What size screws do you use for metal roofing?
Short answer. Most residential exposed-fastener metal roofs use #9 or #10 wood-grip screws with a bonded EPDM rubber washer, about 1 to 1 1/2 inches long through the panel into wood, with longer screws at the ridge and trim where they pass through more layers. Side laps take short #12 or #14 stitch screws, and standing seam uses hidden clips with low-profile screws instead.
Choosing the screw
- Diameter. #9 and #10 are the usual panel-to-wood sizes. #12 and #14 are thicker and used for stitching laps, fastening into steel, or replacing a stripped screw.
- Length. Long enough to bite fully into the purlin or deck after passing through the panel and any closure or trim. Ridge caps and trim over foam closures often need 2 inches or more.
- Point. Sharp points for wood, self-drilling tips for steel framing.
- Washer. A bonded EPDM washer seals the hole. It should compress slightly, not squash out past the edge.
- Head. Painted to match the panel color, with a corrosion-resistant coating suited to the panel metal.
Where they go
Panel makers publish fastener schedules showing whether screws go in the flat or on the rib, and how many per row. Rib fastening keeps the washer out of standing water but requires a longer screw. Follow the schedule for the panel you have, since it drives the wind rating.
Common mistakes
- Overdriving. Crushes the washer, which cracks and leaks.
- Underdriving. Leaves the washer unseated, an open path for water.
- Missing the purlin. The screw holds nothing, and the hole leaks.
- Angled screws. The washer cannot seal against the panel.
Maintenance
Exposed washers age in Georgia sun. Over the years, inspect and replace loose or cracked ones, using an oversized screw where a hole is stripped.
More in metal roofing in Georgia and common roof leak causes.