Chimney Flashing Repair: Why Caulk Always Fails and What Correct Flashing Looks Like

The anatomy of correct chimney flashing

A chimney is a rigid masonry column punched through a flexible roof plane. The two move independently — masonry expands and contracts differently from a wood-framed roof deck, and the whole assembly cycles through Ontario's freeze-thaw dozens of times each winter. No sealant survives that indefinitely. The only durable solution is overlapping metal that lets the two parts move while water still runs downhill and out.

Base flashing (apron)

An L-shaped metal piece at the downslope face of the chimney. One leg lies on the shingles, the other runs up the brick. It catches water coming down the roof toward the chimney and diverts it around.

Step flashing

Individual rectangular pieces of metal, one per shingle course, bent at 90 degrees and woven into the courses up each side of the chimney. Each piece overlaps the one below like shingles do. Critically, step flashing is never a single continuous piece of metal — that is a common shortcut and it fails because water tracks behind the top edge with nowhere to escape.

Counter-flashing

The second layer, and the one almost everyone gets wrong. A reglet — a groove roughly 25 mm deep — is cut into the mortar joint with a grinder. The top edge of the counter-flashing is folded and inserted into that groove, then wedged and sealed. It hangs down over the step flashing, so water running down the brick face is shed onto metal, not into the joint. Surface-mounting the counter-flashing to the brick face with caulk instead of cutting a reglet is fast, cheap, and guaranteed to leak once the caulk cures out.

The cricket (saddle)

A small peaked structure built on the upslope side of the chimney that splits water and snow around it. Building code requires one on chimneys wider than 30 inches measured across the slope. Countless GTA homes have a wide chimney with no cricket, and the result is a permanent snow and debris dam on the upslope face that no flashing can outlast.

Signs your chimney flashing has failed

• Water stains on the ceiling or wall near the chimney chase, especially after wind-driven rain rather than every rain.
• Damp, crumbling, or white-stained brick inside the attic where the chimney passes through.
• Rust streaks on the shingles below the chimney — the flashing itself is corroding.
• Visible gaps, lifted metal edges, or cracked caulk lines at the brick-to-metal joint.
• A musty smell near the fireplace, or damp firebox and rusty damper — water is coming down the chase.
• Peeling paint or blistered drywall on the wall that contains the chimney, on any floor.

Chimney flashing repair costs in the GTA, 2026

The honest way to read this table: the difference between the $250 reseal and the $900 proper job is about six years of service life and whether your roof deck stays dry. On a chimney you plan to keep, the cheap option is the expensive one.

Aluminum, steel, copper, or lead?

How to check yours from the ground

1. Use binoculars or the zoom on your phone from the yard. Do not climb on a sloped roof for an inspection.
2. Look for the horizontal line of the counter-flashing along the brick. If you can see a straight cut line into a mortar joint, that is a good sign.
3. Look for a thick black or grey bead instead of a clean metal edge. That is roof cement.
4. Check the upslope side for a peaked cricket if the chimney is wider than about 30 inches.
5. Look for rust streaks running down the shingles from the flashing line.
6. Then check the attic from inside, in daylight, for staining on the chimney masonry and the surrounding sheathing.