Chimney Crowns and Counterflashing
Chimneys are the most common source of roof leaks on houses that have one, and the reason is structural rather than accidental. A chimney is a heavy masonry structure passing through the roof plane, made of a material that absorbs water, moving independently of the timber frame around it.
The complication for a homeowner is that a chimney can leak in four distinct places, only one of which is a roofing problem. Working out which is which determines who you should be calling.
The four failure points
1. The crown
The crown is the concrete or mortar cap on top of the chimney, around the flue. It should be a proper sloped concrete slab that overhangs the brickwork slightly with a drip edge, shedding water clear of the masonry.
What is very often there instead is a thin skim of mortar, troweled flat. Mortar is not designed for horizontal exposure. It cracks within a few years, water enters the cracks, freezing and thawing widens them, and the whole cap begins to break up. Water then runs straight down inside the chimney structure.
A failing crown is one of the most common chimney problems and one of the least visible from the ground. It is masonry work rather than roofing work.
2. The masonry itself
Brick and mortar are porous. Older or degraded mortar joints absorb water directly, and in cold markets freeze-thaw cycling spalls the brick faces. Water absorbed high up on a chimney can appear inside the house well below the roof line, which makes it look like a roof leak when the roof is not involved at all.
This is repointing and masonry repair. Again, not a roofer.
3. The flashing, this one is ours
Where the chimney passes through the roof, two components work together, and the distinction matters:
- Step flashing is woven into the shingle courses up each side of the chimney, one piece per course, carrying water down the sides.
- Counterflashing is the upper layer. It is let into a groove cut in the mortar joint, bent down over the top of the step flashing, and sealed into that groove. It stops water running down the face of the masonry from getting behind the step flashing.
The correct detail requires cutting a channel into the mortar, called a reglet or a kerf, and seating the counterflashing in it. The common shortcut is to skip the channel and simply run a bead of sealant where the metal meets the brick. That relies entirely on sealant, which fails within a few years, and it is why so many chimneys are leaking despite having flashing that looks present.
There is also the cricket, a small peaked structure built on the upslope side of a wide chimney to divert water around it rather than letting it pile against the masonry. On chimneys above a certain width this is required rather than optional, and its absence causes persistent problems that no amount of flashing repair will solve.
4. The cap and flue
A missing or damaged rain cap lets water straight down the flue. Cheap to fix, frequently overlooked, and it produces symptoms (damp, staining, odor) that look nothing like a roof leak.
How to tell which one you have
- Staining on the ceiling around the chimney, worst after driving rain. Flashing, most likely.
- Damp or efflorescence on the chimney breast inside the house, some way below the roof. Masonry absorption or the crown.
- White powdery deposits on the brickwork outside. Water moving through the masonry.
- Damp in the firebox, or a smell of soot and damp. Cap or flue.
- Visible cracks in the cap on top. The crown.
- Rust staining running down the brick from the flashing line. The flashing itself is corroding.
An inspection should establish which before anyone starts work, because the wrong repair is a complete waste. Replacing flashing on a chimney whose crown has failed changes nothing.
The shortcut to be aware of during a roof replacement
Chimney flashing is one of the most commonly reused components on a new roof. The step flashing gets left in place, new shingles are woven around it, and old metal ends up under a new roof at the single most leak-prone point on the house.
Ask explicitly whether chimney flashing is being replaced and whether the counterflashing is being properly let into the mortar. It should be a named line on the estimate, see how to read a roofing estimate. If the chimney is wide and there is no cricket, ask about that too.
Who does what
Being straight about the boundary: flashing, crickets and the roof-to-chimney junction are roofing work, and we handle them. Crowns, repointing, spalled brick, flue liners and caps are masonry and chimney work, and while we will identify them and tell you plainly what we are looking at, a mason or chimney specialist is the right trade for those.
A chimney that needs both is common, and the sequence matters, masonry repairs first where the counterflashing has to be cut into fresh mortar.
Our roofing repair team rebuilds chimney flashing as a standalone job, and a free roof inspection will tell you which of the four problems you actually have before you spend anything.
Chimney leak questions
Chimneys leak in four distinct places: the crown on top, the porous masonry itself, the flashing where the chimney meets the roof, and the flue or rain cap. Only the flashing is a roofing problem, which is why identifying the source before repairing matters so much.
The concrete cap on top of the chimney around the flue. It should be a sloped slab overhanging the brickwork with a drip edge. What is often there instead is a thin flat skim of mortar, which cracks within a few years and lets water run down inside the chimney.
The upper layer of chimney flashing. It is let into a groove cut in the mortar joint and bent down over the step flashing, stopping water running down the masonry from getting behind it. Sealing metal to brick without cutting that groove is the most common shortcut and the usual cause of failure.
A small peaked structure built on the upslope side of a chimney to divert water around it instead of letting it pool against the masonry. On chimneys above a certain width it is required rather than optional, and its absence causes leaks that flashing repairs will not solve.
Yes, and it is one of the most commonly reused components instead. Old step flashing left under a new roof puts aged metal at the most leak-prone point on the house. Ask for it to be named on the estimate as new.
It depends which of the four failure points is involved. Flashing and crickets are roofing. Crowns, repointing, spalled brick, flue liners and caps are masonry and chimney work. Many chimneys need both, and masonry usually goes first so counterflashing can be cut into fresh mortar.