Sealer, Recast or Precast Cap: Repairing a Cracked Chimney Crown in Albany

Three ways a chimney top gets built

Most chimney tops are one of three things. A mortar wash is mortar troweled over the top course; CSIA calls sand-and-mortar washes the more commonly found kind, and the Brick Industry Association (BIA) allows them while warning that they may not be as durable. A cast-in-place crown is concrete formed and poured on the chimney. A prefabricated, or precast, cap is made off the roof and set in place, and the BIA's chimney note recommends it, saying it provides better durability and is more easily made water-resistant than a cast-in-place cap.

The terms overlap. The BIA and the model code call this masonry lid a cap, masons usually call it the crown, and on a quote a chimney cap more often means the metal hood over the flue. Whatever the name, IRC section R1003.9.1 calls for a concrete, metal or stone cap, a drip edge and a caulked bond break around any flue liners, and it requires the cap to be sloped to shed water. A thin mortar wash troweled flush with the brick, with no drip edge and no bond break around the tiles, falls short of that description before it ever cracks.

The details that decide how long a crown lasts

A handful of details matter more than the choice of material. Each one targets the two things that break crowns: water running where it should not, and movement between materials that expand and contract at different rates. In Albany, where the temperature crosses freezing on roughly 95 days in an average year, water left standing in a hairline crack gets many chances to freeze.

• Overhang: CSIA recommends that a crown project 2 inches beyond every side of the chimney to help prevent erosion of the brick and mortar below.
• Drip edge: the model code requires one, and where a cap does not overhang, the BIA suggests corbeling the last two courses of brick out to form a drip.
• Bond break: the BIA calls for a break between the brickwork and the setting bed so the cap can respond to differential movement, and says flashing under a cast crown can serve as that break.
• Flue joint: NFPA 211, as cited in CSIA's inspection procedure, requires a bond break where the top tile passes through the crown, allowing vertical expansion of the flue.
• Flue projection: the BIA says the liner should extend at least 2 inches above the top of the cap.
• Edges: feathering a crown thin at its edges should be avoided, the BIA says, because it cuts the thickness at the edge and raises the potential for deterioration.
• Sealant: at the joint between cap and liner, the BIA recommends a good grade polysulfide, butyl or silicone rubber sealant and rules out oil-based sealants.

What a crack sealer can and cannot do

Elastomeric crown sealers are the least invasive option. One manufacturer's data sheet describes its product as a fast-setting elastomeric coating that forms a permanently flexible waterproof membrane, and that flexibility is the point on a crown whose hairline cracks move with the seasons. The same sheet sets conditions. The surface must be structurally sound, not loose, and clean and dry, and larger cracks and voids are filled with patching cement or latex caulk before the coating goes on.

Temperature is the other limit. The data sheet calls for at least 45 degrees F at application, no more than 95 degrees F, and no application if the temperature will drop below 45 degrees F within 4 hours. National Weather Service normals put Albany's average daily high below 45 degrees F from December through February, so coating season effectively pauses for the winter, and in late fall a mild afternoon followed by a cold evening can rule out a day. What the sheet does not say is when a crown is too far gone to coat; that call comes from the crown, not the label.

When the answer is a recast or a precast cap

A coating cannot add thickness, slope, overhang or a bond break that was never there. A crown that is loose, crumbling, feathered thin at the edges or troweled flush with the brick is a candidate for replacement, and CSIA calls a cast crown with an expansion joint an ideal long-lasting replacement for the more commonly found sand-and-mortar washes. Because concrete shrinks as it cures, the BIA highly recommends flashing under cast-in-place caps; its stated preference is still the prefabricated cap.

Age is the local factor. Census figures show 42.9 percent of Albany's housing was built before 1940, and many older chimneys were finished with a trowel of mortar rather than a formed crown. Common brick, as CSIA puts it, is like a sponge, absorbing water and wicking moisture to the chimney interior, so a failed top feeds water to every course below it, where freeze-thaw action spalls brick faces. On a party-wall chimney shared by two rowhouses, one crown may cover flues serving both houses, which puts the neighbor's flue projection and bond break in the same job. A written scope for chimney crown repair should name the type of top being built and list each detail above.

Timing the work and checking the height

Crown work belongs in the same mild-weather window as other masonry, since fresh material has to cure before it freezes; the National Park Service's repointing guidance, for instance, calls for protecting completed work from freezing. A recast also changes the top of the chimney, which makes it a sensible moment to confirm the stack still meets the 3-2-10 chimney height rule, especially if a roof or addition within 10 feet has gone up since the chimney was built.

Owners gathering Albany chimney repair quotes for a crown can ask every bidder the same questions. Is the existing top a mortar wash, a cast crown or a cap? Will the new one overhang 2 inches with a drip edge? Where is the bond break, and how far will the flue tile stand above the finished surface? If a sealer is proposed, what temperatures do the day and the following evening need to hold? Matching answers make the bids comparable.

More from the blog

• Is Chimney Repair Covered by Homeowners Insurance in Albany?
• The 3-2-10 Rule: How Tall an Albany Chimney Has to Be
• When to Schedule Chimney Repair in Albany: Mortar, Frost and the Burn Season