Most specimens had moderate to high edge-seam solder |
| 2 | Same general preparation without pre-tinning | Average 59 years; range 19–75; 5 of 11 reached the benchmark | Outcomes were more variable than Group 1 |
| 3 | No pre-tinning, faster solder travel and about 20–25% less solder; intended to reach only Leg 1 | Average 37 years; range 7–75; 1 of 10 reached the benchmark | Limited penetration produced early failures in most samples |
| 4 | One-side pre-tinning; intended to be partially sweated into Leg 3 | Average 63 years; range 14–75; 13 of 17 reached the benchmark | Every inspected specimen with full or partial Leg 3 penetration reached the benchmark; all four without Leg 3 solder failed early |
| 5 | One-side pre-tinning, faster travel and about 20–25% less solder; intended to reach only Leg 2 | Average 10 years; range 5–17; 11 of 23 failed before 10 years | All early failures involved copper fracture in Leg 2 and/or Leg 3 |
The table should be read with sample counts and ranges, not averages alone. For example, Group 4’s 63-year average hides a sharp split: specimens with solder in Leg 3 reached the benchmark, while the four specimens without that penetration failed early. Conversely, Group 2’s 59-year average spans results from 19 to 75 years.
Why solder reaching Leg 3 mattered
The folded seam has multiple copper legs. Solder that is merely visible at the edge does not prove that the interior of the seam has been filled. The authors found that solder penetration into the third leg stiffened the folded assembly and transferred thermal loads with less harmful concentration and differential movement. In their words, “The most important factor in determining the quality of a copper soldered seam is the extent of the solder sweated through the seam.”
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This mechanism explains the contrast between Groups 4 and 5. Group 4 was allowed to reach Leg 3, even partially, and most samples completed the full benchmark. Group 5 was intended to reach only Leg 2; its average simulated life was 10 years, and its early failures were copper fractures in Legs 2 and/or 3. The result is not that every Leg 2 seam will fail at 10 years in a building. It shows that, under this particular modeled cycle history, stopping short of Leg 3 left the copper carrying damaging strain.
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Dressing extent, joint gaps and solder voids had relatively little effect within the variation the authors tested when solder reached Leg 3. Edge-seam solder sometimes increased stiffness, but the results were highly variable. A large bead at the seam edge is therefore not a substitute for solder that has been sweated through the folded joint.
Failure modes recorded during cycling
- Copper fracture in Leg 3: cracking occurred in the third folded leg after repeated thermal movement.
- Copper fracture in Legs 2 and 3: the two legs fractured together, a pattern seen prominently in the low-penetration Group 5 specimens.
- Edge-seam solder failure: the exposed soldered edge separated without the same copper-fracture pattern.
- Buckling of straight hook legs in compression: the copper legs deformed under compressive movement; buckling was also one reason the finite-element model included panel deformation.
These modes are useful for inspection because a failed bead at the edge and a fractured folded leg indicate different weaknesses. A seam can look heavily soldered at its exterior while still lacking the internal penetration needed to distribute movement.
What the findings mean for specifications and quality control
Specify the joint, not just the appearance
Design documents should identify the required flat-lock seam configuration, soldering procedure and inspection criteria using applicable industry standards. A requirement such as “provide a continuous solder bead” does not by itself establish how far solder must travel inside the folded legs. The study authors recommend detailing the joinery clearly and defining quality-control procedures rather than relying on visual appearance alone.
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The authors recommend experienced sheet-metal installers, soldering tests before production and periodic retesting during construction. The purpose is to verify that the crew can consistently heat, feed and move solder so it penetrates the joint, rather than simply depositing material at the edge.
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Build and inspect a mockup
A mockup gives the design and inspection team a physical reference for dressing, hook-leg alignment, solder coverage and the expected interior condition. It should be made with the same copper, tools, flux, solder and sequence intended for the roof. Approval of a mockup should not replace production checks; it establishes the workmanship standard against which later samples are judged.
Destructively inspect test seams
Because the critical condition is inside the folded seam, the authors recommend cutting test seams into 2-inch strips. Inspect the cut faces for:
- the extent of seam dressing;
- gaps between the hooked copper edges;
- solder voids;
- the depth of solder penetration into each leg; and
- the geometry and continuity of solder at the seam edge.
The authors recommend rejecting a sample when solder does not reach at least 10 percent into the third leg. That threshold is a construction-quality-control recommendation from this study, not a universal code requirement. Project specifications and the authority having jurisdiction may impose different or additional criteria.
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Roofing inspection should occur while seams are being fabricated and installed, not only after the roof is closed in. Periodic destructive checks can reveal drift in travel speed, solder quantity, heating technique or pre-tinning. Records should identify the location of each sample and the disposition of any rejected work so deficient areas can be corrected before they are concealed.
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How to apply the results without overclaiming
The study supports a clear workmanship priority: verify that solder has entered the third leg. It does not prove that solder penetration alone makes a roof watertight or durable. The article explicitly places thermal-movement joints, attachment, underlayment, slope and drainage, corrosion and staining outside its scope. Those design and construction issues can control roof performance even when a seam passes a penetration check.
Nor does the article provide an independent, population-level field-failure rate or a typical service life for flat-lock copper roofs. The five groups were prepared specimens tested under one modeled load history. Their results should guide detailing and inspection, not be converted into a promise that a particular roof will last the same number of years.
Relationship to ASTM B593-21
ASTM B593-21 is titled Standard Test Method for Bending Fatigue Testing for Copper-Alloy Spring Materials. Its stated scope covers repeated or reversed bending-fatigue testing of copper-alloy flat sheet or strip spring materials on fixed-cantilever, constant-deflection machines. It is not the test method used for a formed, soldered flat-lock roofing seam assembly. The standard may provide adjacent background on copper-alloy material fatigue, but it should not be presented as the governing method for the seam study.
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A practical inspection sequence for a project
- Before production: put the specified seam detail, solder, flux, copper thickness and required Leg 3 penetration in the project quality plan.
- Qualify the crew: require soldering test seams from each installer or crew and review the cut sections before authorizing production work.
- Approve a mockup: document dressing, hook-leg fit, pre-tinning practice and the appearance of the completed seam.
- Sample during installation: select seams at an agreed frequency and locations, including changes in crew, weather, material batch or work method.
- Cut 2-inch sections: expose the interior and record dressing, gaps, voids and penetration into the third leg.
- Apply the acceptance rule: reject or repair samples that do not show at least 10 percent solder penetration into Leg 3, unless the project’s approved specification sets a stricter requirement.
- Investigate failures: determine whether the cause was insufficient heat, excessive travel speed, inadequate solder quantity, poor cleaning or joint geometry before restarting work.
- Coordinate the rest of the roof: separately verify attachment, movement joints, underlayment, drainage, slope, corrosion control and staining protection.
Bottom line for designers, contractors and inspectors
The cyclic tests point to internal solder penetration—not a large visible bead—as the most useful indicator of a robust flat-lock copper seam. In the tested specimens, seams that reached the third leg generally completed the 75-year simulated-equivalent benchmark, while seams limited to the second leg performed poorly. The practical response is to qualify the soldering crew, approve a representative mockup, and use destructive 2-inch seam checks to verify Leg 3 penetration during construction. Treat the 75-year figure as the endpoint of this study’s Boulder-based modeled loading, and evaluate the rest of the roof assembly independently.
For the original article and its complete methodology, see Dunn, Ceruti and Grey, “Cyclical Testing of Flat Lock Copper Seams”. For the separate fatigue-standard scope, see ASTM B593-21.
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