Installing Metal Roofing Over Asphalt Shingles

Posted on April 14, 2016 by Jason Allen

When an asphalt shingle roof wears out, one of the most long-lasting solutions is to retrofit the roof with metal panels, and not remove the asphalt shingles.  While it’s a change of appearance, the metal roofing panels will provide a long service life.

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Installing Metal Roofing Panels Over Shingles

There are a number of considerations when installing metal panels over an existing asphalt shingle roof.  First, the building codes allow metal over shingles if there is only one layer of asphalt shingles.  A third roof is not allowed, so there must only be one shingle roof in place.

Choosing the Correct Metal Roofing System

Fastener choice is important for wind resistance.  As is the pull-out resistance of the deck. The panel thickness (e.g., 24 gauge), panel width, height of the standing seam, spacing between clips, and clip strength all help determine the overall wind resistance of a metal panel recover installation.  Wind design loads are specific to geographic location and height of the building; work with the metal panel manufacturer to determine the design specifications.  Importantly, use fasteners that are long enough to penetrate through the asphalt shingles and the deck by at least ½” to ensure proper strength.UCI_2

 

Because metal panels run from eave to ridge, the flatness of the existing roof can affect the appearance—the waviness—of the metal panels.  Three tab shingles are quite flat versus laminated dimensional shingles.  Consider installing a base sheet or a #30 underlayment over the shingles before installing the re-cover metal panels.  The uneven surface of the asphalt shingles can be telegraphed to the metal panels, leading to an uneven or wavy surface of the new metal panels.  And, like oil canning, it’s not a performance issue, but homeowners don’t want an unsightly roof.  Spending a few dollars on a heavy base sheet/underlayment is cheap insurance to ensure an aesthetically pleasing roof, and that means a satisfied homeowner who is willing to pay for the new roof!

Benefits of Installing Metal Panels

There’s a green aspect to installing metal over existing asphalt shingles.  Not removing the asphalt shingles not only saves money but also reduces the amount of waste sent to a landfill.  Roofing tear-off is one of the largest contributors to landfill waste in the U.S. And, while there’s a lot of discussion about the reuse of asphalt shingles (i.e., downcycling) in asphalt roads and bike paths, the reality is that much of the shingle tear-off is not actually reused.  For example, the Colorado Department of Public Health and Environment no longer considers asphalt shingle waste to be a recyclable material.  Not because it’s not recyclable, but because there is no significant market for its use.  These ideas will influence some homeowners, so use them when selling the idea of recovering with metal.

Installing metal panels over existing asphalt shingles is a smart choice.  Design it right, and the new metal roof could last the life of the house.

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