DEAR TIM: Eight years ago, I purchased an existing home. I have no idea how old the standard three tab asphalt shingles are. At this point, I do not have any leaks. How can I tell when my asphalt shingles are worn out? Where do leaks on shingles originate? What causes shingles to fail prematurely? Can minor repairs extend the life of my roof? Beverly K. Minneapolis, MN

DEAR BEVERLY: It sounds to me that you are already fairly roof savvy. Not many people would know the correct name for their shingled roof. A three tab shingle refers to the common shingle that many people have. Its distinguishing characteristics are vertical slits that are usually 5 inches long between each shingle tab. These vertical lines are usually offset by 6 inches on each course of shingles.

These vertical slits between shingles are the first place I look for deterioration. It is not uncommon for leaks to develop in the top 3 inches of each vertical slit. This area is the weak link in a 3 tab asphalt shingle roof. Because of the way your shingles were designed and overlap, only a single shingle thickness stands between your plaster ceiling and the worst rainfall in this narrow 3 inch part of the slit. A majority of the surface area of your roof has at least two shingle thicknesses protecting you from rain and snow. Some areas of the roof have 3 layers!

The deterioration in this area often manifests itself by missing granules. These are the ceramic color coated pieces of crushed rock that protect the asphalt from the destructive ultraviolet (UV) rays of the sun. If exposed to sunlight, the asphalt within the shingle begins to rapidly deteriorate. I have my own idea as to what causes granule erosion in this narrow slit area. I actually believe it is caused by the force of the water that drips from the shingle above. The granules at the top of each slit receive twice as much energy from the dripping water as the granules just one half inch away on either side of the slit. Over time, this makes a big difference.

You should also look for cracks on the shingle tabs and within each slit. The cracks often happen as the asphalt gets older and loses the oils that give it its flexibility. The oil loss can result from overheating or poor quality asphalt from the get-go. Not all shingle manufacturers use the highest quality asphalt. Shingles can overheat if your roof ventilation is sub-standard or non existent. The presence of soffit ventilation and upper roof ventilation allows cooler air to replace the super-heated air that bakes the underside of the shingles.

If you discover missing granules in the top of the vertical slits, you can extend the life of your roof quite easily. All it takes is a little time and pieces of 40 pound tin that measure 2 inches wide by 5 inches long. It will also help to have a pair of lightweight leather gloves and a wide, flat pry bar.

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Try to work on the roof when it is cooler. You will become less fatigued, and the shingles will be easier to work with. Using the flat pry bar, pry up the corners of the shingle tabs at the bottom of slits that have missing granules. Align the piece of tin so that the 5 inch length is parallel with the 5 inch long vertical slit. Center the tin strip and slide it up under the two shingle tabs on either side of the slit. Continue to slide the tin strip up until the top one half inch disappears under the shingles above. The friction of the shingles will hold the metal strip in place. There is no need for caulk, roofing cement or nails. If you bent the shingle tabs up too much, the sun will most likely flatten them within a couple of days. Do not do this work on a cold roof. You can fracture shingle tabs.

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Comments: Welcome! I, Tim Carter, don't answer questions here. If you post a question here in the Comments Area, perhaps another visitor will help you. You need to go to the Ask Tim page if you want a question answered. Once there, look closely at how many weeks behind we are. Please be patient as you use this free service. If you have an emergency and need to talk to me, there is an option there for you. JOHN MOONEY 20 Jul 2009, 15:25 Does the dark mold/mildew on asphalt roof shingles reduce their life? Thanks. Joseph Berk 30 Dec 2009, 09:47 Tim mentioned that he believed that "shingle deterioration is caused by the force of the water that drips from the shingle above". This is well established and the best way to minimize this kind of deterioration is with a 5 inch offset instead of a 6 inch offset. When the offset is 5 inches both cutouts and butt joints align only every 8 courses as opposed to 6 inch offset which aligns cutouts every other course.      View all comments *Name: Email: Notify me about new comments on this page Hide my email *Text: Security Image: Security Image Text:   Remember, Tim Carter doesn't answer questions here in the comments. You need to go to the Ask Tim page.