You've decided your old roof is beyond repair and it's time to choose a new one. There are many options. New materials are now available that were not yet invented when your old roof was installed.

Roofs can be categorized into two basic types: low slope and steep slope. Low slope roofing varies from level up to a slope of four-in-twelve, which means the roof rises vertically four inches for each twelve inches of horizontal run. Roofing with a four-in-twelve slope or greater is categorized as steep roofing. The main difference between the two roofing categories is that low slope materials have a watertight membrane, whereas steep slope materials are water shedding and will leak unless applied on a slope of sufficient steepness.

For low-slope roofs, the homeowner can either go traditional or high tech. The most common low slope roofing is tar and gravel, for which the proper name is gravel-surfaced, built-up roofing. A built-up roof consists of layers of hot asphalt and roofing paper or felt. The top coating may be gravel, a flood coat of hot asphalt, or a rolled roofing cap sheet. Built-up roofs have the advantage of history. Built-up roofing has been the standard low slope roofing for over a hundred years. Most experienced built-up roofers can consistently provide a surface that will last 15 to 20 years.

The high-tech alternative is a polymer modified rolled roofing called modified bitumen. The "mod-bit" roof membranes are heat-welded together with a flame to provide a very strong system. Modified roofing materials may last up to 20 or even 40 years, but require skilled and well-trained installers who make relatively few or no mistakes.

Steep slope roofing materials include wood shingles and shakes, tile, slate, and composition shingles. Concrete tile, clay tile, and slate are expensive and durable roofing materials. Composition shingles (also called asphalt shingles) are the most common and practical of the steep slope materials. Composition shingle roofing comes in a variety of thicknesses and has a potential life ranging from 20 to 40 years, depending on the quality of the product. Most homeowners do not wish to accept the potential fire hazards related to wood shakes or shingles and these materials are rapidly loosing favor in many areas.

If your roof has asphalt composition shingles or wood shingles, you may have the option of installing a second roof over your original roof. If old roofing must be stripped off, it may be necessary to add plywood sheathing on top of the original skipped sheathing, which adds to the cost of reroofing. Many areas do not allow more than two layers of roofing. Multiple layers can overload a roof structure. A second roof cannot be installed over wood shakes (except for some metal tiles that can be placed over existing wood shakes); the wood shakes must be removed before a new roof can be installed.

A good way to become familiar with the various roofing products on the market is to visit a roofing showroom. Your local roofing contractor may show you two or three sample materials. A well-stocked showroom on the other hand may have 30 to 40 samples of various roofing materials and should have staff available to help you examine and compare the different products available.
Manufacturers' warrantees often provide little value to the consumer, so it's very important to select your roofer carefully. The warranty may be valid only if the roof has been installed according to the manufacturer's specifications and, as any experienced home inspector will tell you, this is rarely the case. Ask your roofer to provide you with the roofing materials manufacturer's specifications and insist that they be followed.