Start With Age
Age sets the frame for everything else.
Under ten years, almost anything is a repair. Something failed early, usually an installation detail or a piece of flashing, and fixing it makes sense. Between ten and eighteen years you are in judgment territory, where the damage pattern decides. Past twenty years, the calculation flips. Even if the current problem is small, you are spending money on a roof that is going to need replacing within a few winters anyway.
In Connecticut, where each winter does real work on a roof, an old roof does not improve while you wait. If you do not know the age, granule loss and shingle flexibility give an experienced eye a decent estimate.
Look at the Damage Pattern
One problem in one place is a repair. Storm damage on a single slope, a cracked vent boot, a valley that opened up. These are isolated failures on an otherwise working roof.
Scattered problems across multiple slopes mean something systemic. When shingles are curling on the south face, granules are gone at the ridge and two different areas have leaked, the roof is not failing at one point. It is failing everywhere at slightly different speeds. That is the clearest signal you can get.
Ask yourself how many separate repairs have happened in the last three years. Three is usually the number where continuing to patch stops making financial sense.
Layers, Decking and What Is Underneath
This is the part you cannot see from the ground and it changes the math.
If there are already two layers of shingles up there, most Connecticut towns will not allow a third. That means the next roof is a full tear off regardless, so a repair today is purely buying time.
Decking condition matters just as much. Soft spots, sagging between rafters or plywood that has delaminated all mean the surface under the shingles is compromised. You cannot fix that with a repair. Any honest inspection tells you what is under the shingles, because that is where the real cost lives. A quote given from the driveway skips this entirely. Chimney flashing is another spot that hides more than the driveway view suggests.
Run the Numbers Over Five Years
Compare the two options across five years, not today. A repair is a few hundred to a couple thousand depending on the scope. A replacement is a much larger number once.
If the roof has three or four years left, the repair wins clearly. If it has one or two, you are likely paying for both within a short window, plus whatever the interior damage costs when the next leak lands on drywall instead of plywood.
There is also the sale question. A twenty year old patched roof shows up in every home inspection and becomes a negotiating point. A recent roof replacement removes that conversation entirely and travels with the house.