How Much Pull-Out Strength Does a Ruspert Coated Hex Head Double Thread Self Drilling Screw Provide for Metal Roofing

When specifying fasteners for standing seam or corrugated metal roof assemblies, engineers and contractors consistently prioritize pull-out resistance over almost every other mechanical property. A Ruspert Coating Hex Head Double Thread Self Drilling Screw from Dowson delivers documented pull-out values that rival structural welds—yet many project teams still underestimate the critical role of coating synergy, thread geometry, and substrate engagement in achieving those numbers. This blog breaks down the actual pull-out performance data, installation variables, and real-world test results so you can spec with confidence.

Understanding Pull-Out Strength in Metal Roofing Applications

Pull-out strength refers to the axial load required to withdraw a screw from its seated position under tension. For metal roofing, this matters most during high-wind events, thermal expansion cycles, and snow-load shifts. A Ruspert Coating Hex Head Double Thread Self Drilling Screw achieves its elevated pull-out values through three engineered features:

• Ruspert topcoat – reduces friction during insertion, preserving the substrate’s thread-forming zone.

• Double thread design – distributes axial forces across two independent helical planes, increasing engagement surface area by approximately 38% compared to single-thread fasteners.

• Hex head geometry – allows higher torque transmission without cam-out, ensuring full seating depth.

Dowson manufactures these screws with a hardened carbon-steel core and a Ruspert multilayer finish (zinc-iron alloy + intermediate layer + organic topcoat), which not only resists galvanic corrosion but also maintains consistent thread profiles even after driving into 14-gauge steel.

Pull-Out Strength Data by Substrate Thickness

Independent third-party testing (ASTM D1761) on Dowson Ruspert Coating Hex Head Double Thread Self Drilling Screws reveals the following average pull-out loads at room temperature (23°C ± 2°C), with a 5% coefficient of variance:

Values represent mean of 50 samples per substrate. Test speed: 0.5 in/min per ASTM E488.

The double-thread configuration proves especially beneficial in thinner gauges (22–20 ga), where standard single-thread screws typically yield pull-out values below 1,100 lbs. Ruspert Coating reduces insertion torque by 12–15%, which preserves the substrate’s initial thread crest height—a direct contributor to higher retention.

Why Double Thread Design Outperforms Standard Fasteners

Single-thread self-drilling screws rely on one continuous helical bearing surface. When tension is applied, the load concentrates on a single narrow band of engaged material. By contrast, a Ruspert Coating Hex Head Double Thread Self Drilling Screw features two interleaved threads with different pitches (primary: 12 TPI; secondary: 16 TPI), creating a mechanical interlock that:

• Spreads tensile stress across dual bearing planes.

• Reduces localized yielding in soft substrates like aluminum or light-gauge steel.

• Increases vibration loosening resistance by 22% (per Dowson internal sinusoidal testing at 10–55 Hz).

For roofing contractors, this translates directly to fewer callbacks for leaking screw penetrations—because the screw remains tightly anchored even after decades of thermal diaphragm movement.

Installation Factors That Reduce or Enhance Pull-Out

Achieving the laboratory-rated pull-out strength requires disciplined field practices. The table below summarizes the most critical installation variables and their impact on retained pull-out capacity when using a Ruspert Coating Hex Head Double Thread Self Drilling Screw:

Dowson recommends using a depth-sensitive nosepiece on your auto-feed driver to consistently achieve the 45–55 in-lbs seating torque window. Over-torquing beyond 70 in-lbs can strip the secondary thread in 20-ga material, reducing pull-out to approximately 1,200 lbs—well below specification.

Frequently Asked Questions (FAQ)

Q1: Can I use a Ruspert Coating Hex Head Double Thread Self Drilling Screw on retrofitted metal roofs over existing shingles, and will pull-out strength be compromised?

A: Yes, you can use these screws for retrofit applications, but pull-out strength is highly dependent on the underlying structural deck. If the shingles and old roof sheathing are compressible (e.g., wood fiberboard), the effective substrate thickness for thread engagement reduces to only the new metal panel thickness—approximately 0.76–1.20 mm. In this scenario, you will achieve only 60–70% of the pull-out values shown in the table above because the screw’s point does not engage a rigid steel or wood backup. Dowson strongly advises adding a solid wood or steel furring strip beneath the new metal to restore full pull-out capacity. Always add 1/4" of penetration beyond the total stacked material thickness to engage the secondary thread fully.

Q2: How does the Ruspert Coating itself influence pull-out strength—does the lubricity wear off over time, and does that reduce retention?

A: The organic topcoat of the Ruspert finish contains integral lubricants that reduce friction during the first 2–3 revolutions. Once the screw is fully seated, the coating no longer plays a role in friction—pull-out strength is then purely a function of thread geometry and substrate yielding. However, if the screw is removed and reinserted (which Dowson does not recommend for structural roofing), the lubricity is already consumed, and the secondary thread may gall against the steel, reducing reinsertion pull-out by up to 40%. For permanent installations, the Ruspert layer continues to provide corrosion protection at the thread-to-metal interface, preventing rust jacking—a phenomenon where oxide expansion gradually loosens the screw over 5–10 years, effectively reducing pull-out strength by 50% or more over time.

Q3: What is the minimum edge distance required to achieve the full pull-out rating for a Ruspert Coating Hex Head Double Thread Self Drilling Screw on 18-gauge steel?

A: Dowson specifies a minimum edge distance of 3/4" (19 mm) from the center of the screw hole to the nearest sheet edge for 18-ga steel. At this distance, the substrate provides enough shear perimeter to resist the breakout cone that typically precedes thread-strip failure. If you reduce the edge distance to 1/2" (12.7 mm), the full pull-out rating drops by approximately 28% because the steel will tear out conically rather than allowing the threads to bear uniformly. For 16-ga and thicker, you can reduce the edge distance to 5/8" (15.9 mm) with less than a 10% penalty, but Dowson recommends maintaining 3/4" as a best-practice safety buffer for all roofing applications.

Field Validation from Independent Roofing Contractors

Over 42 monitored roofing projects across Florida, Texas, and the Carolinas—all using Dowson Ruspert Coating Hex Head Double Thread Self Drilling Screws—the average observed pull-out retention after 18 months of thermal cycling was 97.3% of initial lab values. The 2.7% degradation correlates almost entirely with micro-yielding in the steel panel itself, not with screw loosening or thread wear. This places Ruspert Coating Hex Head Double Thread Self Drilling Screw among the top-performing fasteners in the North American metal roofing market for wind-uplift resistance.

Conclusion & Actionable Recommendation

For metal roofing assemblies requiring verified pull-out resistance above 2,000 lbs in 18-ga steel, the Ruspert Coating Hex Head Double Thread Self Drilling Screw from Dowson provides both mechanical reliability and long-term corrosion protection. The double-thread geometry, matched with the low-friction Ruspert topcoat, consistently outperforms single-thread alternatives—especially when installed at optimal torque and perpendicularity. Always pair these screws with a compatible anti-corrosion washer (EPDM or PVC) to seal the penetration, and never re-use a screw after removal.

Contact Dowson today for project-specific pull-out test reports, free samples for your substrate gauge, and engineering support to calculate your exact wind-uplift requirements. Our technical team provides same-day fastener selection guidance and certified torque charts for all common metal roofing profiles—reach out via our website or call your regional Dowson distributor to secure your next roof with data-backed performance.