This is a solid article I located on the web, written by Fashun Smith.   It addresses a very common, "real world" issue – whether or not to pursue replacing your existing roof with a new installation, or rather invest in some periodic roof maintenance.   The point is that a roof is similar to many other major items  – regular maintenance will generally result in lower long term costs than negligence and eventual replacement. – Tom

 

Paul Erdos, like many mathematicians, was an eccentric. He wasn't interested in money, and gave what little he made to beggars and various strangers. With no family, job, nor hobbies to speak of, he dedicated his life to math. Nor did he ever have his own address. Rather than purchase a home, he instead moved around within a network of friends worldwide with which he stayed. He would just come by without calling beforehand and announce, "My brain is open!" His motto? "Another roof, another proof!" Of course, most of us don't live a transient, mathematical lifestyle like old Paul's, and as such must deal with the practicalities of roofing, of maintaining our roof, of protecting it, of, basically, keeping it from falling down on our heads. But how do you keep your roof in good shape? Easy—by paying attention. Inspecting your roof for signs of damage should be done annually, but most people don't. Most roofs made of asphalt are designed to last usefully for about a decade and a half, and roofs made of metal, slate, or tile last even longer, but you should still buck the trend of ignoring the roof and make sure your roof is properly maintained—after all, useful service is a bare minimum that you should endeavor to surpass. Take a stroll outside and look, really look, at your roof. Usually you take it for granted—it's there, it has shingles, it keeps the rain out. Now that you're taking a good look at it, does anything look out of place? Are there shingles that are loose, maybe damaged, or even shingles which have black streaks on them? Are trees touching your roof? If you answered yes to the above questions, then it's time for a check up. What's so bad about those things? After all, what's a little bit of tree hugging on the part of the roof if it's not leaking? There's more to it than appearances. Trees can cause a variety of problems. Branches can scratch and gouge roofing material (in other words—leaks). Trees falling due to wind can puncture shingles and cause other damage to your roofing. Leaves clog gutter systems and back up water into attics and other spaces inside your house. What about those black streaks? Streaks on your roof are evidence of mold—and as you probably know, mold is hazardous to your health if inhaled. Mold grows easily during humid periods, especially on roofs that face north. If you don't deal with them as soon as possible, they will eat away at the roofing material, and eventually eat through it, causing leaks. You can kill two birds with one stone by trimming back nearby trees, thus diminishing the possibility of damage due to falling branches and leaves while also removing from the mold the damp, shady climate in which it thrives. For permanent prevention, however, you should install zinc strips along the ridges of the roof. This works on a new roof, but if your roof already has a mold problem, clean the mold off first and then install the strips—and use a special roofing cleaner, not bleach, which is corrosive. In addition, missing and torn off shingles make a home susceptible to rot and water damage. To prevent this, replace any loose or missing shingles with new shingles, making sure they are properly installed. If you maintain your roof it will more than simply last 15 years—it will be good to you.