A chimney is one of the most vulnerable parts of a structure because it is exposed to weathering on all sides. If the proper steps are
not taken, both on the drawing board and in the field, water can enter the chimney and cause efflorescence and eventually freeze thaw damage. However, if industry guidelines are followed, chimneys can be designed to
be watertight for decades. As with most forms of masonry construction, steps should be taken to prevent moisture damage.
Roof / Chimney intersection
The intersection between the chimney and the roof is a critical spot where several trades have to come together to provide a watertight
joint. Base flashing should be installed on top of the roof sheathing, extending horizontally under roof shingles for a minimum of 4 inches. As shown in figure 1, flashing should be turned up a minimum of four
inches before it is inserted into a raked mortar joint and caulked. As shown with the dotted lines, the stepped flashing segments should be lapped at least three inches and sealed. Prefabricated corners are also
suggested to properly protect the intersection.