Asphalt shingles often don’t bend tight enough to sit flat in the center of a roof valley, which leaves an unsupported hollow between the shingles and the roof sheathing. This is more likely to occur when using heavy laminated shingles or in cold weather when shingles are stiff. The risk with the hollow spot is that the shingles can split if a worker steps in the center of the valley. To reduce that risk, I rip an angled fillet from a 2x strip of wood and fasten it in the center of the valley. The fillet eases the sharp crease of the valley center so that the shingles have support.
Mike Guertin, East Greenwich, RI