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How to Flash a Chimney on the Side of Your House: A Definitive Guide

Flashing a chimney on the side of your house is a crucial waterproofing step that prevents water intrusion and costly structural damage. Proper flashing creates a watertight barrier between the chimney and the roof, directing water away from the house. Improper flashing can lead to leaks, rot, and even structural failure, making it a critical area for both homeowners and contractors to understand.

Understanding the Importance of Chimney Flashing

Chimney flashing acts as a shield, preventing rainwater, snow, and ice from seeping into the junction between the chimney and the roof. This area is particularly vulnerable because of the difference in materials and the varying expansion and contraction rates they experience with temperature fluctuations. Without effective flashing, water can penetrate the roof, leading to:

Water damage: This includes stains on interior ceilings and walls, rotting wood, and mold growth.
Structural damage: Prolonged exposure to moisture can weaken the roof structure and the chimney itself, requiring expensive repairs.
Reduced energy efficiency: Damp insulation loses its effectiveness, leading to higher heating and cooling bills.

Essential Materials and Tools

Before you begin, gather the necessary materials and tools. Using high-quality materials will ensure the longevity and effectiveness of your flashing.

Materials:

Flashing material: Choose between aluminum, copper, or stainless steel. Copper is the most durable but also the most expensive. Aluminum is affordable but may corrode in some environments. Stainless steel offers a good balance of cost and durability. For most residential applications, 26-gauge galvanized steel flashing is a common and cost-effective choice.
Roofing cement: A high-quality, flexible roofing cement is essential for sealing the flashing and preventing leaks.
Shingles (matching your existing roof): You’ll need shingles to replace any damaged or removed shingles during the process.
Step flashing: These are pre-formed pieces of flashing that overlap each other, creating a watertight seal along the sides of the chimney.
Counter flashing: This is embedded into the chimney mortar joints and overlaps the step flashing, providing an additional layer of protection.

Tools:

Hammer: For nailing shingles and flashing.
Nail puller: For removing old nails and staples.
Tin snips or metal shears: For cutting flashing to size.
Utility knife: For cutting shingles.
Trowel: For applying roofing cement.
Caulking gun: For applying caulk to seal gaps.
Safety glasses: To protect your eyes from debris.
Gloves: To protect your hands.
Ladder: To safely access the roof.
Measuring tape: For accurate measurements.
Grinder with masonry blade: For cutting reglets into the chimney.
Chisel and hammer: For creating mortar joints to embed counter flashing.

Step-by-Step Guide to Flashing a Chimney

This guide assumes you are installing new flashing during a re-roofing project. If you’re replacing existing flashing, carefully remove the old flashing and shingles surrounding the chimney.

Step 1: Preparing the Area

Remove shingles: Carefully remove the shingles around the chimney, working your way up the roof.
Clean the area: Remove any debris, old nails, and roofing cement from the roof deck and the chimney surface. A clean surface is crucial for proper adhesion.

Step 2: Installing Step Flashing

Start at the bottom: Begin installing step flashing at the bottom of the chimney, where it meets the roof.
Overlap method: Each piece of step flashing should overlap the previous piece by at least 2 inches.
Nail in place: Secure each piece of step flashing to the roof deck with roofing nails, being careful not to nail through the flashing into the chimney itself.
Embed in cement: Apply a bead of roofing cement under each piece of step flashing before nailing it down.

Step 3: Installing Counter Flashing

Cut reglets: Use a grinder with a masonry blade to cut shallow grooves (reglets) into the chimney mortar joints, approximately 1 inch deep. These reglets will hold the counter flashing.
Insert counter flashing: Bend the counter flashing to fit into the reglets. The counter flashing should overlap the step flashing by at least 4 inches.
Secure with mortar: Use mortar to secure the counter flashing within the reglets. Ensure the mortar is packed tightly to prevent water from entering.
Seal with caulk: Once the mortar has cured, apply a bead of caulk along the top edge of the counter flashing where it meets the chimney to further seal the joint.

