A valley is a point where two slopes of a roof meet. These area are prone to leak if not roofed right. To make sure you have a valley on your roof that wont leak there are a few things you have to do. First Its always important to put an underlayment. Second you always got to make sure you nail 6″ out of the valley. Finally proper shinglingof the chimney by a professional with plenty of experience is a must.