Valleys in winter
A valley is the busiest square metre of any roof: it takes the water of two slopes and moves it through a single channel. In summer that is simply a lot of water. In a Minnesota winter it is meltwater running under a snowpack, over whatever autumn left behind, and freezing every night for four months.
Why winter is hard on a valley
Snow bridges a valley rather than filling it evenly, so meltwater runs through a covered channel where it cannot evaporate and the sun cannot reach it. Add leaves or needles left from October and the channel slows further; slowed water freezes, and ice in a valley lifts the shingles or metal on either side of it. Each cycle opens the joint a little more. By the time the trouble is visible from the ground, the valley has usually been working badly for several seasons.
This is also where the two most common construction choices show their difference. An open valley with metal in it sheds ice and debris far better than a woven or closed-cut valley made of shingle alone — the smooth surface gives water somewhere to go even when the sides are frozen. In this climate that is not a style preference; it is a durability decision.
What to ask for
On a replacement, three questions cover it. Is ice barrier membrane running the full length of every valley, not just at the eaves? Is the valley being built open with metal, and if not, why? And what is the plan where the valley meets the eave — the point where the fastest water on the roof arrives at the coldest edge, and the single most common place for winter damage to start. Between replacements, the maintenance is unglamorous and effective: clear the valleys and gutters after the leaves come down, before the first snow that stays.