Back when I started roofing 22 years ago, I was a perfectionist. I had come from a tool design background where we routinely worked to precise tolerances and I did not yet know what tolerances applied to roofing. As such, I was obsessed with getting every shingle in the exact right place… and I chalked a lot of lines to do it.
I remember the boss stopping by a shingle job I had started a few hours before, only to find me and the roof covered with red chalk, but not one shingle yet laid. I was on piecework, so that didn’t really bother him, but he had a good laugh at my expense. I remember him asking me how much I was being paid per line I snapped.
Since that time I’ve learned to shoot for excellence, which is possible on every job, instead of impossible perfection. I still chalk lines though, just not as many as I used to. Some roofers claim they can run shingles straight without lines, but I don’t believe it. I guess it depends on your definition of straight. I’m reminded of a quote by Robert Hughes:
“The greater the artist, the greater the doubt. Perfect confidence is granted to the less talented as a consolation prize.”
Anyway, it’s just easier to do a good job with some chalk lines to guide you. You don’t see carpenters and masons trying to work without lines. And it’s especially important to use lines when you’re first learning how to install a roof.
Use red for lines on the underlayment. The red dye is permanent and won’t easily wash off, which is what you want on the underlayment. Fill another chalk box with blue chalk. Since the blue will wash away, that is the one to use when chalking lines on the shingles for hips, ridges and valleys.