Chimney Flashing Layers and How They Fail

Chimney Academy · How it works · Updated October 2026

Why flashing has two systems, not one

Brick and shingles move differently. The roof deck flexes with temperature and settles over time, and the chimney is a rigid column. A single piece of metal glued to both would tear. The solution used in masonry guidance is to split the job in two so the parts can move independently while still shedding water.

The Brick Industry Association’s technical note on flashing chimneys describes the pair this way. Base flashing goes on first, at the roof, and turns up the chimney face. Counterflashing is anchored into the masonry and hangs down over the top of the base flashing like a shingle laps a shingle. Water running down the brick lands on the counterflashing, drops onto the base flashing, and then onto the roof surface, never going behind either.

That overlap is the whole idea. Every leak path discussed below is a place where the overlap stopped working.

The layers, from the roof surface up

Names vary between roofers and masons, so here is a plain walk-through of what each part does and where it sits.

Step flashing runs along the sides of the chimney, where the roof slopes past it. Individual pieces are woven in with each course of shingles, stepping up the slope. Each piece sheds water onto the shingle below it. Pieces are not meant to be one continuous strip, because the stepped overlap is what lets the roof surface flex.

Apron (front flashing) is the piece on the downhill face, where water pours straight into the chimney. It lies over the shingles below it and turns up the brick.

Back flashing and the cricket. The uphill side is the hard part, because water runs straight into it. A cricket, also called a saddle, is a small ridge-shaped roof section built behind the chimney to divert water around it. Pro Builder’s guidance, citing the residential code, notes that a cricket is called for behind a chimney wider than 30 inches, and explains that without one, water backs up on the uphill side and seeps into the flashing. Flashing up the chimney face behind it is integrated with the roof’s underlayment.

Counterflashing is the upper layer, covering the top edge of the base pieces. Brick Industry Association guidance has it set into a mortar joint about three quarters to one inch deep, lapping the base flashing by at least 3 inches. The base flashing itself should extend at least 4 inches along the roof and 4 inches up the chimney face. A cut slot, called a reglet, in the brick or mortar is one way to receive it.

Crown flashing and sealant. At the very top, backer rod and sealant close the gap between the crown and the flue liner. That is a different seam from the roof flashing, and it matters just as much. See chimney crown repair, resurfacing or replacement for that area.

To tie the parts to the rest of the structure, parts of a chimney explained shows where each sits.

Metal does the work, sealant only backs it up

The standard for dependable flashing is metal that overlaps, not caulk that fills a gap. Caulk, tar and roofing cement can slow a leak for a season or two, but they age. The Brick Industry Association’s maintenance note lists sealant joints as lasting roughly 5 to 20 years, and metal copings and flashing roughly 20 to 75 years, though those ranges assume proper materials and workmanship and say chimneys with harsher exposure may need repair sooner.

That gap in service life explains a pattern many homeowners know: a bead of sealant that was replaced three times and a leak that came back each time. Sealant is a reasonable detail where the design calls for it, such as sealing the laps and edges of the metal. It is not a stand-in for the overlap itself.

The flashing technical note is explicit that every joint and lap in base and counterflashing should be completely sealed, and that flashing must be continuous with any bends or breaks sealed. So sealant has a role, as a supplement to a lapped metal system. The failure comes from using it as the primary barrier.

Which metal also matters.

Our guide to chase cover materials goes further into metal choices on other parts of the chimney top.

How flashing fails, and what that looks like

The failures cluster into a short list. Each has a visible or measurable sign, though you should treat these as clues and not diagnoses.

• Unsealed or open laps. The technical note calls these a failure point in base and counterflashing. Look for lifted edges or gaps at corners.
• Counterflashing that no longer sits in the masonry. If the mortar joint holding it has eroded, the metal can loosen and drop, leaving a path behind it. Flashing that stops short of passing into the wall performs little better than none.
• Surface-mounted flashing. A strip stuck on the face of the brick with a bead of sealant, instead of tucked into a joint, relies entirely on that sealant.
• Corrosion. Rust streaks, white powdery deposits on aluminum, or green copper staining. Wrong-metal combinations and old galvanized pieces are the usual suspects.
• Missing or undersized cricket. Staining on the uphill side, or ceiling stains that follow heavy rain.
• Roof work that disturbed it. Reroofing can bend or reuse old pieces. See roof replacement and your chimney.
• Wind-driven or snow-related water. Some flashing leaks only in a certain weather. Chimney leaks only in wind-driven rain and ice dams, snow and chimney leaks cover those.

Inside the house, a flashing leak may show up as a stain on a ceiling or wall near the chimney, damp insulation in the attic, or a musty smell. Water that runs down inside the masonry can also appear far from the entry point. If you want help sorting those out, stains on walls and ceilings near the chimney is the right next read.

What to do while you wait for a repair

You do not need to go on the roof, and you should not. A few safe steps help.

1. Note when the leak appears: during any rain, only in wind, only after snow melts, or only in heavy storms. That narrows the possible causes more than almost anything else.
2. Take dated photos of any stain, including a coin or tape measure for scale, and look at the attic with a flashlight from the access hatch only if it is safe and easy to reach.
3. Place a bucket or towels, and move belongings away from the wall.
4. Do not put roofing tar or caulk on it as a quick fix. A temporary layer can hide the real path and complicate the repair.

The repair route depends on the findings. Replacing counterflashing and resealing laps is a narrower job than rebuilding a leaking cricket or addressing masonry that is taking in water. Chimney flashing repair and roof leaks covers the repair side, and chimney repair is the service page. We explain findings and options before any work begins.

When to call a professional, and what this guide cannot tell you

Call when water enters the house, when metal near the chimney is loose or visibly lifting, or after a storm or roof project. Water in a ceiling near an electrical fixture is a reason to switch off that circuit and call an electrician as well.

Frequently asked questions

Related reading

• Chimney flashing repair and roof leaks
• Why is my chimney leaking?
• Chimney flashing and crickets
• Roof replacement and your chimney
• Chimney repair