There are times when you have just one hand free to both hold the nail in place and hammer. One hand nailing is easier than it sounds. Simply wedge the nail between the hammer's claw with the point facing forward and the nail head resting on the hammer head base. Swing the hammer with the nail/claw facing forward. The force should give the nail a start into the surface. Remove the claw from the nail, and finish the job with the hammer head side.
The number one cause of problems with roofing products is faulty installation and the leading installation problem is incorrect nailing of roof shingles.