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What are the compelling reasons for installing a metal roof?

Metal vs. asphalt shingles: Are the reflective shingles that qualify for the energy star rebate a good value? I wonder about the long term reflectivity. Is there any information about the copper additive that is supposed to retard mildew washing off - especially given the acid rain in the east and the fact that they only warrant the shingles against mildew for a limited time? If I go with metal, do the clear coats on the galavalume last as long as the paint finishes?

Asked by Tom Stephens
Posted Sat, 12/04/2010 - 04:15

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Tom,
Q. "What are the compelling reasons for installing a metal roof?"

A. Longevity and recyclability, among others.

Q. "Are the reflective shingles that qualify for the Energy Star rebate a good value?"

A. It depends. Certainly, if you live in a cold climate without air conditioning, you don't need reflective shingles or a "cool roof." If you are in a hot climate with high air conditioning bills, a "cool roof" may help, but only if you have a poorly designed house with ductwork in an unconditioned attic. If you have a new, well designed home with ductwork inside the conditioned space (where it belongs), a "cool roof" isn't going to make any difference in your energy bills.

Q. "Is there any information about the copper additive that is supposed to retard mildew washing off - especially given the acid rain in the east and the fact that they only warrant the shingles against mildew for a limited time?"

A. The problem you are referring to is not mildew, it is algae. Different species of algae cause staining of asphalt shingles, and some species are more resistant to the copper granules blended into "algae-resistant" shingles than others. "Algae-resistant" shingles will not remain algae-free forever. You can read an article I wrote about the problem here:
http://www.jlconline.com/cgi-bin/jlconline.storefront/4cfa1b240de1923327...

Q. "If I go with metal, do the clear coats on the Galvalume last as long as the paint finishes?"

A. I don't know.

Answered by Martin Holladay, GBA Advisor
Posted Sat, 12/04/2010 - 05:47

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A concern with asphalt shingles is the amount it contributes to landfills. The square footage per home is large; figure a 20 year product that gets thrown out and replaced; multiply by the number of homes that use it and you get very large quantities of waste with likely some bad stuff in it that represents further environmental pollution.
Metal roofs will last over 2x as long as asphalt if not much, much more. As Martin points out I believe they can be melted down for their metal. (Someone please correct me if I am wrong.)
A good design principle is to reduce the square footage of roofs as much as possible. Sprawling single story homes may be nice for accessibility and convenience but for issues stated above (and thermal envelope issues) they represent more lifecycle inefficiencies and waste. Multi-unit residences on the other hand represent more efficiencies and less waste.

Answered by J Chesnut
Posted Sat, 12/04/2010 - 11:36

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I will start by disagreeing with Martin's statement that "If you have a new, well designed home with ductwork inside the conditioned space (where it belongs), a "cool roof" isn't going to make any difference in your energy bills."

It will have a smaller impact, but reducing summertime solar radiant gain is always advantageous both in terms of energy efficiency and in the reduction in the heat island effect (and global warming).

Given that there are now composite shingles which carry the same duration of warranty as a metal roof, longevity may no longer be the determining factor, and there are a small but growing number of asphalt recycling plants (albeit mostly used in paving roads).

Metal roofs are more wind resistant, and more resistant to external fires but also more likely to prevent spontaneous ventilation (and heat release) from an internal fire. Metal roofs make maintenance, repair and remodeling more difficult (including climbing up to clean the chimney). Their snow-shedding quality, while allowing somewhat lighter framing, creates an avalanche zone under the eaves which can make ground-level snow removal challenging and can damage shrubbery, vehicles and people.

A metal roof may be a better collection surface for rain-water reuse, depending on the alloys in the coating (galvalume has aluminum and zinc - aluminum is associated with Alzheimer's). But a metal roof makes it more difficult to install a functional ridge vent with exterior wind baffles to prevent intrusion of wind-driven rain and snow. They both have very high embodied energy.

So the choice involves trade-offs and in my perspective there is no clear winner.

Answered by Riversong
Posted Sat, 12/04/2010 - 12:34

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