There are two main reasons.
How to install standing seam metal roof panels.
Fasten panel side with number ten pancake head screws and the top of the panel with three number fourteen mill points.
Place the first panel with overlap leg along the alignment line.
This video shows how to install standing seam panels when hemming the eave and rake.
The connecting point where the two separate metal panels meet forms a vertically pointed rib or a seam that is one inch to inch and a half high.
Unlike steel and aluminum shingles standing seam panels should not be installed over the existing asphalt shingles.
Overlap the end a minimum of six inches when starting a new roll of underlayment.
A typical standing seam panel can be approximately 12 16 and 19.
Allow each consecutive course to overlap the previous one by at least four to six inches.
First of all if you install a standing seam metal roof over asphalt shingles there will be a so called telegraphing effect where shingles can punch horizontal lines in standing seam panels.
This technique provides the cleanest look and shows no exposed fasteners.