How do I know when I need to replace my roof?

There are several ways:

Age – If your roof is around 20 years old, it is probably nearing the end of its useful life.  There is some variation in life due to shingle color, sun exposure and ventilation under the roof.

Appearance – If you have shingles that have broken off, or look buckled or warped, or have lost some of the granule surfacing, it is probably time to replace it.

Leaks – Not the best way to find out.  By the time you see evidence of water leaking inside the house, it has probably been leaking for quite a while prior to that.  This leaking can cause expensive structural damage to the rafters and the boards on top of them, not to mention the damage to your ceilings and walls.  It is usually much less expensive to replace a roof before you see leaks.

Can the new roof be installed on top of my old roof?

It depends.  If you have only one layer of roofing in decent condition, you can probably install a new roof on top of it.  If you have two or more roofs, if the existing roof is badly buckled, if there is a need to inspect or replace decking, or if there is an accumulation of moss or algae, the old roof should be removed.