This decision has real money attached, which is why it deserves more than the opinion of whoever is currently on your roof.
The straightforward repair cases
A handful of shingles lost in a windstorm on a roof installed eight years ago. A single flashing detail that has failed. Damage from a fallen branch confined to one area.
These are repairs. The roof has life remaining, the problem is localised, and replacing the whole thing would be replacing a great deal of sound material.
The straightforward replacement cases
Widespread granule loss, visible as bald patches and as granules accumulating in gutters. Shingles that are curling, cupping or cracking across large areas. Leaks appearing in multiple unrelated locations. A roof at or beyond 20 to 25 years in Western Washington.
Multiple leaks in different places is the clearest signal. One leak is an event; several simultaneously means the roof system as a whole is failing rather than a specific detail.
The Washington moss consideration
Heavy moss coverage complicates the assessment in Western Washington, and it usually means the roof is functionally older than its age suggests.
Moss lifts shingle edges and holds moisture against the deck, so a fifteen-year-old roof under heavy canopy with substantial growth may be in worse condition than a twenty-year-old roof on an exposed lot.
Important: do not pressure wash an asphalt roof to assess or clean it. It strips the protective granules and takes years off the roof’s life. If someone proposes this, that tells you something about them.
The middle ground
Most real cases sit here — a roof in its teens with a leak or two and some visible wear. Useful questions:
How much longer do you need it to last? If you are selling within two years, a sound repair may be entirely appropriate. If you are staying fifteen, a repair on a roof with five years left is money spent twice.
Is the deck sound? If leaks have been running, the decking may be compromised, which pushes toward replacement since the repair requires opening things up anyway.
How many layers are up there? A roof already carrying two layers cannot take another, so a future replacement will be a full tear-off regardless. That affects the economics of repairing now.
Get a second opinion
Particularly where the recommendation is replacement. Roofers are not the only people who can assess a roof, and a paid inspection from someone not bidding on the work removes the obvious conflict.
Be especially cautious with unsolicited assessments following a storm. A contractor who knocked on your door and then found significant damage is a pattern worth recognising rather than dismissing.
Ask what they actually looked at
A recommendation based on walking the roof, checking the attic for daylight and staining, and examining flashing details is worth considerably more than one based on a look from the driveway or photographs from a drone. Ask which you are getting.