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How to Put Flashing on a Chimney? A Comprehensive Guide

The key to properly flashing a chimney involves creating a watertight seal where the chimney intersects with the roof, preventing water intrusion and costly damage. This process involves installing layered pieces of metal, strategically placed and secured, to divert water away from the chimney and down the roof.

Understanding the Importance of Chimney Flashing

Chimney flashing is a critical component of a well-maintained roof, often overlooked until problems arise. Without properly installed or maintained flashing, rainwater and snowmelt can seep into the roof structure, leading to leaks, wood rot, mold growth, and costly structural damage. Furthermore, improper flashing can compromise the integrity of the chimney itself, accelerating deterioration and requiring expensive repairs. Think of flashing as the roof’s final defense against the elements around a vulnerable point.

Common Problems Resulting from Poor Flashing

Ignoring the importance of flashing, or attempting a subpar installation, can lead to a myriad of issues:

Roof Leaks: The most obvious consequence is water entering your home.
Wood Rot: Water intrusion encourages fungal growth that destroys wooden roof components.
Mold and Mildew: A damp environment is perfect for mold and mildew proliferation, posing health risks.
Chimney Damage: Moisture can freeze and thaw, cracking and damaging the chimney masonry.
Increased Energy Bills: Moisture can reduce the effectiveness of insulation, driving up energy consumption.

Essential Materials and Tools for Chimney Flashing

Before you begin, gather the necessary materials and tools. Accuracy and using the correct materials are paramount for a successful, long-lasting flashing job.

Materials You’ll Need

Flashing Material: Options include aluminum, copper, and stainless steel. Copper is considered the premium choice due to its durability and aesthetic appeal, but aluminum and stainless steel are more affordable and still effective.
Step Flashing: These are small, L-shaped pieces of flashing installed in overlapping layers along the sides of the chimney.
Counter Flashing: This is embedded into the chimney mortar joints and overlaps the step flashing, directing water away from the joint.
Base Flashing (Apron Flashing): This is a single piece of flashing that covers the bottom of the chimney where it meets the roof.
Cap Flashing (Head Flashing): This is similar to the base flashing but is installed at the top of the chimney.
Mortar or Sealant: Used to secure counter flashing and seal gaps. Choose a sealant specifically designed for roofing and masonry.
Roofing Nails: For securing flashing to the roof deck.
Ice and Water Shield Membrane: Used as an underlayment to protect the roof deck from water intrusion.

Tools You’ll Need

Hammer: For nailing flashing in place.
Tin Snips: For cutting flashing to size.
Mortar Chisel: For removing old mortar when installing counter flashing.
Trowel: For applying mortar.
Measuring Tape: For accurate measurements.
Pencil: For marking cut lines.
Safety Glasses: To protect your eyes.
Gloves: To protect your hands.
Ladder: For accessing the roof safely.
Caulk Gun: For applying sealant.
Grinder (optional): May be required for creating clean cuts in brick for counter flashing installation.

Step-by-Step Guide to Chimney Flashing Installation

Flashing a chimney is a meticulous process requiring attention to detail. Prioritize safety and follow each step carefully for a successful outcome.

Step 1: Remove Old Flashing (If Applicable)

Carefully remove any existing flashing around the chimney. Use a flat bar or chisel to gently pry the old flashing loose. Inspect the underlying roof deck for any damage and repair it as needed. Rotted wood must be replaced before proceeding.

Step 2: Prepare the Roof Deck

Clean the area around the chimney, removing any debris, dirt, or old roofing material. Apply ice and water shield membrane around the chimney base, extending several inches up the chimney sides and onto the roof deck. This provides an extra layer of protection against water intrusion.

Step 3: Install Base Flashing (Apron Flashing)

Position the base flashing at the bottom of the chimney, ensuring it extends several inches onto the roof deck. Secure it to the roof deck with roofing nails, being careful not to puncture the chimney itself. The base flashing should cover the entire bottom edge of the chimney where it meets the roof.

Step 4: Install Step Flashing

Begin installing step flashing along the sides of the chimney. Each piece of step flashing should be bent into an L-shape, with one side extending up the chimney side and the other lying flat on the roof deck. Overlap each piece of step flashing by a few inches, securing them to the roof deck with roofing nails.

