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Adding photovoltaic panels to a roof

Scott Gibson, Contributing Editor

I'm thinking of adding panels on my wood shake roof for generating electricity. The roof could have been 35 years old when I bought the house in 2002 (it's in Modesto, California). At the time, the inspector said it would be good for at least two more years. Suggestions?

Great idea. Given very high energy costs and a very unstable world, I'll bet half the country is wishing they could put photovoltaic panels on their roofs.

You're in a good spot for it, too. Modesto, about 90 miles east of San Francisco, is in a part of the country with lots of solar potential. It adds up to something like four or five kilowatt hours per square meter every day. Only parts of the desert southwest do better than that.

But before diving in, you really should do something about that roof.

solar power

This photovoltaic skylight system is integrated into the main entry of the Thoreau Center at the Presidio National Park, in California. Electricity generated by this system supplements power supplied by the local utility. Photo: Lawrence Berkeley Lab

Shake roofs don't last forever

Shakes, which are split, are thicker than sawn shingles. That's an advantage because there is more material to start with. And because of the age of the roof, you may very well have lucked into good quality heartwood, either redwood or western red cedar. That's good, too.

But 35 years is a mighty long time for a wood roof to sit baking in the sun, getting drier and more brittle all the time. Even if you weren't thinking of solar panels this would probably be a good time to start saving your pennies for a new roof.

Panels take time to install

I know next to nothing about installing solar panels so I called someone who does -- Warren Lauzon, part-owner of a company called Northern Arizona Wind + Sun, which has installed thousands of solar systems since the company opened in 1979.

There are a variety of solar systems on the market, including ones that look just like asphalt shingles. But for the more conventional photovoltaic panels we're used to seeing, installers begin by attaching special rails to the roof. Fasteners go right through the roof and into the framing. Then the panels are added.

A "typical" system (I know, there probably isn't such a thing) would produce 2,000 watts of electricity and cover an area of about 140 square feet, says Warren. These numbers will vary slightly depending on what kind of solar panel you buy.

A system like this would take a do-it-yourselfer between two and four days to install and cost something like $7 per watt. A pro is going to do it much faster, but charge between $9 and $10 per watt.

It's not cheap.

And whether you or a pro does the installation, you're going to have some real money tied up in the job. If the foundation on which they rest -- your roof -- is nearing the end of its useful life, you'll be faced with the expense of removing and re-installing the panels much sooner than you'd like.

Start with a roof in good repair

According to Warren, solar panels have a very long life -- "basically forever" as long as they are not damaged. Most are guaranteed for 25 years.

Some extremely durable roofing materials -- slate, for example -- may actually last for a century with periodic repairs. But it would be a miracle worthy of Vatican notification if asphalt or wood shingles lasted that long.

Even in the best or circumstances, solar panels will have to be removed at some point to allow new roofing to be installed. But minimize the pain by making sure your roof is in reasonably good condition before spending all that dough on solar panels. After all, at least part of the object here is to save money.

There are lots of solar installers in your local phone book. Give a few of them a call and get some expert opinion. If the roof is that old, my bet is they'll recommend new roofing before you go solar.

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