Metal Roof Screw Length Calculator – Roofing Fastener Guide
Calculate the right screw length for metal roof installations
Table of Contents
How to Use
- Enter the metal panel thickness
- Enter underlayment thickness (if any)
- Enter sheathing thickness (if any)
- Select the substrate/purlin type
- Click calculate to get the recommended screw length
Why Screw Length Matters
Choosing the correct screw length for metal roofing is critical for both structural integrity and weather resistance. Screws that are too short won't provide adequate holding power, while screws that are too long can damage underlying structures or create safety hazards.
The right screw length ensures proper penetration into the substrate for maximum pull-out resistance while accommodating all roofing layers including panels, underlayment, and sheathing.
Penetration Requirements
Different substrates require different minimum penetration depths:
- Wood purlins/decking: Minimum 1 inch (25mm) penetration into solid wood
- Metal purlins: Minimum 3 full threads engaged (approximately 1/2 inch)
- Through-bolting: Screw should extend just past the material for nut attachment
Metal Roofing Screw Types
Metal roofing screws come in several types designed for specific applications:
- Self-drilling screws - Have a drill bit tip for metal-to-metal applications
- Self-tapping screws - Require pre-drilled holes, used for wood substrates
- Stitch screws - Shorter screws for panel-to-panel connections at overlaps
- Pancake head screws - Low-profile for exposed fastener applications
Common Metal Roofing Screw Lengths
| Application | Common Length | Notes |
|---|---|---|
| Panel to wood purlin | 1.5" - 2" | Most common residential application |
| Panel to metal purlin | 1" - 1.5" | Self-drilling type recommended |
| Panel overlap (stitch) | 3/4" - 1" | Connects panels at side laps |
| Through insulation | 2" - 4" | Varies with insulation thickness |
Installation Tips
- Always use screws with EPDM rubber washers for weatherproofing
- Drive screws perpendicular to the panel surface
- Don't over-tighten - the washer should compress slightly but not bulge
- Use stainless steel or properly coated screws to prevent corrosion
- Follow manufacturer's recommended screw pattern and spacing
Frequently Asked Questions
- What screw length do I need for standard metal roofing on plywood?
- For metal panels over 1/2" plywood sheathing, typically 1.5" screws are recommended. This provides adequate penetration into the wood while accommodating the panel thickness.
- Can I use regular wood screws for metal roofing?
- No, you should use screws specifically designed for metal roofing. These have EPDM washers for weatherproofing, appropriate thread patterns for the substrate, and corrosion-resistant coatings that match the panel finish.
- How do I determine screw length when installing over existing shingles?
- Add the thickness of the existing shingles to your calculation. Typically, asphalt shingles add about 1/4" to 3/8" of thickness. Ensure the screw still achieves minimum penetration into the decking.
- What's the difference between #10 and #12 metal roofing screws?
- The number refers to the screw diameter. #12 screws are larger in diameter than #10 and provide more holding power. Use #12 for main panel attachment and #10 for trim and lighter applications.
- Should screws go through the high or low rib of metal panels?
- This depends on the panel type. Exposed fastener panels typically screw through the flat areas. Standing seam panels use concealed clips. Always follow the manufacturer's installation guidelines for your specific panel profile.