Why crowns crack in the first place
The National Park Service lists the usual causes of cracks in brick masonry: drying shrinkage, expansion and contraction from temperature and moisture changes, differential movement between building materials, and freeze-thaw cycles. A crown sits at the most exposed spot on the house, and it is often a different material from the brick it rests on, so it moves differently every time the weather changes.
Freeze-thaw is what turns a hairline crack into real damage. The Park Service's brief on concrete explains that when concrete is saturated with water and then exposed to freezing temperatures, the water expands as it freezes and pushes on the surrounding material. Each winter in a climate like the Philadelphia area's gives that cycle many chances to repeat.
Some crowns were built to fail early. Preservation Brief 47 says the masonry cap should slope to drain water away from the flue, and it warns against mortar washes, the thin skim coats some repairs use, because they tend to crack, let moisture in and promote spalling (brick faces flaking off). A flat or skim-coated top holds water instead of shedding it.
What the crack lets in, and where it shows up
The water doesn't stay at the top. It soaks into the brick and mortar joints below, and that is where you see the damage. A Department of Energy guide on masonry walls notes that caps with inadequate slope, overhang or drip edge let bulk water collect on the masonry below, and it shows severe freeze-thaw damage at a chimney with no projecting cap and drip edge. A drip edge is the lip that hangs past the brick so runoff falls clear instead of running down the face.
So the signs of a failing crown are often a few feet lower than the crown itself:
- Brick faces flaking or popping off near the top of the chimney, which our guide to flaking brick explains in more detail.
- Mortar joints just below the crown that are eroded, cracked or missing.
- White mineral streaks or dark wet staining running down from the top.
- Damp patches inside, in the attic or on the chimney breast, after long rain or a thaw.
A leak at the chimney isn't always the crown. The joint where the chimney meets the roof is protected by flashing, which the Building America Solution Center says must be integrated with the roof flashing to keep water out. If you're not sure which one is failing, our guide to why a chimney leaks even after a new roof walks through telling crown, flashing and mortar leaks apart, and this one covers flashing failures in general.
What you can check without climbing
Stay off the roof. From the ground or a window, use binoculars or your phone's zoom and look for missing chunks, open cracks across the top, a top that sits flush with the brick instead of overhanging it, and loose or leaning brick. Photograph the same spots every few months so a contractor can see whether things are changing. Indoors, write down when dampness shows up compared with rain, snow and thaws.
If pieces are loose or the chimney looks out of line, keep people and cars away from the area below and get it looked at soon. Preservation Brief 47 recommends professional services to repair chimneys and caps; this isn't a ladder job.
What to ask the mason
- Is the crack only in the crown, or does damage continue into the brick and joints below? A crown patch won't fix spalled brick.
- Should the crown be repaired or rebuilt, and why? Ask to see photos of the top up close.
- Will the new crown slope away from the flue and overhang the brick with a drip edge?
- What mortar will you use on joint repairs? For older brick, Brief 47 says repointing mortar should be slightly weaker than the adjacent masonry.
- Is a skim coat or mortar wash part of the plan? If so, ask why, given the Park Service's warning about them.
- Will you check the flashing while you're up there, and how will you handle damage you find once work starts?
Don't skip the safety inspection
A sealed crown says nothing about whether the flue inside is safe. The Consumer Product Safety Commission estimates fireplaces and chimneys were involved in an average of 15,400 fires a year from 2020 to 2022, and says fireplaces can spark fires when chimneys are cracked, blocked or coated with creosote. It recommends a professional inspection of chimneys, flues and vents every year. If the chimney serves a fireplace or furnace, ask for that inspection along with the crown work, and see our pre-season gas fireplace checklist if you burn gas.