Closed-cut valleys are very popular on roofs due to the fact that they have a very clean look and do not have the typical metal valley flashing exposed. They do work great but they need to be done correctly. Several homes I have inspected have had this valley cut the wrong way. How is it supposed to be cut? Well, the waterproofing of this method requires the shingles to be lapped the correct way. The roof plane that is larger/steeper should lay on top of the smaller/more gradual roof plane. The larger/steeper plane shingle is the one that gets the cut. In the pic above, the right plane is the larger/steeper side. This one is laid on top and cut to ensure that the bottom shingle coverage extends up under it to protect when water rushes down the roof. It if were the opposite way water could rush down the roof and up under the cut getting to the end of the shingles under the lap which could allow for leakage. It is best to cut the shingles approximately 2″ up the valley side. For a real clean look, one course of shingles can be turned perpendicular to the valley and run up that edge under the surface shingles. If these are not installed the cut can be irregular and the cut line can wander a bit depending on how careful the installers are. For extra protection, a bead of roofing cement can be applied under the cut edge to seal it down to the other shingles. This type of valley can require more maintenance by the homeowner in the event that they live in an area with more tree coverage. This valley does not allow leaves, seeds, and branches in the valley to slide as easily down and off the roof as metal flashing so occasionally they need to be cleaned out. If they are not cleaned the debris can trap moisture and shorten the life of the shingle.
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