Get a cracked chimney crown looked at before winter, not after. Every freeze-thaw cycle with water in the masonry does incremental and cumulative damage.
The crown is the sloped masonry surface on top of a chimney, and its job is to throw water clear of the flue tile and off the brickwork below. When it works, the stack sheds water. When it cracks, water runs down inside the masonry and does its damage where nobody can see it, so a crown is so often the actual cause of deterioration found much further down. A great many crowns in older housing were never really crowns at all. They were a thin skim of mortar spread across the top, with no reinforcement, no overhang and no drip edge, sitting hard against the flue tile. That construction cracks within a few Vermont winters because the mortar and the tile move at different rates and there is nothing to accommodate it. A proper crown is cast, reinforced, overhangs the brick and has a drip edge.
Checks you can do safely
- Look at the top of the stack from the ground with binoculars
- Look for white chalky staining on the exterior masonry below
- Note any water in the firebox or on the damper after rain
- Check for spalled brick or open joints on the upper courses
- Photograph what you can see from the ground and date it
- Do not climb onto the roof to inspect it
Cracked crown: causes in the order worth checking
Not an emergency; do not leave it for a season
| Possible cause | Rank |
|---|---|
| A thin mortar wash used instead of a correctly cast crown | Most common |
| No expansion allowance where the crown meets the flue tile | 2 |
| No overhang or drip edge, so water runs back onto the brickwork | 3 |
| Freeze-thaw cycling opening small cracks into large ones | 4 |
| Structural movement of the stack | 5 |
| A previous repair skimmed over a failing crown instead of replacing it | 6 |
Services this maps to
Related problems
- The Brick on My Chimney Is Flaking and Falling Off
- Water Is Getting In Around My Chimney
- There Is an Animal or a Bird in My Chimney
- Buying a House With a Fireplace, Stove or Unused Chimney
If what you have is not cracked crown after all, the full list of chimney problems is grouped by urgency, and the service triage tool takes a symptom and returns the stop condition and the service it maps to.
Common questions about cracked crown
What is the difference between a crown and a cap?+
The crown is the masonry top surface that sheds water off the stack. The cap is the metal cover over the flue opening itself. Each fails in its own way and is put right in its own way, so a stack can urgently need one of them while the other is doing its job perfectly well.
Why do so many crowns crack?+
Because many were built as a thin mortar wash, not a cast crown. A skim of mortar with no reinforcement, no overhang and no expansion allowance against the flue tile has no realistic chance of surviving many freeze-thaw cycles in this climate.
Can a cracked crown be repaired or does it need replacing?+
Where the crown is otherwise sound, fine cracks can be bridged with a flexible crown coating. Where it has broken up, shed material, or was only ever a wash, it gets recast. Patching something that failed because it was too thin simply schedules the same failure again.
How serious is a hairline crack?+
More serious than it looks, because water only needs a path. In a climate that freezes and thaws repeatedly through a winter, a hairline crack is a hairline crack that is getting bigger, and the damage is happening inside the masonry instead of at the surface.
What does a proper crown look like?+
Cast, not skimmed, with reinforcement, sloped to shed water, overhanging the brickwork by enough to throw water clear, with a drip edge underneath and an expansion joint where it meets the flue tile.
Is the crown why my chimney is spalling?+
Frequently, yes. A failed crown puts water into the masonry from above, and that water freezes inside the brick and breaks the faces off. Repointing and replacing brick without fixing the crown is repairing a symptom.
Can it be repaired in winter?+
Not correctly. Crown material needs temperatures that let it cure, so casting waits for the warmer months. What can be done in cold weather is temporary protection to limit water entry until the work can be done correctly.
Do I need staging for this?+
Commonly some form of access, and how much depends on the height and the roof. One reason crown work is often combined with cap and flashing work. The access is the same and doing them in one visit avoids paying for it twice.
How long should a crown last?+
A correctly cast and detailed one is a long-term component. A mortar wash may last a handful of winters. The gap between those two outcomes is the reason it is worth asking what is being built instead of just what it costs.
Should the cap be replaced at the same time?+
Almost always worth doing, because the access is already there and the two work together to keep water out. Fitting a new cap into a failing crown, or recasting a crown and leaving a rusted cap, is half a job.
Will waterproofing help?+
Only after the crown is sound, and only with a vapour-permeable repellent. Applying a sealer over masonry that is being fed water from a cracked crown traps that water inside, where it still freezes and still breaks the brick.
What happens if I leave it?+
Water keeps entering the masonry and each freeze does a little more damage. The usual progression is crown crack, then spalled brick and open joints, then failed courses, then a rebuild: over years, and considerably more expensively than the crown would have cost.