How long a roof actually lasts
The number on the wrapper is a laboratory figure. What a roof actually delivers depends on how it was installed and what the attic underneath it is doing.
Rated life versus real life
Architectural asphalt is commonly sold as a twenty-five to thirty-year product and commonly delivers rather less in practice. Three-tab is shorter. Metal, slate and tile last far longer and cost accordingly. The rating assumes adequate ventilation and correct installation, and those two variables move the outcome more than the product grade does.
Ventilation is the quiet variable
An attic that runs hot cooks the shingles from below, and the underside of the deck never gets a chance to dry. Intake at the soffits and exhaust at the ridge, sized to the attic, keeps the deck close to outdoor temperature. Roofs on poorly ventilated attics routinely fail years early, and the failure looks like premature ageing rather than like anything dramatic.
Which slope fails first
The south and west faces take the most ultraviolet exposure and age fastest. A north slope under mature trees ages more slowly but grows moss and algae that hold moisture against the shingle. In the older suburbs across this region, where the canopy is seventy years old, that difference between slopes on the same house can be substantial.
Next step: Roof inspection