The Importance of Proper Flashing on Slate Roofs:
What Every Homeowner Should Know
When it comes to slate roofing, many people focus on the beauty and longevity of the stone itself—but flashing is just as important to the long-term performance of the roof. Flashing is the protective metal installed in areas prone to water intrusion, such as around chimneys, valleys, vent pipes, and walls. Without correctly installed flashing, even the highest-quality slate roof can fail.
Here's what you need to know about proper flashing practices, common mistakes, and why choosing the right material matters.
Why Flashing Matters
Flashing is your roof's first line of defense in areas where water naturally concentrates or penetrates:
Chimneys
Pipe penetrations
Valleys where roof sections meet
Dormers and sidewalls
Skylights
Transitions to lower roof slopes
In these areas, even the most perfectly laid slate won't stop water on its own. Without durable and properly installed flashing, water can seep under slates, damage the roof deck, and lead to interior leaks, mold, and costly structural repairs.
Recommended Flashing Material: 16 oz. Copper
For slate roofs, 16-ounce copper is widely regarded as the standard for flashing. Here’s why:
Durability: Copper outlasts most other flashing materials like aluminum or galvanized steel.
Compatibility: Copper is compatible with slate and won't cause staining or corrosion.
Malleability: It can be shaped precisely for tight fits around chimneys, valleys, and curved profiles.
Aging Gracefully: It develops a natural patina over time, enhancing the historic character of the roof.
Even though copper flashing won’t last as long as your slate, it still offers decades of performance when installed correctly. Expect 40–60 years of service from copper flashing in most North Carolina climates—far longer than alternatives.
What NOT to Do: No Caulk or Tar
Too often, inexperienced roofers or handymen rely on caulk, roof tar, or sealants to “repair” flashing. While this may temporarily stop a leak, it’s a major red flag:
Short lifespan: These products break down within a few years due to UV exposure and weather cycles.
Water traps: Improper sealing can hold water rather than shed it, leading to faster deterioration.
Aesthetic issues: Tar and caulk look sloppy and out of place on a high-end slate roof.
The correct method is always mechanical flashing with copper, properly integrated into the roofing and masonry—not glued on.
Critical Areas That Require Proper Flashing
🔸 Chimneys
Step flashing and counter flashing are essential at the chimney base.
Counter flashing should be tuck-pointed into the masonry, not glued with caulk.
Flashing should wrap all four sides, especially the back where pooling often occurs.
🔸 Valleys
These areas handle the highest water volume.
A pre-formed open copper valley is best.
Closed or woven valleys can trap water under slate and lead to rot.
🔸 Pipes and Roof Penetrations
Vent pipes and plumbing stacks must be surrounded by copper flashing collars.
Rubber “boot” flashings used on asphalt roofs degrade quickly and should be avoided.
🔸 Sidewalls and Dormers
Use step flashing up the wall where the roof meets vertical surfaces.
Counter flashing should be let into brick or siding and overlap the step flashing.
🔸 Skylights
Pre-manufactured skylight flashing kits are rarely designed for slate roofs.
Custom copper flashing should be fabricated for a secure, lasting fit.
Signs of Failing Flashing
Staining on interior walls or ceilings near chimneys or valleys
Visible tar or sealants around flashing edges
Rusted, corroded, or lifting flashing material
Water runoff behind siding or chimney mortar joints
Moss or algae streaks forming where water is not shedding properly
Final Thoughts
Slate roofs are known for lasting 100+ years, but their performance is only as good as the details—and flashing is one of the most important. Using high-quality materials like 16 oz copper, and avoiding shortcuts like caulk and tar, ensures your investment is protected for decades to come.
At Carolina Slate, we specialize in the restoration and repair of slate, clay tile, and copper roofs across North Carolina. Whether you’re maintaining a historic structure or preserving a custom roof, we can evaluate your flashing and recommend long-term solutions tailored to your home.
Need a professional evaluation of your slate roof’s flashing?
📍 Serving North Carolina | www.carolinaslate.com
🔧 Expert craftsmanship. No shortcuts. Historic care.