True or False: A fully closed valley features shingles interwoven across the valley extending a minimum of 12 inches onto the adjacent plane.

A fully closed valley is a roofing technique where shingles are laid in an interwoven pattern across the valley area between two roof planes. This method is designed to provide both a watertight seal and a visually appealing appearance. The requirement of extending the shingles at least 12 inches onto the adjacent roof plane ensures sufficient overlap, which is crucial for preventing water intrusion. A proper installation provides a robust barrier against leaks, especially at a critical junction like a valley where water runoff is concentrated. Therefore, the statement is confirmed as true, given that the guidelines specify that this interweaving should occur with the minimum extension mentioned to ensure the integrity of the roofing system.