Choosing the correct fastener length is critical for the structural integrity and weatherproofing of a metal roof. A screw that is too short will not achieve adequate pull-out strength, putting the roof at risk during high winds. A screw that is excessively long can be difficult to drive and may damage insulation or plumbing behind the decking.
The Formula
To determine the correct screw length, you must account for three specific layers:
Rib Height: If you are fastening through the "high" part of the corrugation (the rib), you must add the height of the rib to your calculation. This is common for exposed fastener systems to minimize leak risks.
Insulation or Spacer: Add the thickness of any blanket insulation, rigid foam boards, or thermal spacers installed between the metal panel and the deck.
Substrate Penetration: This is the depth the screw needs to embed into the structure below.
Solid Wood (2×4 purlins): Minimum 1 inch embedment.
Plywood/OSB: The screw should penetrate fully through the backside of the sheathing by at least 1/2 inch.
Steel Purlins: The screw should extend past the steel by at least 3 thread pitches (approx. 1/2 inch).
Standard Metal Roof Screw Sizes
Metal-to-wood screws generally come in specific increments. Once you calculate the exact minimum length needed, you should round up to the nearest available size.
Standard Screw Length
Common Application (No Insulation)
1 Inch
Fastening in the flat (pan) to wood deck.
1-1/2 Inch
Fastening through a 3/4″ rib into wood purlins.
2 Inch
Fastening through a 1-1/4″ rib, or 3/4″ rib with thin insulation.
2-1/2 Inch +
Required for high ribs combined with insulation layers.
Top of Rib vs. In the Flat
Top of Rib: Placing the screw on top of the corrugation places the penetration point above the main water drainage path. This reduces the chance of leaks if the washer fails. This requires a longer screw (Rib Height + Penetration).
In the Flat: Placing the screw in the flat pan ensures a tighter seal against the wood and is less prone to over-tightening distortion, but the washer is constantly submerged in rain flow. This uses a shorter screw (Only Penetration).