How to Fix Broken Chimney Flashing: Step-by-Step Guide

Frequently Asked Questions

How do I know if my chimney flashing is the source of a leak?

Water stains on the ceiling or walls near the chimney are the most common sign. During rain, go into the attic with a flashlight and look for active drips or water trails along the chimney masonry. If the leak only happens during rain and stops shortly after, flashing is far more likely than a cracked chimney cap or damaged bricks.

What is the difference between step flashing and counter flashing?

Step flashing consists of individual L-shaped metal pieces woven between the shingles along the chimney sides. Counter flashing is embedded into the chimney mortar joints and overlaps the step flashing to create a two-layer seal. Most chimney leaks originate at the counter flashing, which is exposed to the weather and relies on mortar that eventually cracks or crumbles.

Can I just use roofing caulk to fix chimney flashing?

Roofing sealant is a legitimate short-term repair for hairline gaps and minor separations between the counter flashing and the mortar joint. However, caulk alone is not a permanent fix because it deteriorates in UV exposure and thermal cycling. Repointing with proper mortar and re-embedding the flashing edge is the lasting solution.

What materials are chimney flashing made from?

Most residential chimney flashing is made from galvanized steel, aluminum, or lead. Copper is premium and extremely durable but expensive. Aluminum is the most common DIY replacement material because it is easy to cut, bend, and work with basic tools. Avoid thin aluminum tape products sold as flashing — they fail quickly.

How long does chimney flashing repair last?

A properly done flashing repair using metal flashing embedded in fresh mortar should last 20 to 30 years. A sealant-only repair may last 3 to 5 years before needing re-application. The quality of the mortar repointing and how well the flashing is mechanically secured both determine longevity.

Is chimney flashing repair a job I can do myself safely?

Yes, for single-story homes and roofs with a moderate pitch (6 in 12 or less). You need to be comfortable working on a ladder and on the roof surface. For steep roofs, two-story homes, or tall chimneys, hiring a roofer is safer. Always use roof brackets or a roof ladder hook for stability and never work on a wet or icy roof.

Fixing chimney flashing: (1) Diagnose the leak: use a hose to wet the chimney while someone watches the attic — water appearing near the chimney confirms flashing is the source, not the roofing itself.

Chimney flashing is a narrow strip of metal that seals the joint between your chimney and the surrounding roof surface. When it separates, cracks, or corrodes, rainwater runs directly into the gap and eventually into your home. The good news: most chimney flashing problems are fixable in a single afternoon with basic hand tools and materials from a hardware store.

Understanding the Problem

Chimney flashing fails for three main reasons. First, the mortar holding the counter flashing in the chimney joint erodes over time, allowing the flashing edge to lift. Second, the sealant bridging the gap between metal and masonry deteriorates from ultraviolet exposure and freeze-thaw cycles. Third, the flashing itself may corrode or deform, especially on older homes with galvanized steel that has reached the end of its service life.

Before climbing onto the roof, inspect from the ground with binoculars and from the attic after a rainstorm to pinpoint exactly where water is entering.

Tools and Materials

• Roofing pry bar or flat bar
• Wire brush and cold chisel
• Caulking gun
• Trowel or grout bag
• Tin snips
• Hammer and masonry nails
• Replacement aluminum or galvanized flashing stock
• Roofing sealant (polyurethane or butyl)
• Hydraulic mortar or pre-mixed mortar
• Safety glasses and work gloves
• Roof ladder hook or roof brackets

Step 1: Access the Roof Safely

Set up an extension ladder against the house and secure it properly. Use a roof ladder hook over the ridge or install temporary roof brackets to give yourself a stable platform near the chimney. Never step directly onto wet shingles or work without a firm footing. Wear rubber-soled shoes.

Step 2: Inspect the Existing Flashing

Look at the counter flashing embedded in the mortar joint. Press along the top edge — if it flexes away from the mortar, the mortar joint has failed. Check the base flashing (also called apron flashing) at the front of the chimney at the lower edge. Look for rust, holes, lifted edges, or separated sealant along any seam.

Mark the problem areas with chalk or tape before you start repairs.

Step 3: Clean Out Failing Mortar Joints

Use a cold chisel and hammer to rake out cracked or crumbling mortar from the joint where the counter flashing is embedded. Remove mortar to a depth of about 3/4 inch. Brush the joint clean with a wire brush and blow out loose dust. The goal is a clean, sound channel to accept new mortar.

Step 4: Reseat or Replace the Counter Flashing

If the existing counter flashing metal is in good condition, press it back into the cleaned joint. If it is corroded or deformed, cut a replacement piece from aluminum flashing stock using tin snips. Bend a 90-degree lip along the top edge (about 3/4 inch) that will embed into the mortar joint. Slide it into place so the lip sits fully in the channel and the face laps over the step flashing below.

Step 5: Repoint the Mortar Joint

Pack the joint with fresh mortar using a trowel or grout bag, working it firmly behind the flashing lip. Tool the mortar to a slight concave profile to shed water. Allow mortar to firm up (about 30 minutes) before applying any sealant on top. Follow the mortar manufacturer’s cure time before exposing the joint to rain — typically 24 to 48 hours.

Step 6: Apply Roofing Sealant

Once the mortar has cured, run a bead of polyurethane roofing sealant along the top edge of the counter flashing where it meets the mortar, and along any seams in the base flashing. Smooth the bead with a wet finger. Sealant here acts as a secondary defense — the mortar is the primary seal.

Inspect the step flashing along both sides of the chimney. If individual step flashing pieces are lifted, press them flat and seal the edge. Badly corroded step flashing must be woven back under the shingles — carefully lift the shingle tabs with a pry bar, remove the old piece, slip in a new one, and re-nail the shingle.

Step 7: Test and Monitor

After the next rainfall, go into the attic and check for drips. If the leak has stopped, the repair was successful. If water still appears, trace the wet trail back to its highest point — there may be a second leak point, such as a cracked chimney cap or damaged crown.

Preventing Future Flashing Failures

Inspect chimney flashing every fall as part of your annual roof check. Look for sealant that has pulled away, metal that has lifted, or rust staining on the shingles below the chimney. Catching small separations early and re-sealing them takes 15 minutes and prevents the water damage that can cost thousands to repair once it reaches the ceiling or wall framing.

A chimney cap that keeps rain from entering the flue also reduces the amount of water running down the chimney exterior, reducing stress on the flashing system over time.