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A well-built and well-maintained roof is critical to the health of every structure. Builders, designers and manufacturers are constantly working to develop problem-free roof systems.

Most new roofs are designed to meet or exceed the load standards for a particular geographic area. The standards address what happens when the snow falls straight down or when it blows and drifts, for instance.

The trend is to make existing roofing materials better. There are roofing materials of metal, clay and fiberglass that won't chip or peel and are light, strong, durable and fire-resistant.

Fiber-reinforced cement, for example, is noncombustible roofing material that is lightweight as well.

In the old days, the fiber in a shingle was asbestos. It was a source of pollution as shingles deteriorated, but they did last 50 to 75 years. Now, shingles made of eucalyptus cellulose fibers impregnated with silica and Portland cement are being manufactured to mimic slate at about one-third less cost.

While fiber-cement shingles have about the same 50-year life expectancy as Pennsylvania gray slate shingles, Vermont slate lasts 100 to 150 years.

But fiber-cement is about 30 percent to 50 percent lighter than slate and easier to install.

Wooden roof-truss assemblies nowadays are produced in factories that are often hundreds of miles from building sites. The advantage to the mass production is that standardized parts are of uniform quality, they cost less than on-site assembly, they use less wood, there is less wasted material, and they are quicker to install.

Amid concerns about the renewability of lumber, many builders have turned to steel in roof construction. Besides cost and availability, the steel industry maintains that steel roofs offer the advantage of being more durable in areas vulnerable to earthquakes and hurricanes.

New products are being manufactured to facilitate roof ventilation. The products are used to insulate and ventilate asphalt-shingle roofs installed on underlayment nailed to plywood or oriented strand board.

In typical roof construction, decking is nailed to the truss assembly and the underlayment to the decking, with the shingles on top.

What the underlayment, typically roof felt, does is prevent rain or melting snow from leaking through the roof deck into the interior of the house.

By ventilating and insulating an area between the roof decking and the plywood sheathing, moisture buildup is prevented. Asphalt shingles have a 20-year life under normal conditions; proper ventilation ensures it.

Underlayment prevents moisture from entering a building. But what this barrier also does is prevent water vapor from a variety of sources from exiting outdoors through the decking.

In the winter, moisture is carried upward by warm air. When it comes into contact with the attic side of the ice-cold sheathing, it condenses. The warm air passes through the decking, but the moisture remains on the underside of the decking and accumulates.

This action creates two problems that are about to combine to drain your bank account. The wood used in the roof trusses and sheathing is like a sponge, and will sop up and retain every bit of moisture it comes into contact with. The trapped moisture will rot and warp the decking and leaks will develop.

Outside, on the roof surface, something else is happening, especially if it's winter. The warm air that is escaping through the roof decking from the attic melts the snow on the roof, which trickles down to the frozen gutter and re-freezes. This process continues until a dam of ice is built up that exerts pressure at the edge of the roof and the lower tier of shingles.

As this ice expands and backs up closer to the warm part of the roof, it melts once more and finds its way through the underlayment and decking by way of leaks created by moisture buildup inside.

The solution to this problem is proper ventilation, proper insulation and some added insurance -- a product known generically as ice-and-water shield, a self-sealing bituthene waterproofing membrane with a strong adhesive underside.

As much as 70 percent of home heat can be lost through lack of insulation. The best time to install insulation is in new construction, when it adds only one percent to the cost of a house. The kind of insulation -- batts and blankets, rigid-board or loose-fill -- and the amount is determined by the anticipated heat loss or gain, cost, and code requirements.

In addition, a vapor barrier should be installed on the side of the insulation that remains warm in winter, especially if the outdoor temperatures average 40 degrees Fahrenheit or below in the coldest winter months, and the relative humidity within the house is a constant 50 percent.

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