Flashing where a chimney passes through the roof typically consists of:

Chimney flashing has to solve a hard problem: two materials that move differently must stay watertight where they meet. The solution is two overlapping layers rather than one seal. Base flashing is fastened to the roof deck and turns up the masonry; counter flashing is let into a mortar joint, folded down, and laps over the base flashing's vertical leg. Water running down the masonry is intercepted by the counter flashing and shed onto the base flashing, which carries it onto the roof surface. Critically the two pieces are not fastened to each other, so the chimney and the roof can move independently without tearing the assembly apart. That is why a single bead of sealant fails: it bonds two moving surfaces rigidly and cracks within a season or two, and roofing cement smeared over the joint does the same thing more thickly.

Correct chimney flashing uses base flashing turned up the masonry and counter flashing let into a mortar joint and lapped over it. The two-part arrangement sheds water while allowing the chimney and roof to move independently.

Chimney-to-roof transitions use base flashing secured to the roof and counter flashing set into a reglet or mortar joint above it. Step flashing is woven with shingle courses along the sides, and a cricket is used on the upslope side of wide chimneys.

Two Pieces, Not One Seal. Base goes up from the roof, counter comes down from the brick, and they overlap without touching.

Check the upslope side for a cricket on wide chimneys. Its absence lets water and debris pond behind the masonry and is a commonly tested defect.

• •Accepting roofing cement as a flashing system
• •Expecting the two flashing pieces to be fastened together
• •Overlooking a missing cricket on the upslope side