A little education on metal roofing.
Profiles, gauges, etc. vary but there are two primary types of metal roofing "Screw Down", and "Standing Seam".
Screw Down:
a. Typically 36" wide corrugated sheets with 5/8" ribs on approx. 9" centers.
b. You lay it over the deck and underlayment, then run screws with weatherseal washers through the panels, into the structure to hold it in place.
c. This is pole barn stuff, and is now seeing extensive use re-roofing homes.
d. It's inexpensive, goes down quick but has some inherent design flaws, Lots of screws punching through the panels being the main one.
e. The open rib corrugations are plugged with foam fillers which aren't the greatest long term solution.
f. This roof type can be a real bear to install at hips and valleys.
Standing Seam:
a. Typically 12" to 18" wide flat sheets with a vertical rib / seam between each panel.
b. A special "clip" is concealed in the panel joint to hold the roofing down to the structure. No penetrations through the panels.
c. The panel to panel joint is typically seamed or crimped.
d. These systems are far superior, typically cost more and most run of the mill roofers don't install them because they don't have the tools or understand the details.
e. The flat "pan" profile of these panels makes flashing details at hips, valleys, etc. much better and more watertight.
General:
a. Screw down will probably work fine for something at least 4/12 slope in a simple gable configuration. If you have a bunch of hips, valleys, dormers, slope changes, etc. stay away from it.
b. None of these roof should be installed directly over shingles. Shingles should be stripped, a new vapor barrier installed, then the metal roof placed on a smooth, flat surface.
c. Even the cheapest metal roof will cost you at least double the cost of a good shingle roof. A good standing seam roof might be $800 per square installed.
d. Paint finish is critical so it doesn't chalk & fade. You want Kynar paint, not silicone polyester.
e. Ask about warranty. Some of the better systems can provide 10-20 year leak free guarantees.
f. Leaks often occur at penetrations through the metal; vent stacks, chimneys, etc. The devil is in the details at these penetrations.