Roof flashing is a critical component in protecting a building from water intrusion, especially at vulnerable areas like valleys and chimneys. These junctions require precise techniques to ensure long-term durability and weather resistance.

For valley flashing, the most common method is using continuous metal flashing, typically made of galvanized steel, copper, or aluminum. The key technique is to install the flashing over the roof underlayment and beneath the shingles on both sides. The metal strip should be at least 36 inches wide, with a center crease to direct water flow. Shingles are then cut to overlap the flashing by at least 6 inches on each slope. A proper sealant, such as a high-quality polyurethane or butyl tape, should be applied along the edges to prevent capillary action. For steep valleys, a W-shaped flashing pattern can improve water diversion.

Chimney flashing involves two main parts: base flashing and step flashing. Base flashing is installed at the lowest point of the chimney where it meets the roof. It is bent to extend 4 inches up the chimney face and 6 inches onto the roof deck. Step flashing consists of individual pieces that are woven into each row of shingles as they are installed. Each piece must be embedded in a bead of roofing cement and secured with corrosion-resistant nails. Counter flashing, or cap flashing, is then inserted into a reglet (a groove cut into the mortar joint) to cover the top of the step flashing. This prevents water from running behind the system.

An additional technique for chimneys is using a cricket or saddle on the uphill side. This is a small peaked structure that diverts water away from the chimney base, reducing the stress on the flashing. The cricket is typically framed with wood and covered with metal or shingles, then integrated with the step flashing.

Sealants play a vital role in both valley and chimney flashing. Use elastomeric sealants that can expand and contract with temperature changes. Avoid using foam or tape alone, as they may deteriorate over time. Finally, always inspect the flashing annually for corrosion, cracks, or loose seams, especially after severe weather.

In summary, successful roof flashing relies on careful material selection, proper overlap lengths, and meticulous sealing. Whether working with valleys or chimneys, these techniques will help create a watertight envelope that extends the life of the roof.