Built-up roofing (BUR) systems are made of multiple layers of asphalt and felt. Valley leaks in these roofs often happen because valleys—the areas where two roof slopes meet—are places where water flow intensifies. If the flashing, seals, or the roof layers in the valley become damaged, cracked, or poorly installed, water can seep through and cause leaks. Common causes include wear from foot traffic, ice dams pulling at edges in colder climates, or deterioration over time. To diagnose a valley leak, start by visually inspecting from the attic during or right after rain; look for dark spots or mold that indicate moisture intrusion. On the roof, check for cracks, blisters, or gaps in the tar or gravel surface in and around valleys. You can also perform a gentle water test with a hose to see if water penetrates these areas.
For safe DIY checks, focus on clearing debris from valleys and making sure no loose gravel or tar spots are missing or cracked, but avoid climbing on the roof if it’s steep or slippery. Touching up small cracks with roofing tar can be a temporary fix, but for persistent leaks or if you see extensive damage, it’s best to call a professional roofer. When you contact a contractor, ask if they have experience specifically with BUR valleys and what method they recommend for repairing or replacing the flashing without voiding any existing roof warranty. Also inquire about how they will ensure proper water flow and long-term durability in the valley area to help prevent future leaks. This will give you a clear picture of their understanding and approach.