Pull-out and pull-over are different tensile failure modes in sheet-metal screw connections. They are often discussed together because either one can govern when the connection is loaded away from the support, but they occur at different locations and are controlled by different parameters.
Pull-out occurs at the screw-to-support thread engagement. Pull-over occurs in the connected sheet around the screw head or washer. Both need to be checked for the actual connection.
Follow the load path
Under tensile loading, force moves from the connected sheet into the screw head or washer, through the screw shank and thread, and finally into the supporting steel. Failure can occur at either end of that path.
This is why screw material strength alone is not enough to define connection capacity. The sheet, washer arrangement and supporting steel all contribute.
Pull-out
Pull-out occurs when the engaged thread can no longer develop enough resistance in the supporting material. The steel around the thread deforms or strips and the screw is withdrawn from the support.
Support thickness and strength are usually major factors. Screw diameter, thread pitch, thread profile, hole condition and the amount of effective engagement also matter. In thin steel, even a modest change in support thickness can change the available thread-bearing area significantly.
Pull-over
Pull-over occurs in the connected sheet around the screw head or washer. The sheet deforms locally and passes over the bearing area while the screw may remain fully engaged in the support.
The result is sensitive to connected-sheet thickness and strength, effective head or washer diameter, local profile geometry and installation condition.
The weaker mode governs
Pull-out and pull-over are alternative failure modes for the same tensile action. Their resistances are not added together. The governing design resistance is the lower applicable value after the required safety format is applied.
For example, increasing washer diameter may improve pull-over without changing pull-out. Increasing support thickness may improve pull-out while the outer sheet remains vulnerable to pull-over.
What affects pull-out resistance
- Supporting-steel thickness: controls the available thread engagement zone.
- Supporting-steel strength: affects local thread bearing and stripping resistance.
- Screw diameter and thread geometry: control how force is transferred into the support.
- Hole and thread formation: oversized holes or damaged threads reduce effective engagement.
- Installation: overdriving or thread stripping can reduce the resistance of an otherwise suitable connection.
What affects pull-over resistance
- Connected-sheet thickness: thinner sheet is more susceptible to local deformation.
- Connected-sheet strength: influences bearing and tearing around the fixing.
- Head or washer diameter: changes the effective bearing area.
- Profile and fixing location: crowns, ribs, curvature and unsupported width can change the local response.
- Installation condition: excessive compression or local sheet damage can alter the bearing zone.
Sealing washers also participate in load transfer
On roofing and cladding screws, the washer is usually selected first for sealing, but its metal backing also affects how tensile force is introduced into the sheet.
A larger washer can improve local bearing, but it does not automatically increase the capacity of the complete connection. The washer, sheet, screw and support still have to work together.
Do not confuse drilling capacity with tensile resistance
A self-drilling screw can drill the steel successfully and still have inadequate pull-out or pull-over resistance. Drilling capacity tells us whether the point can penetrate the specified build-up. Connection resistance tells us how the installed fixing carries load afterwards.
Clamping range, corrosion protection and sealing also remain separate checks.
Installation can change the governing mode
Underdriving may leave the washer or head poorly seated. Overdriving can deform the sheet, damage the washer or strip the engaged thread. A screw installed at an angle can also create uneven bearing around the fixing.
Where resistance is based on test data or an approval, the site installation should reproduce the validated condition as closely as practical.
Assessment sequence
- Define the connected-sheet thickness, grade and profile.
- Define the supporting-steel thickness and material strength.
- Confirm drilling capacity and clamping range for the selected screw.
- Determine pull-out resistance for the screw-to-support condition.
- Determine pull-over resistance for the connected sheet and bearing geometry.
- Check screw tension, shear or interaction where required.
- Apply the safety factors or design format required by the governing standard, approval or project specification.
- Use the lowest applicable design resistance.
Use values for the actual connection
Pull-out and pull-over are not fixed properties of a screw diameter. They belong to a specific connection made up of the fastener, connected sheet, support, washer arrangement and installation condition.
For JOPAL project selection, use the applicable technical data, verified calculation method, approval or project-specific test data for the fixing being assessed.
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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