Just in case....
If not Ice and Water, or Lead:
[tab=30]Cut valleys so they meet and lap, takes a little effort, but get them fairly tight.
[tab=30]Install Felt over join
[tab=30]Install a Triangle piece of metal (cut from 1 side of the valley top) over the felt, triangle point going over the Ridge
[tab=30]Instal Caps over the whole works.
Works like a charm, and you need not bring anything extra along.