Chimney Flashing in the Seattle Area, WA.

Chimney flashing is a two-part system and understanding that explains most of what goes wrong with it. Step flashing is a series of L-shaped pieces woven into the shingle courses as the roof is laid, so each one laps the one below. Counter-flashing is a separate piece let into a cut groove in the mortar joint, bent down over the step flashing. The roof can move, the masonry can move, and the joint stays closed because the two halves overlap rather than being bonded to each other.

What we find instead, over and over, is a single piece of metal run up the brick and held there with a heavy bead of sealant or roof cement. That works for a few seasons. Then the sealant shrinks, cracks and lets go – it is a consumable, not a flashing – and the chimney leaks again. The same goes for flashing nailed face-on into the brick, which puts fasteners through the water barrier, and for flashing reused from the old roof when the roof was replaced.

There is also the matter of what is above the chimney. On a wide stack set low on a slope, snow and water run down the roof and pile against the uphill face. That is what a cricket is for: a small framed saddle behind the chimney that splits the flow around it. A chimney wide enough to need one and not have one will keep failing at the uphill flashing whatever metal you put there.