Several questions need to be answered when determining the approximate lifespan of a roof that has been correctly installed. To start, one must examine the type of a roof. Whether the roof is sloped or flat, it has two basic elements: the roof deck and the wood planks, plywood, or oriented strand board, otherwise known as particle board. These elements are usually used in residential construction and have a weather-resistance or waterproof finish. A sloped roof can be finished with asphalt composite shingles, wood shingles/shakes, clay tiles, metal, or slate tiles.
A flat roof's membrane typically includes several layers or plies of felt with a coat of tar between each ply. The surface of the roof is then covered with gravel to protect the membrane from the sun. Other flat roof finishes include a rubberized asphalt membrane, or plastic and rubber membranes. Even a flat roof is slightly sloped to allow the roof to drain properly and dry. The slope of the roof must also be taken into account. The weather resistant finish of a sloped roof can be compared to an umbrella - designed to shed water or snow and dry before the materials become saturated. The steeper the slope, the better the roof sheds water. Therefore, a typical asphalt composite shingle may last longer on a roof with a greater slope, as it allows the shingle to dry faster. Other factors that affect the roof's lifespan include its proximity to weather, including sun, wind, rain and snow. Take exposure to the sun as an example: On a sloped roof finished with asphalt composite shingles, the portion of a roof that faces south or southwest typically has more exposure to the sun. The sun's rays can cause the shingles to become brittle and age prematurely. This is why some areas may show more deterioration than others. The conditions in an attic space can also affect a roof's life. If insulation and ventilation levels are inadequate, air leaks from an interior living space can cause a build-up of warm, moist air. Under certain weather conditions, moisture condenses in an attic space, potentially causing mold and mildew accumulation on the roof deck and framing, which can lead to wood rot.
The following summary provides a typical life expectancy of various roof finish materials:
• Asphalt composite shingles: 15 - 25 years
• Wood shakes/shingles: 15 - 35 years
• Slate tiles: 35 - 100 years
• Built up roof (tar and gravel): 10 - 30 years
• Modified bitumen: 15 - 25 years
• Rolled roofing (selvage): 5 - 10 years
It takes a trained eye to properly evaluate a roof and to understand the many facts that affect the performance of a roof system. A properly trained home inspector can provide a homeowner with an objective opinion regarding the current conditions of a roof. ***This article was prepared by the Pillar To Post Home Inspection Team***