Valley areas are installed using the "Florida Cut" specification.  Other roofing companies use the "W" valley metal system which saves time and material.  We have found this detail to fail prematurely due to the exposure of the valley metal, allowing water to seep in where the metal meets the shingles.  Along with water seepage the "W" detail allows the valley metal to get extremely hot which makes it expand and contract causing a normal #30lb underlayment to tear and leak.  For our detail we fasten the specially designed valley metal to the deck, then seal both sides of the valley metal using a modified adhesive.  The second step is to layer one full shingle up and through the valley to the adjacent roof plane, then we cove the valley yet again with a second layer of shingles.  This second layer of shingles is trimmed neatly and secured completely using modified bitament adhesive.  Water is prevented from even entering the valley, leaving your home protected from leaks.