For the past few decades, many people have viewed their houses as financial investments. Buy one, hold on to it for a few years, sell it at a fat profit and buy a bigger one.

But due to today's housing slump, general economic outlook and resource depletion, those days might be over for the foreseeable future. People will increasingly view their houses as their long-term abodes and maintain, repair and improve them accordingly. They will use longer-lasting paint because they -- not someone they sell the house to -- will have to paint it next.

In this new economic regime, the more expensive an item is, the greater the incentive to invest in durable products. Serendipitously, durability and sustainability often go hand in hand, so a good financial investment is also a good investment for the planet.

Premium metal roofing, for example, is a high-ticket item with an initial installed cost of two to four times that of asphalt shingles, the "standard" roofing solution. But metal roofing also has two to five times the lifespan of asphalt shingles, and it incorporates many other economic and environmental benefits that make it competitive in the long run.

Asphalt shingles have the lowest initial cost of any roofing material designed for a house with a pitched roof. They typically last 20 years (30 + years for premium asphalt shingles) and while they don't boast first-rate curbside appeal, they don't reduce the asking price of most houses. These features have made them America's most popular shingle, used on over 50 percent of our residential roofs.

However, asphalt shingles have hidden costs. They are made from petroleum, which is subsidized by the U.S. government (thus, some of our tax dollars can be added to their price.) Their cost is bound to rise as petroleum becomes scarcer. They are impossible to reuse, hard to recycle and heavy, so 11 million tons of asphalt shingles are transported to landfills each year, creating more externalized cost.

Metal roofs vary in quality from the corrugated "tin" (galvanized steel) panels used to cover sheds to the premium "standing seam" (ribbed) or metal-shingle roofs used for occupied buildings. Overall, metal roofing panels have a minimum recycled content of 28 percent, and many have 50 percent or more. Also, metal roofs, which can outlast the houses they cover, sometimes can be reused and are readily recycled.

Almost all the steel used for metal roofs is manufactured or recycled in the U.S. -- much of it close to its final destination, due to shipping expenses. By and large, pollution from steel fabrication has been reduced to standards set by the EPA decades ago.

Premium metal roofs have specialized finishes that contribute significantly to the energy-efficiency of the buildings they cover. The finishes can be designed to reflect solar energy in hot climates, absorb it in cold climates, and balance gain and loss in neutral climates.

The reflective coatings of both metal roofing and asphalt shingles are white, to reflect visible light, but premium metal coatings can include an additional pigment that reflects invisible, infrared "heat rays." The percentage of reflectance of a premium coating far exceeds that of a reflective asphalt shingle.

A University of Florida study demonstrated a 23 percent reduction in air conditioning costs for a test house covered by a premium reflective roof. The EPA has given some premium metal roof materials an Energy Star rating because of their solar-reflective properties. A federal tax credit of up to $500 is in place through 2008 for qualifying metal roofs on new houses. A similar credit for retrofitted metal roofs has expired but might be renewed next year.

Metal roofs are lightweight and can often be installed directly over existing roofs, eliminating the need for tearoff and dumping. They are also highly fire resistant; embers from nearby fires seldom burn through metal panels. (They are not "fireproof;" they will melt in a major fire.) Their fire resistance and high wind resistance qualifies some homes with metal roofs for up to a 35 percent discount on their house insurance.

Metal is not the only roofing material with sustainable properties. However, its combined durability and sustainability could make it a good investment for your ecological house.

Philip S. (Skip) Wenz is a freelance writer specializing in ecological design issues. He was a general contractor, residential designer and teacher in the San Francisco area and founded the Ecological Design Program at the San Francisco Institute of Architecture.