If you find yourself asking this question, it is time to thoroughly investigate the shape of your roof. Here are some warning signs that it is time for a new roof:
When shingles curl they are no longer able to keep water from seeping through the roof, causing serious damage. Do not attempt to walk on your roof if the shingles are curled; this will cause them to break off. There is no way to reverse this damage, so the best thing to do is replace them. Checking on the shingles regularly and replacing them as necessary will help you avoid having to get an entirely new roof.
Missing shingles create openings in the roof through which water can enter. They should be replaced as soon as possible, whereas loose shingles can be sealed with flashing cement. If you plan to handle the cement yourself, read the instructions thoroughly, as some contain hazardous materials.
All roofing materials will deteriorate over time. Deteriorating shingles are easy to spot if you look for splitting, cracking, chipping or peeling and replace them as necessary.
Shingles blister when there is too much moisture build up inside and create bubbles on the surface. They may not necessarily be the death sentence of your roof, so long as the bubbles do not crack open. Keep an eye on shingles that appear to be blistering.
Flashing can be damaged by wind, snow, hail, rain or fallen branches. It is important to repair them properly, so if you are unsure of how to fix any damaged flashing, call a professional.
Rotting is most common in wooden shingles, but can also occur in asphalt roofs as well. If several shingles begin to rot, the rest may begin soon. Rather than individually replacing rotting shingles, it is a better idea to just replace them all.
Algae will discolor shingles but is not harmful to your roof. Spraying your roof with a strong hose should get rid of most of the algae. You may want to replace thick algae buildup with algae-resistant shingles.
Courtesy of Water Damage Defense