Step 4: Installing Head Flashing (Top of Chimney)

Form the head flashing: Cut a piece of flashing long enough to extend at least 6 inches on either side of the chimney and 6 inches up the roof.
Bend the flashing: Bend the flashing to create a 90-degree angle.
Position and secure: Position the head flashing at the top of the chimney, ensuring it overlaps the side flashing. Nail it to the roof deck and seal it with roofing cement.
Insert into reglet (if applicable): Some installations also involve cutting a reglet into the chimney at the top to install the head flashing similar to the counter flashing.

Step 5: Installing Base Flashing (Bottom of Chimney)

Form the base flashing: Similar to the head flashing, cut and bend a piece of flashing to fit the bottom of the chimney.
Position and secure: Position the base flashing, ensuring it overlaps the side flashing. Nail it to the roof deck and seal it with roofing cement.

Step 6: Replacing Shingles

Reinstall shingles: Reinstall the shingles around the chimney, overlapping the step flashing as you go.
Cut shingles to fit: You may need to cut shingles to fit snugly around the chimney.
Secure shingles: Nail the shingles in place, being careful not to nail through the flashing.

Step 7: Final Sealing

Inspect all seams: Carefully inspect all seams and joints in the flashing.
Apply sealant: Apply caulk or roofing cement to any areas that appear vulnerable to water penetration. This is particularly important around nail heads and where the flashing meets the shingles.

Frequently Asked Questions (FAQs)

1. What type of flashing material is best for a chimney?

Copper is generally considered the best option due to its durability and resistance to corrosion. Stainless steel is a good compromise between cost and durability. Aluminum is the least expensive but may corrode over time, especially in coastal areas or where it comes into contact with dissimilar metals. The specific choice depends on your budget, climate, and aesthetic preferences.

2. How often should chimney flashing be inspected?

Chimney flashing should be inspected at least once a year, preferably in the spring and fall, to check for signs of damage or deterioration.

3. What are the signs of damaged chimney flashing?

Common signs include water stains on interior ceilings or walls, visible rust or corrosion on the flashing, missing or loose flashing pieces, and crumbling mortar joints.

4. Can I repair chimney flashing myself, or should I hire a professional?

Minor repairs, such as sealing small gaps with caulk, can often be done by homeowners. However, significant damage or replacement of flashing is best left to a qualified roofing contractor or chimney professional. Improper installation can lead to serious water damage.

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 anywhere from $300 to $1,500 or more for a complete replacement.

6. What is the difference between step flashing and counter flashing?

Step flashing is installed in overlapping pieces alongside the chimney, interwoven with the shingles. Counter flashing is embedded into the chimney mortar joints and overlaps the step flashing, providing an additional layer of protection. They work together to create a watertight seal.

7. How do I prevent ice dams from damaging my chimney flashing?

Proper roof insulation and ventilation are key to preventing ice dams. Ensuring adequate attic insulation helps to prevent heat from escaping and melting snow on the roof, which can refreeze and form ice dams. Maintaining proper attic ventilation helps to keep the attic cool and dry, further reducing the risk of ice dam formation. Installing heat tape along the roof edge can also help prevent ice dams.

8. What is a chimney cricket, and when is it needed?

A chimney cricket is a small, peaked structure installed behind a chimney on a roof. It’s designed to divert water and debris around the chimney. Crickets are typically recommended for chimneys wider than 30 inches.

9. What happens if I don’t fix damaged chimney flashing?

Ignoring damaged chimney flashing can lead to significant water damage, including rotting wood, mold growth, and structural damage to the roof and chimney. This can result in expensive repairs and compromise the safety of your home.

10. Can I use aluminum flashing with copper?

No. Dissimilar metals can react electrolytically, causing corrosion. Avoid mixing aluminum and copper. Use stainless steel as a neutral option if you’re concerned about compatibility.

11. How do I seal around a chimney liner when flashing?

When installing counter flashing, carefully seal around the chimney liner pipe using high-temperature silicone caulk specifically designed for chimney applications. This will prevent water from entering the space between the liner and the chimney.

12. What is the best way to dispose of old chimney flashing?

Old chimney flashing can often be recycled at a local metal recycling facility. Check with your local waste management services for specific disposal guidelines.

By following these steps and understanding the importance of proper chimney flashing, you can protect your home from water damage and ensure the longevity of your roof and chimney. Remember to prioritize safety and consult with a professional if you are unsure about any part of the process.

How to Flash Chimney on Side of House?