The Basics
Metal roof trim is used to cover the existing drip edge on this metal roof.
Covering an old shingle roof with a metal roof can be easily done (once you’ve secured your permit) if you’re skilled enough to climb around on the roof and you’re not afraid of heights—and possibly part monkey. Metal roofing is the perfect cover for asphalt shingles as it lasts for decades—possibly centuries longer than conventional roofing system—and it looks great doing it.
Covering a shingle roof with a metal roof begins with 1×3 wood strips that are used to cover the existing shingle roof and provides a base for the metal roofing and screws. It also acts as an added layer of insulation by providing a layer of static air between shingle and metal roofing.
Getting Wood
Attach the 1×3 pressure treated wood strips with two-10d ring shank galvanized nails to the underlying trusses when possible. It’s crucial to find each truss and pull a layout across the old shingles with a chalk line. Have a helper (with monkey blood) snap chalk lines 24” on center across the old shingles along the hidden trusses.
Trusses are typically 24” apart but can also be 16”, 19” or any measurement where an offset truss is located. Use a 12d nail to poke through the roof to be certain you’re on a truss when you snap lines.
Attach the 1×3 strips flush around the perimeter of all roof lines. That includes gable ends, ridges, drip edge, fascia, penetrations (plumbing pipes and vents to clarify any dirty thoughts) and hip lines. Ridge lines, valleys and hips should have 1×3 strips on either side, spaced apart 1” minimum.
For peaks and ridges that will get a ridge cap, measure the overall length of each side of the metal cap and then transfer that measurement to the 1×3 deadwood spacing on the existing ridge.
Gable to gable sides should be measured to make sure that an equal length is met on each side. It is important you attach the metal roofing square over the existing shingles correctly, or it might look as if the three stooges were on the job.
Use the 3-4-5 method to be certain your layout is square. Measure horizontally three feet, vertically four feet and diagonally five feet. Adjust the horizontal and vertical axis of the three and four-foot lines until the diagonal measurement equals five feet.
Heavy Metal
Now that the deadwood is complete, you can begin installing the metal roofing. This is easily done by beginning on the windward side of your roof. Have Moe, Larry or Curly hold one end of the panel and adjust it until it is square on the 3-4-5 layout.
Once in place, secure a waterproof self-tapping grommet screw through the metal roof panel and into the center of the deadwood pieces below. Depending upon the metal roofing manufacturer, you may need to attach screws on either side of the v-rib or directly on the v-rib. Always check the specs before you begin installation to avoid being called Shemp by the rest of the crew and possibly voiding the metal roof warranty.
Work your way across the roof, overlapping each piece of metal and screwing through the deadwood. Keep checking the measurements are consistent and the metal panels are still square, or you may have to back up and re-install the panels.
Monkey Business
This is where it gets tough and your monkey skills come into play. It is crucial that you walk on the deadwood only while on the metal roofing. It can bend the metal, possibly causing a leak. Install one panel at a time, finishing the last panel before you set the next one. Never stand on any unsecured metal!
Cut out any opening with tin snips and attach a new metal vent/boot according to the manufacturer’s specifications. Use a butyl caulk to seal the penetration, and then attach the boot/vent with four screws in each corner.
Finishing it Off
The last step for covering a shingle roof with a metal roof is to attach the trim around the drip edge, gable barges and fascia. Starting from the top of the highest peak, use tin snips to cut a break along the front of the trim, the same pitch as your roof. Set this piece aside, then go back down to the lowest point on the roof and begin there.
Attach the drip edge trim with a screw in each metal roofing panel. Work your way from low point to high point, overlapping each piece of drip edge trim 2” (minimum) until you reach the top piece. Insert the last two pieces of drip edge trim underneath the top piece and secure it in place.
Finish the job by attaching the ridge cap every 6” (3” from the ends) with a 2” overlap between pieces, being uber-careful not to step on the metal roofing where it is not supported underneath. With a little help from your friends, some common sense and a little monkey genetics, you can save a ton of money on re-roofing your asphalt shingle roof with a metal roof.
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