Up on the roof, however, the addition became more problematic.
Most additions create at least two.
Because valleys collect water from both roof planes, they're prone to leaking. If you're having an addition put on, discuss how valleys will be treated with your roofer. In most cases, he'll recommend installing 28-ga. If your roofing is tile, wood or another rigid material, you're limited to the latter two options. The woven style provides the best weather shield, as the shingles overlap across the valley.