A roof does more than keep a home dry. It usually has an impact on the overall appearance of a home and hence, its value. And, because a new roof is one of the single largest home-improvement expenses, it makes sense to choose material that will enhance a home's architecture and last a long time.

Wood, asphalt, metal, slate and cement are the most common choices. For the most part, roofing material is a durable, yet decorative, layer that covers a waterproof membrane which is the real stopgap from water entering the home. The roofing material simply protects the membrane from becoming damaged. Most membranes consist of an asphalt-impregnated building felt with the seams lapped for proper water shed. The membrane is available in various thicknesses (weights) and widths.

Asphalt shingles are by far the most popular roofing choice. There are two types - organic and fiberglass. Organic shingles consist of a wood-fiber base that is saturated with asphalt and coated with colored mineral granules. A fiberglass shingle consists of a fiberglass mat, top and bottom layers of asphalt and mineral granules.

One advantage that a fiberglass shingle has over the organic type is fire resistance. Fiberglass has a UL Class A rating while organic shingles have a UL Class C in the Underwriters Laboratories (UL) system for classifying fire resistance. Roofing materials are rated Class A, B or C, with A having the highest resistance to fires originating outside the structure.

Organic or fiberglass, asphalt shingles come in a variety of weights, styles and colors. In general, the thicker and heavier the shingle, the longer it will last. The three-tab shingle is the most common, lightest and least expensive. The architectural grade or layered shingle is heavier, more attractive and pricier.

Wood shingles were once the rage, but their popularity has waned due to concerns about their ability to resist fire. Wood shingles have a UL fire rating of Class B, C or none at all. Wood shingles and shakes are made from cedar and sometimes redwood, southern pine and other woods. The appearance and lasting quality of a natural wood roof can be greatly enhanced if it is treated regularly (every three to five years) with a shingle and floor oil. A good wood roof care program consists of cleaning with a power washer, select replacement of damaged material and oil treatment. A pigment in the oil will offer greater protection from damage by ultraviolet rays, one of the biggest causes of deterioration of wood roofing.

Because consumers love the look of wood, manufactures are producing shake look-alike material. These impostors look like shake, but, in most cases, have a UL Class A fire rating. Look-alike roofing materials include metal tiles and concrete or fiber-cement shakes.

Tile-clay or concrete - is a durable but fairly expensive roofing material. Mission-style and Spanish round-topped tiles are widely used in the Southwest and Florida. Flat tiles are also available to create a French and English look. Tile is available in an array of colors and finishes. One concern with tile is its weight. If it is being used to replace another type of roofing material it is important to verify that the structure will support the load. An engineer can help here.

Slate, one of our favorites, literally is stone. Quarried in Vermont, New York, Pennsylvania and Virginia, it comes in different colors and grades, depending on its origin. Considered virtually indestructible, it is more expensive than other roofing materials. Furthermore, its application requires a highly skilled craftsperson. As with the shake and tile, slate has its look-alike cousins which are significantly less expensive and can be installed by an experienced roofer. Slate look-alike material is made from portland cement which is reinforced with cellulose fibers.

Metal, once reserved for commercial roofing, has found a growing niche in the residential market. The two most common types of metal roofing are standing-seam and corrugated panels. The standing-seam material is generally factory finished with paint or a powder coating. The corrugated panels and tile look-alike material are subject to a rigorous factory priming process followed by a granular mineral finish. Metal is durable, but its life expectancy ultimately depends on the properties of the metal selected, its coating and its installation.

The price of a new roof can vary widely. To get a good idea of the cost of your roof, get three or four estimates from reputable contractors in your area. Keep in mind that cost is only one factor, and it must be balanced against the quality of the materials and workmanship.

How long should a roof last? Different materials have different life expectancies. The American Society of Home Inspectors provides the following estimates: asphalt shingles, 15-30 years; wood shakes/shingles, 10-40 years; clay/cement tiles, 20-plus years; slate, 30-100 years; and metal 15 to 40-plus years. These estimates depend on local conditions, installation, quality of materials, proper design and maintenance.