There are 2 different methods to roof valleys: open vs. closed.  What are the differences?  Are there advantages to either application?  Open valleys refer to trimming back the shingles along the valley metal, hence exposing the flashing and allow water to run-off.  In closed valley applications the field shingles are installed across the valley.  Closed valley applications can pose some challenges in the Alberta climate.  If water does get behind the shingles in the valley it can freeze and cause the shingles to crack (freeze-thaw cycle).  Water that collects in these valleys can lead to the shingles deteriorating at a faster pace, as they are in the photo below.