A metal roof is an alternative to a shingle covered roof, but a metal roof can actually be installed over shingles. However, you should only install a metal roof over shingles if you have less than four layers of shingles on the roof. Otherwise, you will need to remove the shingles.
Make a slat board base on top of the shingles. You will need to lay some 1-by-4 inch pine boards lengthwise every two feet on your roof. Move from bottom to top. Attach them to your roof with 3-inch wood screws in each corner.
Install the metal edging. Install your metal edging around the perimeter of your roof. Use 1 1/4-inch roofing nails every 4 to 6 inches to nail it to the roof.
Install metal roofing sheets. Metal roofing sheets have to be installed one at a time. Place a sheet from top to bottom on a section of the roof. Use 1/4-inch metal wood screws to attach the metal sheets to the wood slat board base.
Place screws on each side spaced two feet apart. On your first metal sheet only, you will need to put screws in the center as well. Your next sheet will overlap your first sheet and that pattern will continue each time you install a new metal sheet.
Install overlapping screws. Once the roof is covered in metal sheets you have to install 5/16-inch lap screws every place the metal sheets overlap at a distance of every four feet.
Install the metal trim. Your metal trim will go along the roof's edges as well as the top ridge. Install with 1/4-inch metal wood screws at a distance of every two feet. Use notching scissors to notch the metal in the middle so that it will bend over the ridge.
Close the bottom of the sheets. The only part of your metal roof not completely attached will be the bottoms of the metal sheets. You can attach those to the roof using 1/4-inch metal wood screws at a distance of every two feet.
Install closures. The closures will be self-adhesive on one side so you can just push them into the openings and they will self seal. This will prevent animals and water from getting into openings.