How to Flash a Chimney With a Metal Roof: A Definitive Guide

Flashing a chimney with a metal roof requires precision and the correct materials to ensure a watertight seal and prevent leaks. The process involves carefully integrating flashing components – step flashing, counter flashing, and base flashing – with both the metal roof panels and the chimney masonry, creating a durable barrier against water penetration.

Understanding the Importance of Proper Chimney Flashing

Improperly flashed chimneys are a leading cause of roof leaks, resulting in costly repairs and potential structural damage. A metal roof, while incredibly durable and long-lasting, requires specific flashing techniques due to its inherent thermal expansion and contraction properties. The goal is to create a flexible, watertight seal that accommodates these movements without compromising the integrity of the flashing system. This involves using appropriate materials, understanding the proper overlap and integration points, and considering the specific geometry of both the chimney and the metal roofing profile.

Necessary Materials and Tools

Before starting, gather all the essential materials and tools. Using high-quality materials designed for metal roofs is crucial for longevity and effectiveness.

• Flashing Materials: Opt for metal flashing that is compatible with your roof material. Aluminum, copper, or galvanized steel are common choices. Consider the gauge (thickness) of the metal; a heavier gauge offers greater durability.
• Sealant: Invest in a high-quality sealant specifically designed for metal roofs and chimney applications. Look for a product that is UV-resistant, flexible, and adheres well to both metal and masonry.
• Fasteners: Use stainless steel or aluminum fasteners to prevent corrosion. The type of fastener will depend on the flashing material and the surface it’s being attached to (e.g., screws for metal, masonry anchors for brick).
• Tools: You will need a measuring tape, metal shears or snips, a folding tool or brake (for bending flashing), a hammer drill (for masonry), a caulking gun, a level, a pencil or marker, and appropriate safety gear (gloves, safety glasses).

Step-by-Step Flashing Installation

This section outlines the general steps involved in flashing a chimney with a metal roof. Always consult local building codes and consider hiring a professional if you’re uncomfortable with any aspect of the installation.

Step 1: Prepare the Area

Clean the area around the chimney thoroughly, removing any debris, old flashing, or loose mortar. Inspect the chimney masonry for cracks or damage and repair them before proceeding. Ensure the metal roof surface is also clean and dry.

Step 2: Install Base Flashing

The base flashing is the first layer of protection, installed where the roof meets the base of the chimney. It typically consists of a continuous piece of metal bent to conform to the roof profile and extending up the chimney wall.

1. Carefully measure and cut the base flashing to the required length.
2. Bend the flashing to create a 90-degree angle, with one side resting on the roof and the other extending up the chimney.
3. Slide the base flashing under the upper edge of the roof panels if possible, ensuring it overlaps the lower panels by at least 6 inches.
4. Secure the flashing to the roof using appropriate fasteners and sealant. Pay particular attention to sealing the top edge of the flashing against the chimney.

Step 3: Install Step Flashing

Step flashing consists of individual pieces of metal flashing that are interwoven with the roof shingles or panels as they rise up the side of the chimney.

1. Cut individual pieces of flashing, typically 6-8 inches wide and long enough to extend up the chimney wall and overlap the adjacent roof panel.
2. Bend each piece of flashing at a 90-degree angle, creating a vertical flange that will be attached to the chimney.
3. Starting at the bottom, install the first piece of step flashing, overlapping the base flashing. Secure it to the roof panel with fasteners and sealant.
4. As you install each subsequent roof panel, integrate the next piece of step flashing, overlapping the previous piece. The vertical flange should extend up the chimney wall.
5. Secure the vertical flange to the chimney using masonry anchors and sealant.

Step 4: Install Counter Flashing

Counter flashing is installed over the step flashing, providing an additional layer of protection and preventing water from getting behind the step flashing.

1. Cut the counter flashing to the same length as the step flashing pieces.
2. Insert the counter flashing into reglets (slots) cut into the chimney mortar joints. These reglets should be at least 1 inch deep.
3. Bend the counter flashing down to cover the vertical flange of the step flashing, creating a tight seal.
4. Use mortar or sealant to fill the reglets and secure the counter flashing in place.

