I have the ridgeline mount on a metal roof that has vertical ribs -- running down from the ridge -- that are 9" from center to center. The bottom of the mount is the width of 4 of these ridges (approximately 27"). FWIW, Each side contains two heavy blocks (pictured above) that weight 15-20lbs each. So, even atop ridges, this thing isn't going anywhere with 80-ish lbs of dead weight.