Pull-out and pull-over are two different tensile failure modes in sheet-metal screw connections. They are often discussed together because both can govern a connection loaded away from the supporting member, but they occur in different parts of the load path and are controlled by different geometric and material parameters.
Pull-out is a failure of the screw-to-support engagement. Pull-over is a failure of the connected sheet around the screw head or washer. A satisfactory connection requires both mechanisms to be checked independently.
Understanding the complete tensile load path
When a roofing, cladding or sheet-metal connection is subjected to tensile action, load is transferred from the connected sheet into the screw head or washer, through the screw shank and thread, and finally into the supporting steel. A weakness at either end of this load path can control the connection resistance.
This is why the fact that a screw has adequate material strength does not by itself establish the capacity of the assembled connection. The surrounding sheet and the supporting steel are part of the structural system.
What is pull-out?
Pull-out occurs when the screw thread can no longer develop sufficient engagement in the supporting material. Under increasing tensile load, the thread may locally deform or strip the supporting steel until the screw is withdrawn from the support.
Pull-out behaviour is strongly influenced by the thickness and mechanical properties of the supporting steel, together with screw diameter, thread geometry, thread pitch and the amount of effective thread engagement. In thin steel, a relatively small change in support thickness can materially change the available thread-bearing area.
What is pull-over?
Pull-over occurs at the connected sheet rather than in the supporting steel. The sheet surrounding the screw head or washer deforms and is pulled over the bearing area, allowing the sheet to separate while the screw may remain engaged in the support.
The resistance is therefore sensitive to the connected-sheet thickness and strength, the effective bearing diameter under the head or washer, local sheet geometry and the quality of installation. A larger effective bearing area can improve load distribution, but the final resistance remains dependent on the complete tested or calculated connection configuration.
Why the weaker failure mode governs
Pull-out and pull-over should not be added together. They are alternative failure mechanisms for the same tensile load path. The governing connection resistance is controlled by the lowest applicable design resistance after the required safety format has been applied.
For example, increasing washer diameter may improve pull-over behaviour while doing very little for pull-out resistance. Increasing support thickness may improve pull-out while leaving a very thin outer sheet vulnerable to pull-over. Connection optimisation therefore requires identification of the governing mechanism rather than improvement of a single component in isolation.
Main parameters affecting pull-out resistance
- Supporting-steel thickness: thicker support generally provides a larger effective thread-engagement zone.
- Supporting-steel strength: thread-bearing and local deformation behaviour depend on the mechanical properties of the support.
- Screw diameter and thread geometry: these control how load is transferred from the screw into the support.
- Effective thread engagement: the thread must develop correctly in the supporting steel after drilling.
- Installation quality: overdriving, damaged threads or an oversized drilled hole can reduce effective engagement.
Main parameters affecting pull-over resistance
- Connected-sheet thickness: thin sheet is more susceptible to local deformation around the screw head or washer.
- Connected-sheet strength: local bearing and tearing resistance depend on the sheet material properties.
- Head or washer bearing diameter: the effective contact area influences how tensile force is distributed into the sheet.
- Profile geometry and fixing location: local curvature, ribs, crowns and unsupported sheet width can change behaviour.
- Installation depth: excessive compression or local damage can alter the condition of the sheet around the fixing.
Sealing washers have both sealing and mechanical functions
In roofing and cladding fasteners, the washer assembly is commonly selected first for sealing, but it also participates in load transfer. Its metal backing and effective bearing diameter influence how tensile force is introduced into the connected sheet.
This does not mean that a larger washer automatically produces a stronger complete connection. Washer geometry, sheet thickness, screw configuration and installation condition must remain compatible with the validated fastening system.
Pull-out, pull-over and drilling capacity are separate checks
A self-drilling screw may be able to drill the assembly successfully and still have insufficient tensile connection resistance. Drilling capacity addresses whether the point can penetrate the required steel build-up. Pull-out and pull-over address how the installed connection transfers load after installation.
Likewise, a screw with adequate tensile connection resistance can still be unsuitable if its clamping range, drilling capacity, corrosion class or sealing arrangement does not match the application.
Installation can change the failure behaviour
Correct installation is part of the connection performance. Underdriving may leave the washer or head without proper seating. Overdriving can locally deform the sheet, damage the sealing washer or adversely affect thread engagement. Incorrect installation angle can also create non-uniform bearing and sealing conditions.
Where connection resistance is based on testing or an approval, installation should reproduce the relevant validated condition as closely as practical.
Engineering assessment sequence
- Define the connected-sheet thickness, grade and profile condition.
- Define the supporting-steel thickness and material strength.
- Select a screw whose drilling capacity and clamping range suit the assembly.
- Determine the applicable pull-out resistance for the screw-to-support condition.
- Determine the applicable pull-over resistance for the connected sheet and bearing geometry.
- Check other required limit states such as screw tension, shear or interaction where relevant.
- Apply the safety factors or design format required by the governing standard, approval or project specification.
- Use the lowest applicable design resistance as the governing connection value.
Product-specific verification
Pull-out and pull-over capacities are not universal properties of a screw diameter. They belong to a specific connection configuration and depend on the fastener, connected sheet, support, installation and test or design method. Values from another manufacturer or a different sheet/support combination should not be transferred to a JOPAL connection without supporting technical evidence.
For project selection, use the applicable JOPAL technical documentation, approved assessment, verified calculation method or project-specific test data for the actual fixing condition.
This resource explains selection principles. Final product suitability, drilling range and resistance must be verified against the applicable JOPAL technical data for the specific fastener and project condition.
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