Flashing a chimney on a slate roof involves creating a waterproof barrier where the chimney meets the roof to prevent leaks. Two common types of flashing used are counter flashing and step flashing. Step flashing consists of small pieces of metal bent at a 90-degree angle and placed along the chimney’s side where it meets each row of slate shingles. The step flashing layers under each slate but overlaps the one below it, directing water down and away. Counter flashing is installed over the chimney itself, covering the upper edge of the step flashing and cementing the flashing into the chimney mortar joints, creating a seal. The goal is to prevent water from seeping behind the flashing and causing damage. Because slate shingles are brittle and unique in size, careful handling and precise fitting are critical to avoid cracking. Simple diagnostics you can perform include checking for missing or rusted flashing, looking for cracked or displaced slates near the chimney, and observing any water stains inside the attic or on walls next to the chimney.
If you notice any visible damage or suspect leaks around your chimney flashing, it's often best to call a professional roofer experienced with slate roofing. Slate requires special tools and skill to handle properly, and improper flashing installation can void roof warranties or cause expensive water damage. A safe DIY check is inspecting flashing edges with a flashlight, gently tapping nearby slates for looseness, or looking for gaps where flashing meets the chimney. When you consult a contractor, ask questions like, “How will you ensure the new flashing integrates properly with the existing slate?” and “Do you provide warranty coverage on both the flashing and slate work?” A reliable roofer should also explain how they’ll cut and fit the step flashing pieces to match the slate’s thickness, and how they plan to seal and secure the counter flashing without damaging the chimney masonry.