Step 5: Install Counter Flashing

This is the most crucial and challenging step.

Mortar Removal: Using a mortar chisel and hammer (or a grinder for more precise cuts), carefully remove about 1 inch of mortar from the mortar joints along the sides of the chimney, above the step flashing. Ensure the groove is deep enough to accommodate the counter flashing.
Counter Flashing Insertion: Bend the counter flashing to match the shape of the step flashing. Insert the counter flashing into the mortar joints, overlapping the step flashing by at least 2 inches.
Mortar Application: Fill the mortar joints with fresh mortar, ensuring a tight and watertight seal around the counter flashing. Allow the mortar to cure completely according to the manufacturer’s instructions.

Step 6: Install Cap Flashing (Head Flashing)

Install the cap flashing at the top of the chimney, similar to the base flashing. Ensure it overlaps the step flashing and extends several inches onto the roof deck. Secure it with roofing nails.

Step 7: Seal and Inspect

Apply a bead of roofing sealant along all edges and seams of the flashing, including where the flashing meets the roof deck and the chimney. This will provide an extra layer of protection against water intrusion. Carefully inspect all flashing to ensure it is properly installed and sealed.

FAQs About Chimney Flashing

Here are some frequently asked questions about chimney flashing to further clarify the process.

FAQ 1: What is the best material to use for chimney flashing?

The best material depends on your budget and desired lifespan. Copper is the most durable and aesthetically pleasing, but also the most expensive. Aluminum is a cost-effective option, while stainless steel offers a good balance of durability and affordability. Consider the climate and the materials used on the rest of your roof when making your decision.

FAQ 2: How often should chimney flashing be inspected?

Chimney flashing should be inspected at least once a year, preferably in the fall before the onset of winter weather. Early detection of problems can prevent costly repairs.

FAQ 3: Can I replace chimney flashing myself?

Replacing chimney flashing is a challenging task that requires roofing experience and attention to detail. If you’re not comfortable working on a roof or lack the necessary skills, it’s best to hire a qualified roofing professional.

FAQ 4: What are the signs that chimney flashing needs to be replaced?

Signs of failing flashing include water stains on the ceiling around the chimney, crumbling mortar, rusted or corroded flashing, and missing pieces of flashing.

FAQ 5: How much does it cost to replace chimney flashing?

The cost of replacing chimney flashing varies depending on the size of the chimney, the type of flashing material used, and the complexity of the job. Expect to pay between $300 and $1,000 or more for professional installation.

FAQ 6: What is the difference between step flashing and counter flashing?

Step flashing is installed in overlapping layers along the sides of the chimney, while counter flashing is embedded into the chimney mortar joints and overlaps the step flashing. Step flashing directs water away from the roof, and counter flashing provides a watertight seal at the chimney wall.

FAQ 7: Do I need to remove old mortar when installing counter flashing?

Yes, removing old mortar is essential to create a clean and secure space for the counter flashing to be installed. This allows for a watertight seal when the new mortar is applied.

FAQ 8: What type of sealant should I use for chimney flashing?

Use a high-quality, weather-resistant sealant specifically designed for roofing and masonry. Look for a sealant that is compatible with the flashing material you are using.

FAQ 9: How can I prevent chimney flashing from leaking?

Ensure the flashing is properly installed with overlapping layers, a watertight seal, and secure attachment to both the roof and the chimney. Regular inspections and prompt repairs are also crucial for preventing leaks. Also be certain to use proper fasteners as nails can degrade over time.

FAQ 10: Can I use tar instead of flashing?

No. Tar is a temporary fix and will not provide a long-lasting, watertight seal. It’s not a suitable substitute for properly installed flashing. Tar can also trap moisture and accelerate deterioration.

FAQ 11: How long does chimney flashing last?

The lifespan of chimney flashing depends on the material used and the climate. Copper flashing can last 50 years or more, while aluminum and stainless steel may last 20-30 years. Regular maintenance and inspections can extend the lifespan of your flashing.

FAQ 12: Is it necessary to hire a professional for chimney flashing installation?

While DIY installation is possible for those with roofing experience, hiring a professional ensures the job is done correctly and safely. A professional roofer has the knowledge, skills, and tools to properly install flashing and prevent leaks, ultimately saving you time and money in the long run. Consider the peace of mind and warranty a professional provides.