Chimney guides

Chimney Flashing on a Slate Roof

Plenty of Massachusetts housing has a slate roof, and slate changes chimney flashing from a routine job into a skilled one.

Step flashing woven into gray slate courses beside a brick chimney

Short answer

Correct flashing is two layers: step flashing woven in with the roof courses as they climb the chimney, and counter-flashing let into a groove cut in the mortar joint and folded down over it. On slate, each slate has to be carefully lifted and relaid rather than nailed through, and a cracked slate becomes the next leak. Copper is the usual metal because it outlasts the roof.

Why slate is different

Slate is brittle, it is fixed with nails through holes near its head, and it relies on overlap rather than adhesion to shed water. All three facts change the job.

You cannot walk on it

Not without breaking slates. Reaching a chimney on a slate roof means staging or roof ladders and hook ladders, and that is a real cost line rather than padding. A contractor who intends to walk the roof will leave you with broken slates as well as a flashing repair.

Nailing through a slate splits it

Step flashing has to be interleaved with the courses, which means slates come off and go back. Face-nailing through slate to hold flashing is the mark of someone who does not work slate, and every nail hole is a future leak.

Matching slate is hard

If slates do break, replacements have to match in thickness, size and color, and old New England slate is not a stock item. Salvaged slate is the usual answer and it takes sourcing.

What correct detailing looks like

If you can see what is there from the ground with binoculars or a photograph, you can judge a lot of it.

Step flashing, not a continuous apron

Individual L-shaped pieces, one per course, stepping up the side of the chimney. A single long strip of metal up the side is wrong on any pitched roof and badly wrong on slate.

Counter-flashing cut into the mortar

A groove — a reglet — cut into the mortar joint, the metal turned into it and the joint pointed. Metal stuck to the surface of the masonry with sealant fails when the sealant does, which is a few years.

No visible sealant doing structural work

A little at terminations is normal. A bead of mastic as the waterproofing layer is a patch, and on slate it is a patch that will cost more to put right than doing it properly would have.

Coordinating with roof work

The best moment to redo chimney flashing is while the roof is open, and on slate that moment may not come again for fifty years.

If slate work is planned, raise the chimney detail before the roofing contract is signed, and establish in writing who is responsible for it. Split responsibility is how it ends up done at speed by whoever gets there last.

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Frequently asked questions

How do I know my flashing is the problem rather than the crown?

Timing. Flashing leaks appear with wind-driven rain and often stop in ordinary showers. A cracked crown leaks whenever it rains. A hose test working upward in sections settles it.

Copper or aluminum?

Copper on a slate roof, essentially always. It will outlast the roof covering, it patinas rather than corroding, and it is what the detail was designed around.

Can flashing be repaired without disturbing slates?

Rarely, and a repair that avoids disturbing them is usually a sealant patch. If someone offers that as a permanent fix on slate, ask what they intend to do about the reglet.