A chimney that extends up the exterior side of a house is a classic architectural feature, but the point where it meets the roofline is one of the most vulnerable areas for water leaks. Proper flashing is the only way to create a durable, waterproof seal in this critical transition area. Flashing involves layering pieces of metal in a specific way to direct water down and away from the joint between the chimney and the roof. For many homeowners, working on a roof can seem like a job strictly for professionals.
However, for a skilled DIYer with the right tools and a strong focus on safety, learning how to flash chimney on side of house is an achievable and crucial home maintenance project. It is a meticulous process that, when done correctly, protects your home from water damage for decades. This guide will provide a detailed, step-by-step approach to this essential roofing task.
Why Proper Chimney Flashing is Essential
Proper flashing installation is the single most important factor in preventing leaks where your chimney and roof meet. An incorrectly flashed chimney is a primary cause of water intrusion, which can lead to rotted roof decking, damaged rafters, ceiling stains, and dangerous mold growth inside your home. Taking the time to install the flashing correctly, using a layered system of step flashing and counter-flashing, creates a robust barrier. This proper waterproofing technique ensures that rainwater is effectively channeled away from the vulnerable joint, safeguarding the structural integrity of your roof and home.
8 Step-by-Step Guide: How to Flash Chimney on Side of House
Step 1: Gather Tools and Safety Equipment
Before you even think about getting on the roof, your first priority is safety and preparation. You will need a sturdy, well-placed extension ladder, a safety harness, and appropriate footwear with good grip. For tools, gather a hammer, tin snips, a pry bar, a grinder with a diamond blade, a caulk gun, and a measuring tape. Your materials will include pre-bent step flashing pieces, a roll of continuous metal for the apron and counter-flashing, roofing nails, and a high-quality polyurethane roofing sealant. Having everything ready will make the flashing installation process smoother and safer.
Step 2: Remove Old Flashing and Prepare the Area
If you are replacing old, failed flashing, you must carefully remove it first. Use a pry bar and hammer to gently lift the old metal flashing away from the shingles and the chimney. You will likely need to remove a few rows of shingles immediately surrounding the chimney to get clear access. Inspect the roof decking underneath for any signs of water damage or rot. If you find any soft or damaged wood, it must be cut out and replaced with a new piece of sheathing before you can proceed with the new flashing installation.
Step 3: Install the Front Apron Flashing
The flashing process begins at the bottom of the chimney on the downslope side. The first piece to be installed is the front apron flashing. This is a single, continuous piece of metal that is bent to fit tightly against the front face of the chimney and lie flat on the roof shingles below it. The apron should extend several inches up the front of the chimney and out over the shingles. Secure the apron to the roof deck with roofing nails, and ensure it is sealed properly to direct all water over the shingles below.
Step 4: Install the Step Flashing
Step flashing is the heart of the system that protects the sloped sides of the chimney. These are individual, L-shaped pieces of metal that are woven into the shingle courses. Starting from the bottom, place the first piece of step flashing against the side of the chimney, with one side flat on the roof deck and the other extending up the chimney wall. Place a shingle over the roof-side of the flashing. Then, place the next piece of step flashing over that shingle, overlapping the piece below it. Continue this pattern all the way up the side of the chimney.
Step 5: Install the Back Cricket or Saddle
At the top, upslope side of the chimney, you must install a cricket, also known as a saddle. This is a small, peaked structure that diverts water and debris around the back of the chimney instead of letting it pool up. A cricket is a critical component for any chimney wider than 30 inches and is a key part of learning how to flash chimney on side of house. The cricket is built and flashed like a small roof itself, directing water to either side and preventing a major weak point for leaks and debris buildup.
Step 6: Cut a Reglet for the Counter-Flashing
Now you will prepare the chimney itself for the counter-flashing. This second layer of flashing covers the top edge of the step flashing and creates a permanent seal. Using a grinder with a diamond blade, cut a groove, called a reglet, approximately 1 to 1.5 inches deep into the chimney’s mortar joints. This groove should be cut just above the top edge of the step flashing and apron flashing all the way around the chimney. This reglet will provide a secure channel to embed the counter-flashing into the chimney itself.
