Your roof has a limited lifetime. While the actual number of years you can eke out of it depends on a number of factors, including the grade of shingles used and how well they were installed, the average life expectancy for an asphalt shingles is just 15 to 20 years. Mother Nature also comes into play. The weather that your roof endures over its lifetime can greatly impact these numbers. Rain, salt water spray, heavy snow load, hail, strong winds, and ice storms can batter a roof and shorten its life. Luckily, as a homeowner, it is easy to keep tabs on how well your roof is holding up in just a few steps.
Flashing is simply a piece of metal, usually steel, aluminum, or even copper, that is used to protect vulnerable areas of your roof. Most often, flashing is used in valleys where two sloping sections of roof meet or around chimneys and vents in your roof. When flashing becomes damaged, water can get under the shingles and damage the wood underlayment. Climb up on your roof and inspect your flashing for loose, bent, or otherwise damaged segments.
You will also want to look for loose or missing shingles. When installed asphalt shingles are soft and pliable. As they age, however, they can become brittle, causing them to crack, break, and eventually snap off. Do not make the assumption that a loose or missing shingle is simply due to a recent thunderstorm in your area. The International Building Code (Section 1504) states that asphalt shingles in most parts of the country need to be able to withstand 85 mph winds. A typical summer storm cannot knock shingles loose, rather they are reaching the end of their effective lifespan.
While you are walking on your roof inspecting the flashing and looking for loose and missing shingles, pay attention to how your roof feels underfoot. Your roof should feel solid, firm, and sturdy. It should feel like it can easily support your weight. If you call it squishy, soft, or bouncy, then you may have a serious problem with the underlayment. When water gets under the shingles and reaches the plywood or OSB underlayment of your roof, it can wreck a ton of damage. The squishy feeling is rotting wood and it requires you to call in roofing contractors immediately. Left unchecked, the water damage can spread to the rest of your roof's underlayment and to the joists supporting your roof.