Most roof leakage is caused by failing or cracked asphalt shingles. Broken, damaged and missing shingles will in most cases cause water infiltration into the attic space. Whenever there is exposed roof decking you are almost certain to have a leaking roof. Another major source of leakage is at the flashing details. Over time the home settles at different rates and this can cause gaps to develop around the flashing details. Roofing cement is often used as a sealer between flashings and penetrations. Over the various freeze and thaw cycles the roofing cement will get brittle and crack. The cracking will ultimately lead to roofing leaks. The area around the chimney is particularly vulnerable to this type of flashing leakage. Well done flashing details do not rely on roofing cement in order to keep water out.
One type of leak has all been eliminated with the use of modern asphalt shingles. Wind leaks that are caused by shingle uplift in wind and heavy rains has been eliminated by the cement strip that holds the shingles on top of one another. This cement strip make for a tight bond between the layers of roofing shingles. There are times where material defects cause the cement strip not to adhere. If the individual asphalt roof shingles do not adhere leakage will result. Most if not all roofing material manufacturers warranty their products against such defects provided the material has been installed using their recommendations.
There are some roofing materials that are more suitable for particular roofing applications. For instance asphalt shingles are suited for steep pitched roofs because they act as a shedding system. Slates and wood shingles are also suited for steep pitched roof systems. Rolled and rubber type roofing materials are suited for lower sloped roofing applications. These materials are act more as a barrier then asphalt shingles and slates.
One of the other major issues that greatly contributes to roof leaks is unscrupulous roofing contactors installing asphalt shingles with a grater then recommended exposure. Sometimes this is done to save on materials costs other times it is done by not understanding the proper installation requirements for the materials. Inadequate over lap of the materials can let wind driven rain to get under the shingles and cause water infiltration and damage.
When it is time to re-roof due your due diligence and make sure to hire a roofing contractor that has a good reputation. Do your own research as to what roofing materials would be suitable for your individual roofing project. Make sure the roofing contractor follows the manufactures installation requirements to the letter so that if defects or problems occur the material will still be covered during the warranty period.