Step 7: Install the Counter-Flashing
The counter-flashing is installed in sections, starting with the front, then the sides, and finally the back. Each piece of metal is bent to fit over the step flashing, with its top edge tucked securely into the reglet you just cut. The side counter-flashing pieces should also be cut in a step pattern to follow the slope of the roof. Overlap each piece of counter-flashing generously to ensure a watertight seal. This two-layer system is the professional standard for a durable, leak-proof flashing installation. This is the most technical part of how to flash chimney on side of house.
Step 8: Seal the Counter-Flashing
With all the counter-flashing pieces fitted into the reglet, the final step is to seal them permanently. Fill the entire reglet groove with a high-quality polyurethane roofing sealant. This will lock the counter-flashing in place and create a completely waterproof barrier. Once the groove is filled, apply another bead of sealant along the top edge of the reglet for an extra layer of protection. Do not use roofing cement or tar for this step, as it will dry out and crack over time.
Common Flashing Mistakes
- Using Inferior Materials
Opting for low-quality or inappropriate materials, such as thin or non-durable flashing, can lead to premature failure and leaks. - Improper Installation
Incorrect alignment or failure to secure the flashing properly can compromise its effectiveness. Overlapping pieces incorrectly or cutting corners during installation are some common errors. - Insufficient Sealing
Neglecting to apply proper sealant or using substandard products can result in gaps and water penetration. - Skipping Overlap Requirements
Ensuring adequate overlap between flashing pieces is essential for directing water away from vulnerable areas. Skipping this step can lead to water infiltration. - Ignoring Maintenance
Over time, flashing can become damaged or displaced due to weather conditions or building settling. Failing to inspect and maintain flashing regularly can exacerbate issues and lead to costly repairs.
By avoiding these common mistakes, the longevity and effectiveness of the flashing system can be significantly improved.
Frequently Asked Questions
What Is the Difference Between Step Flashing and Counter-Flashing?
This is a two-part system that works together for leak prevention.
- Step Flashing: These are the L-shaped pieces of metal that are woven in with the roof shingles along the sides of the chimney.
- Counter-Flashing: This is the second layer of metal that is embedded into the chimney mortar joint and laps over the top of the step flashing, protecting it from direct exposure to weather.
Can I Use Roofing Cement or Tar Instead of Metal Flashing?
No. While roofing cement or tar is often used for temporary repairs, it should never be used as a primary flashing material. These materials break down under UV exposure from the sun, becoming brittle and cracking within a few years, which will inevitably lead to leaks. Properly installed metal flashing is the only permanent, long-term solution.
What Is a Chimney Cricket and When Do I Need One?
A chimney cricket, or saddle, is a small, peaked structure built on the high side (back) of a chimney. Its purpose is to divert water and debris around the chimney instead of allowing it to pool up against the back wall. Building codes typically require a cricket for any chimney that is 30 inches wide or wider.
How Deep Should the Reglet Joint Be Cut?
The reglet, which is the groove you cut into the chimney’s mortar joint to accept the counter-flashing, should be cut to a depth of approximately 1 to 1.5 inches. This provides enough depth to securely hold the metal flashing in place and allows for a substantial amount of sealant to create a durable, waterproof bond.
What Is the Best Material for Chimney Flashing?
The most common and effective materials for chimney flashing are galvanized steel, aluminum, or copper. Copper is the most durable and long-lasting but also the most expensive. Galvanized steel is a cost-effective and reliable choice for most residential applications. It is important not to mix different types of metal, as this can cause galvanic corrosion.
Conclusion
Successfully learning how to flash chimney on side of house is a challenging but vital project that ensures one of your home’s most vulnerable points is protected from the elements. This meticulous process of layering metal and sealing joints is the definitive way to prevent water intrusion and the costly damage it causes.
By approaching the task with a strong emphasis on safety and a commitment to following each step correctly, a skilled DIYer can achieve a professional-quality, leak-proof result.
Protecting your home starts from the top down. By investing the time and effort to flash your chimney correctly, you are providing a crucial defense against water damage and ensuring the long-term health and safety of your house.