Chimneys are a common source of water infiltration into a roofing system. When a metal roof is installed, it makes sense to address the chimney flashing very carefully.
At Classic Products, we have found the best solution to chimney flashing is to install new, lineal flashings of matching metal around the entire chimney.
It is usually best to leave the existing flashing in place. If it is of dissimilar metal, though, use underlayment to isolate it from contact with the metal and from any contact with moisture or condensation. Whenever possible, make a sawcut at least 1/2” deep into the chimney, parallel on all sides to the roof deck, at least 6” above the roof deck and at least 1” above the current flashing. The new flashing will be bent and inserted into this sawcut and sealed. When a sawcut cannot be made, the flashing will be held to the chimney with a terminator bar or, with a rough stone chimney, the flashing should be brought into the chimney, stenciled to meet the configuration of the stone, cut for a tight fit, and sealed. (A wood collar may be installed around the chimney underneath of the flashing to provide support for the flashing.) On chimneys of 18” or greater width, as well as on chimneys that sit at the bottom of a long roof rafter, a cricket should be made and installed above the chimney. On very wide chimneys, a half-valley style of flashing can be installed down the sides of the chimney so that the water coming off of the cricket has a “chute” around the sides of the chimney. When troubleshooting leaks around a chimney, do not rule out that the water could be coming through the chimney itself. If possible, try “bagging” the chimney masonry with plastic, leaving the flashing exposed, and see if that stops the leak to prove that it is a masonry problem and not a flashing problem.