What is the usual length of the fastener that holds the clip-strip to the decking for a standing seam metal roof?
It depends. There’s several issues: 1) deck thickness, 2) metal panel width, 3) wind loading, 4) slope of roof
You’ll want to follow the manufacturer’s installation instructions, but here are generally acceptable standards.
1) Most manufacturers recommend penetrating the roof deck, unless it’s 2x material (1 1/2” thick). Therefore, 1/2” plywood deck could use a smaller (shorter) fastener than 3/4” thick deck. However, most manufacturers recommend 1” long fasteners. (If you install over an old shingle roof, you’ll need to add sufficient length to go through the shingles.)
2) Depending on the panel width, and thus spacing, a shorter screw could be used for narrower spacing and still develop the same holding power.
3) If you’re located in a high wind area, screw lengths will be longer and spacing is less.
4) Roofs with steep slopes are more susceptible to wind loads (uplift) and size and spacing needs to be adjusted accordingly.
Here is an article that explains it better. Go to page 4 for a typical spacing chart and page 8 for length and general installation recommendations.
Btw, don’t forget to install on an isolation sheet.