That answer depends on the kind of roof you have. Some roofs last about 20 years. Others last upwards of 50 years or more. Check with your roofer to determine how long your type of roof is expected to last.
You can check for a number of signs that indicate it’s time to replace your roof:
Your roof needs more and more repairs
Your roof is at the end of its expected lifespan
Your shingles are in overall bad condition
Your gutters serve to protect your roof from water damage. They also help to prevent foundational damages by redirecting rainwater away from the base of your house.
Every roof is different. In most cases, we can repair that particular section, but all cases are handled on a case by case basis.
The best visual cues for when a new roof is needed is old one repaired is when you notice shingles missing, bubbling, leaking and cracking.
A new roof can be installed atop of the existing roof it the existing roof is in good condition and has less than three (depending on code) layers.