How to Seal Chimney Flashing? | Seal Leaks Fast

Knowing how to seal chimney flashing stops roof leaks at the most common entry point — where the metal meets the chimney brick and roof deck. The fix uses two separate sealing actions: applying elastomeric or silicone sealant to the base flashing where it meets the shingles, and embedding counter flashing into a freshly cut groove in the chimney mortar. This article covers the DIY method for minor leaks and explains when full replacement is necessary.

What Does Sealing Chimney Flashing Involve?

Chimney flashing sealing works as two interdependent barriers. The base flashing sits beneath the roof shingles and directs water outward. The counter flashing overlaps the base and is embedded into the chimney’s mortar joints to stop water from running behind the metal.

For minor leaks where the flashing is still structurally sound, a quality sealant applied after thorough cleaning and surface prep is the standard fix. For severe damage — rusted-through metal, loose or missing sections — replacement is required rather than sealing.

How To Seal Chimney Flashing Yourself (DIY Method)

If the flashing is intact but leaks at the seams, this procedure works for most homeowners comfortable on a roof:

1. Inspect and clean. Remove debris, old mortar, loose caulk, and rust using a wire brush and pry bar. Hammer down any raised nail heads.
2. Prepare the surface. The area must be dry, structurally sound, and dust-free. Grind or wire-brush masonry to expose clean surface for adhesion.
3. Re-secure loose flashing. Fasten it firmly to the roof deck only — never nail into chimney bricks. Nailing into brick prevents proper drainage and damages mortar over time.
4. Apply the sealant. Load a quality elastomeric or silicone sealant into a caulking gun. Cut the tip to size, puncture the inner seal, and apply a generous bead to overlaps, seams, and exposed nail heads. Force the bead into joints from the bottom up to avoid air bubbles. Smooth with a putty knife for a tight seal.
5. Test. Spray the area with a garden hose and check inside the home after several hours.

For a detailed comparison of the best products for this job, check out our roundup of the best chimney flashing sealants tested for real-world performance across different flashing types.

Common mistakes that cause seal failures: applying sealant only over the top of a joint without filling it from below (bubbles form and water seeps through), using standard roofing cement as the outer weather seal (it dries out and cracks within a year or two), and failing to cut a proper reglet for counter flashing, which lets water run behind the metal.

When Should You Call A Pro Instead?

If the flashing is rusted, cracked, or missing sections, sealing alone won’t hold.

Key replacement specs a pro will follow: the reglet must be cut 1 inch deep into mortar joints, base flashing extends at least 4 inches onto the roof shingles, counter flashing height should be at least 6 inches up the chimney (or one-sixth of the chimney’s width, whichever is greater), and a cricket is mandatory for chimneys 30 inches wide or wider to divert water away from the base.

If you’re comfortable on a roof, minor sealing is a straightforward weekend project. For anything requiring flashing removal and replacement, professional installation is the safer — and often cheaper in the long run — choice.