Flashing for a chimney placed in the middle of a roof valley.

 Often a chimney is placed directly in the middle of a roof valley. First off, who in their right mind would design such a likely problematic area. Placing a large penetrating obstruction right where the two opposing roof planes meet and conduct the flow of water from each side of the roof into a center valley. Well, sometimes because of the house design and construction there is no other way to do it. And it's not so much of a problem when the roofing installer is experienced and capable.

If a chimney is placed in the valley path it should have a chimney cricket to divert the water from the roof away from and around the chimney. If there were no cricket, there would essentially be a pocket where the valley flashing meets the chimney. Under heavy rains water would pool behind the chimney possibly working it's way up and under the chimney counter flashing or even backing up under the roofing shingles. Another possibility is dirt and debris collecting behind the chimney exacerbating the damming problem. If the chimney flashing is a metal such as galvanized steel, the dirt and debris will hold moisture for extended periods and corrode or rust out the steel, shortening the flashings lifespan.

Chimney cricket flashings should be fabricated from a monolithic material, such as that used for flat roofing,  that can hold ponding water without leaking. Copper with soldered seams, shown in the picture below, is one of the best materials for this application. The sheet metal is made with seams that are folded, hammered tight and then soldered for a permanent leak free flashing. Copper flashing will also last approximately 60 to 80 years before wearing through and needing replacement. The cricket flashing should be brought higher than normal up the back of the chimney and under the counter flashing to protect against the extra water flowing into the area.

Single ply membranes, EPDM or rubber roofing, and modified bitumens are also good, if not as long lived as copper, materials for cricket flashings. Wrapping the cricket area in ice and water shield and then nailing roof shingles and valley flashing metal over the cricket is a poor installation method and will likely not last long before leaking. Cleveland area chimney flashing installation and repair by Dennis M. Crookshanks Const. Inc.