Age is the starting point. Most asphalt shingle roofs are designed to last 25–30 years under normal conditions, though Montana’s climate — the freeze-thaw cycles, wind, and heavy snow loads — can shorten that window. If your roof is approaching or past that range, it’s worth a professional assessment even if nothing looks obviously wrong from the ground.
Beyond age, watch for a pattern of repairs. One repair is normal. Repairs every season is a sign the roof is telling you something. At a certain point the math shifts — you’re spending repair money on a roof that’s on its way out anyway.
Other factors worth considering: if you’re planning to sell your home, a roof that’s near end of life will come up in inspection and affect your negotiating position. Getting ahead of it on your timeline is almost always better than reacting to someone else’s.
When in doubt, have it looked at. A good inspection gives you a clear picture of where you stand and what, if anything, needs to happen — no pressure, just information.