Step 5: Finishing Touches

Carefully inspect all flashing joints and seams, applying sealant as needed. Ensure that all fasteners are properly secured and that there are no gaps or openings that could allow water to penetrate. Consider installing a chimney cap to further protect the chimney from water damage.

Metal Roof Considerations

Different types of metal roofs (standing seam, corrugated, etc.) require slightly different flashing techniques. Standing seam roofs often require specialized flashing components designed to integrate seamlessly with the panel system. Always consult the manufacturer’s recommendations for your specific roof type. It’s also crucial to consider the slope of the roof and adjust the flashing accordingly to ensure proper water runoff. Steeper slopes require larger flashing pieces and greater overlap.

FAQs: Chimney Flashing and Metal Roofs

Here are some frequently asked questions regarding chimney flashing with metal roofs:

FAQ 1: What type of metal is best for chimney flashing on a metal roof?

Answer: The best metal depends on your roof material and budget. Aluminum is lightweight and corrosion-resistant, making it a popular choice. Copper is durable and aesthetically pleasing but more expensive. Galvanized steel is a cost-effective option, but it needs to be properly coated to prevent rust. Choose a metal compatible with your roof to avoid galvanic corrosion.

FAQ 2: How do I cut reglets into the chimney for counter flashing?

Answer: Use a hammer drill with a masonry bit to carefully cut reglets into the mortar joints. The reglets should be at least 1 inch deep and slightly wider than the counter flashing. Take your time to avoid damaging the bricks.

FAQ 3: What sealant should I use for flashing around a metal roof chimney?

Answer: Use a polyurethane or silicone-based sealant specifically designed for metal roofs and masonry. Look for a sealant that is UV-resistant, flexible, and adheres well to both metal and brick.

FAQ 4: Can I reuse old chimney flashing?

Answer: It’s generally not recommended to reuse old flashing. Old flashing may be corroded, damaged, or not compatible with the new roof material. Using new, high-quality flashing is crucial for a long-lasting, watertight seal.

FAQ 5: How often should chimney flashing be inspected and replaced?

Answer: Chimney flashing should be inspected at least once a year, preferably in the spring and fall. The lifespan of flashing depends on the material and environmental conditions, but it typically needs to be replaced every 15-20 years.

FAQ 6: What are the signs that my chimney flashing is failing?

Answer: Signs of failing flashing include water stains on the ceiling or walls near the chimney, loose or missing flashing pieces, rusted or corroded flashing, and cracked or damaged sealant.

FAQ 7: Can I install chimney flashing myself, or should I hire a professional?

Answer: Chimney flashing installation can be complex and requires specific skills and tools. If you’re not comfortable working at heights or have limited experience with roofing and masonry, it’s best to hire a qualified professional.

FAQ 8: How do I prevent galvanic corrosion between different metals?

Answer: Galvanic corrosion occurs when two dissimilar metals come into contact in the presence of an electrolyte (like water). To prevent it, use compatible metals for the flashing and roof. If using dissimilar metals, use a dielectric barrier between them.

FAQ 9: How do I deal with a chimney that is not perfectly square?

Answer: Non-square chimneys require custom-cut flashing pieces to ensure a proper seal. This may involve creating templates and carefully measuring each piece of flashing. It’s often best to consult with a professional roofer in these situations.

FAQ 10: What is the best way to seal the top of the chimney flashing to the chimney itself?

Answer: The best way is with counter flashing inserted into reglets cut into the mortar. The reglets are then filled with mortar or a high-quality sealant, ensuring a watertight bond between the counter flashing and the chimney.

FAQ 11: How much overlap should there be between the step flashing pieces?

Answer: Aim for at least 3-4 inches of overlap between each piece of step flashing to provide adequate water protection.

FAQ 12: What is a cricket, and why might I need one around my chimney?

Answer: A cricket is a peaked structure that diverts water around the back of a chimney (the uphill side). It prevents water from pooling and potentially leaking. Crickets are particularly important for wide chimneys or chimneys on roofs with low slopes.

By following these guidelines and taking the necessary precautions, you can ensure that your chimney is properly flashed with your metal roof, providing long-lasting protection against water damage. Remember to prioritize safety and consult with professionals when needed.