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How to Put Flashing Around Chimney? A Comprehensive Guide

Properly installing chimney flashing is crucial for preventing water damage to your roof and home. This article provides a step-by-step guide on how to expertly flash your chimney, ensuring a watertight seal and long-lasting protection against the elements. A failing flashing system can lead to costly repairs, so mastering this skill is an investment in your home’s future.

Understanding the Importance of Chimney Flashing

Chimney flashing acts as a protective barrier where the chimney intersects the roof. This intersection is a vulnerable point for water intrusion because the roof and chimney expand and contract at different rates, creating potential gaps. Effective flashing channels water away from this junction, preventing leaks and potential structural damage to your roof deck, attic, and interior walls.

Materials You’ll Need

Before you begin, gather the necessary materials:

Flashing: Typically 26-gauge galvanized steel or aluminum flashing. Copper is also an excellent, albeit more expensive, option.
Base Flashing (Apron Flashing): This goes on the lower side of the chimney.
Step Flashing: Individual pieces that are woven into the shingles along the sides of the chimney.
Counterflashing: Covers the step flashing and is mortared into the chimney.
Roofing Nails: Use nails appropriate for your roofing material.
Mortar: Type N mortar is generally recommended for chimney repairs and flashing.
Caulk: High-quality, exterior-grade sealant specifically designed for roofing.
Tools: Hammer, tin snips, measuring tape, pry bar, chisel, wire brush, pointing trowel, safety glasses, work gloves.

Preparing the Area

Safety is paramount. Before climbing onto the roof, ensure you have proper safety equipment, including a safety harness and secure footing.

Remove Old Flashing: Carefully remove the existing flashing around the chimney using a pry bar and tin snips. Be mindful not to damage the surrounding roofing.
Clean the Area: Thoroughly clean the chimney and roof surface with a wire brush to remove any debris, old mortar, or roofing granules. This ensures a clean surface for the new flashing to adhere to.
Inspect the Chimney: Check for any cracks or damaged mortar joints in the chimney itself. Repair these before installing the new flashing. A sound chimney is essential for a long-lasting, watertight seal.

Step-by-Step Chimney Flashing Installation

The installation process involves several key steps, each critical to achieving a leak-proof seal.

1. Installing Base Flashing (Apron Flashing)

The base flashing is the first line of defense against water coming down the roof.

Cut the Flashing: Measure the width of the chimney on the downhill side and cut a piece of flashing to match, adding a few inches for overlap.
Bend the Flashing: Bend the flashing at a 90-degree angle, creating a lip that will sit against the chimney.
Position and Nail: Slide the base flashing under the shingles, ensuring the bent lip is tight against the chimney. Nail the top edge of the flashing to the roof deck, being careful not to puncture the chimney itself.

2. Installing Step Flashing

Step flashing is crucial for the sides of the chimney.

Cut the Flashing: Cut individual pieces of flashing, typically around 8-10 inches long and 6-8 inches wide.
Bend the Flashing: Bend each piece of flashing at a 90-degree angle in the middle.
Weave the Flashing: Starting from the bottom, weave the step flashing into the shingles, placing one piece against the chimney for each course of shingles. Secure each piece with a roofing nail. The nail should go through the step flashing and into the roof, but be covered by the next shingle in the course above.

3. Installing Counterflashing

Counterflashing provides a secondary layer of protection and covers the step flashing.

Prepare the Chimney: Use a chisel to carefully create grooves (reglets) in the mortar joints of the chimney, about 1-2 inches deep. The grooves should align with the top edge of each step flashing piece.
Cut the Counterflashing: Cut pieces of counterflashing to match the size of the step flashing.
Insert the Counterflashing: Bend the counterflashing to fit snugly into the reglets.
Mortar the Counterflashing: Fill the reglets with mortar, ensuring the counterflashing is securely held in place. Allow the mortar to cure completely according to the manufacturer’s instructions.

4. Installing Top Flashing (Saddle Flashing)

The top flashing, sometimes called saddle flashing, helps divert water around the upper side of the chimney.

Cut and Bend the Flashing: Cut a piece of flashing that extends a few inches beyond the width of the chimney. Bend it to create a saddle shape that directs water away from the chimney.
Install and Secure: Slide the flashing under the shingles and secure it with roofing nails. Apply caulk where the flashing meets the chimney to create a watertight seal.

5. Sealing with Caulk

Finally, apply a high-quality exterior-grade caulk along all seams and edges of the flashing to create a final watertight seal.

FAQs on Chimney Flashing

1. What is the difference between step flashing and counterflashing?

Step flashing is woven into the shingles alongside the chimney, providing the initial barrier against water. Counterflashing, on the other hand, covers the step flashing and is mortared into the chimney itself, creating a secondary, more robust seal.

2. What type of metal is best for chimney flashing?

Galvanized steel and aluminum are common choices, offering good durability and corrosion resistance. Copper is a premium option known for its longevity and aesthetic appeal, but it’s more expensive.

3. How often should chimney flashing be replaced?

Chimney flashing should be inspected annually. Generally, flashing lasts for 15-20 years, but this depends on the material and the climate. Look for signs of rust, damage, or separation.

4. Can I reuse old chimney flashing?

Reusing old flashing is generally not recommended. Even if it appears to be in good condition, it may be weakened or damaged, compromising its ability to provide a watertight seal.

5. What happens if chimney flashing is not properly installed?

Improperly installed chimney flashing can lead to leaks, which can cause significant damage to your roof deck, attic, walls, and even the chimney structure itself. This can result in costly repairs.

6. What type of mortar should I use for counterflashing?

Type N mortar is typically recommended for chimney repairs and flashing. It offers a good balance of strength and flexibility, allowing it to withstand temperature fluctuations.

7. What is the best way to clean the chimney before installing flashing?

Use a wire brush to remove any loose debris, old mortar, or roofing granules. A clean surface is essential for proper adhesion of the flashing and sealant.

8. Can I install chimney flashing myself, or should I hire a professional?

Installing chimney flashing is a challenging task that requires precision and attention to detail. If you are not comfortable working on a roof or have limited experience with roofing repairs, it is best to hire a qualified professional.

9. How do I fix a leaking chimney flashing?

Identify the source of the leak. If the flashing is simply loose, you may be able to re-nail it and apply caulk. However, if the flashing is damaged or corroded, it will likely need to be replaced.

10. What is the purpose of a chimney cricket?

A chimney cricket is a small, peaked structure installed on the high side of a wide chimney to divert water around it. This is especially important for chimneys that are wider than 30 inches.

11. What safety precautions should I take when working on a roof?

Always wear safety glasses, work gloves, and a safety harness when working on a roof. Ensure you have secure footing and avoid working in wet or windy conditions.

12. Where can I find more information on chimney flashing installation?

Consult roofing guides, online tutorials from reputable sources, and local building codes for specific requirements. Consider consulting with a qualified roofing professional for personalized advice and guidance.