How to tell a roof is past repairing

Daylight or damp in the attic

The attic tells the truth earlier than the ceiling does. Staining on the underside of the deck predates any stain you will see downstairs by a long way.

Shingles that curl, cup or go brittle

A roof near the end goes hard. Shingles that were pliable enough to seal down in the sun no longer bond, and the first serious wind takes them.

Sagging along a line

Straight lines going wavy means something underneath has softened. That is no longer a roofing question with a cheap answer.

Repeated repairs in different places

One leak is an event. Three leaks in three different areas inside two years is a pattern, and the pattern means the covering has reached the end of its service life rather than suffered a specific failure.

The age on the paperwork

If the roof is over twenty years old and made of asphalt, an inspection is worth doing whether or not it is leaking. The failure modes above are much cheaper to catch before the water arrives than after.

Granules in the gutter

The granules are the sunscreen. Losing them in volume means the asphalt beneath is being cooked directly, and from that point the roof ages in years rather than decades.

In short

If two or more of these apply, get somebody to look at it and give you a remaining-life number in years. That single number turns the decision from a worry into arithmetic.