Ways to Flash a Chimney

Jeff Littleton
Last Updated February 23, 2024
9:58 am

Have you ever noticed frustrating leaks around your chimney and wondered what’s causing them? You’ve checked your tiles or shingles, cleaned your gutters and swept all the debris out of your chimney. So what’s causing the leak? Well, it could be your chimney flashing. Chimney flashing is basically weather stripping for your chimney. This thin metal or caulking layer is installed where your roof meets the chimney to keep water out. While it sounds straightforward, the devil’s in the details – and the craftsmanship. There are various materials and techniques to install chimney flashing successfully. We will identify the common materials and proper installation techniques to help you identify when your chimney flashing is the cause of your dripping ceiling.

What are the three flashing methods used on chimneys?

The choice of flashing style often depends on the technician’s preference and the specific type of chimney being worked on. Common styles include continuous flashing, which follows a single mortar joint (for a clean appearance), stepped flashing (resembling stairs as it descends along the chimney), and diagonal flashing, which slides through the mortar and brick for a distinctive look.

Despite the different styles, the effectiveness of chimney flashing primarily hinges on proper installation rather than a particular style. The three most common flashing installation methods are base flashing, step flashing, and counter flashing.

Base flashing

This is the initial waterproof membrane installed before your tiles or shingles. Base flashing sits at the joint between a vertical surface (the chimney) and the roofing surface, extending up the side of the wall.

Step flashing

Installed after the base flashing, this L-shaped piece of metal lies beneath your roofing tiles or shingles along the brick masonry of your chimney. Step flashing guides water away from the vulnerable areas of your roof – like the seam between your chimney and the rest of your roof and attic. With a shingle roof, the flashing is woven into the shingles or roofing materials to protect the seam. In certain construction scenarios, a roof with step flashing alone is sufficient, and vinyl siding can be used as counter-flashing (as long as the step flashing is positioned below it). However, counter-flashing is mandatory for proper waterproofing and structural integrity in other situations.

(Photo from Chimneys Plus Gutters)

Counter flashing

Also known as cap flashing, counter flashing is also L-shaped. The difference between step and counter flashing is that counter flashing is designed to cover step flashing and embed it into a cut groove in the chimney brickwork.

There are three methods for installing counter flashing – Through-wall, Surface-mount, and Reglet.

Through-wall – attaches the flashing material to a wall cavity that is concealed by exterior siding or masonry. Generally limited to new constructions as this can be an expensive technique for older buildings.
Surface-mount – attaches directly to your roof’s exterior materials and is sealed with caulk. While the most economical installation, it is also the least likely to be watertight as caulk deteriorates.
Reglet – is a mix between through-wall and surface-mount where the flashing material is secured to a 90-degree cut in the exterior masonry and bent to run along the exterior wall. This type of flashing prevents water from getting between the top flashing material and the exterior wall.

What’s the difference between step flashing and counter flashing?

Both flashing methods ultimately direct water away from the joints. Step flashing involves overlapping layers between your tiles or shingles along the chimney’s masonry. Counter flashing is embedded into a cut groove in your masonry.

What is the most common flashing material?

There are many types of flashing material available with their own set of advantages and considerations. It’s important to consult with a roofing contractor to determine which material(s) work best for your project.

Common Flashing Materials:

Aluminum: Aluminum is popular for its versatility, durability, and ease of use. It can be powder-coated to match the aesthetic of the home. Aluminum step flashing needs to be paired with aluminum counter flashing.

Copper: Similar to aluminum, copper flashing is ideal for its durability, malleability, and weathering resistance. Copper is an expensive option often used in high-end projects to add a polished look.

Lead: Less popular than aluminum and copper, lead flashing is flexible and easy to install (especially in colder climates). It’s important to mention that lead flashing can be toxic if it’s open to weathering through improper installation.

Flash Seal: Flash seal and sealing cloth is a cost-effective (temporary) solution for deteriorating flashing. Easy to install, they are not as durable as metal-based flashing and should only be used when no other alternative is available.

Vinyl or PVC: PVC and vinyl flashing are budget-friendly options but lack the weathering capabilities of metal-based flashing. These are commonly used for counter flashing over step flashing incorporated into shingles.

Corrugated Metal: Corrugated metal flashing is used to seal metal roofing. It uses the same type of metal for roofing and flashing to prevent oxidation issues and create a proper seal.

7 Steps to Identify and Replace Chimney Flashing

You can sometimes repair a leaking chimney by sealing it with new caulk or roofing cement. Replacing is necessary when the flashing is warped, flared, or shows signs of wear, such as rust, corrosion, or bending. This includes loose flashing that moves if touched, gaps or separations between the flashing and the chimney or roof surface. Water can penetrate and cause damage to the underlying structure.

Here are seven steps roofing contractors follow when accessing and replacing your chimney flashing:

Assess the Condition: Begin by visually inspecting the chimney flashing for signs of deterioration, such as rust, corrosion, or bending. Look closely for any gaps or separations between the flashing and the chimney or roof surface. Test the integrity of the flashing by gently pressing on it to see if it feels loose or if there is any movement
Remove Old Flashing: Carefully remove the old flashing using a pry bar, taking care to not damage the surrounding shingles or roofing materials. Once the flashing is removed, thoroughly inspect the area for any remaining debris or old roofing cement that needs to be cleared away. Use a scraper or wire brush to clean the surface and ensure a smooth, flat base for the new flashing installation.
Measure Roof Slope: Measure the slope of the roof to ensure proper installation of the new flashing. The run of the roof is typically measured as the vertical rise of the roof per 12 inches of horizontal distance. For instance, a 4:12 roof slope indicates that the roof ascends 4 inches vertically for every 12 inches of horizontal run.
Cut New Flashing: Measure the dimensions of the chimney and roof. Use tin snips or a metal cutter to cut the base flashing and step flashing according to the measured dimensions. These need to fit snugly around the chimney and align properly with the roof.
Attach New Flashing: Position the base flashing against the chimney so it sits flush and covers the joint between the chimney and roof surface. Secure the base flashing in place with roofing nails then install the step flashing by layering each piece with the tiles or shingles. The step flashing needs to overlap and interweave to divert water away from the chimney and roof.
Install Cap Flashing: Make groove cuts in the chimney’s mortar joints using a grinder or chisel to create space for the cap flashing. Carefully drive the flange of the cap flashing into the grooves using a hammer and wood block to prevent damage to the flashing. Secure the cap flashing in place with masonry anchors in the mortar joints and drive nails through the flange.
Seal With Caulk: Apply a high-quality exterior caulk (one suitable for metal surfaces and your chosen flashing material) around the edges of the cap flashing to create a strong, waterproof seal. Smooth the caulk with a caulk tool or damp finger to ensure proper adhesion and create a neat finish.

(Photo from Northface Construction)

The Vast Inventory of All Points Tile and Slate

Addressing chimney flashing issues promptly can prevent further damage to your home. All Points Tile and Slate offers a wide range of roofing materials, including various types of flashing. Contact All Points Tile and Slate today to explore our inventory and ensure your chimney is properly protected against the elements.

References

Chimneys Plus Gutters

Iko

Northface Construction

Priddy Chimney Solutions

The information on our blog is for educational purposes only and should not replace the advice of a professional roofer. For all roofing questions and needs, please contact our experts at 407-366-2521.