Chimney Crown Failure in Chicago: The Cheapest Repair You Are Not Making

By Affordable Tuckpointing Pros, a Chicago tuckpointing and brick repair company in business since 1996. License number MC6337.

Direct answer

A chimney crown is the slab across the top of a masonry chimney that sheds water away from the brickwork and away from the gap around the flue liner. BIA calls this element the chimney cap, and says a prefabricated one should be used because it provides better durability and is more easily made water-resistant than a cast-in-place cap. [1] Where a cast-in-place cap is poured instead, BIA says it should take the same shape as the prefabricated one, because the thickened sides and overhangs are what reduce the potential for water penetration. [1] Crowns fail because the thin, feathered, unreinforced, flush-edged version that most Chicago chimneys actually got does none of that: BIA says feathering the cap to the edge should be avoided since it substantially reduces the thickness at the edge and therefore increases the potential for deterioration. [1] BIA also puts caps and copings on the short list of components that need periodic inspection and repair, and warns that neglecting them may lead to deterioration of other elements in the wall. [2] That is the whole argument for fixing a crown early: the crown is not the expensive part, but it is upstream of everything that is.

TL;DR

• BIA treats the top slab as a designed component, not a smear of mortar. A prefabricated cap should be used; it provides better durability and is more easily made water-resistant than a cast-in-place cap. [1]
• A cast-in-place cap should conform to the same shape as the prefabricated one, because the thickened sides and the overhangs are what reduce the potential for water penetration. [1]
• Feathering the cap to a thin edge should be avoided. BIA says it substantially reduces the thickness at the edge and therefore increases the potential for deterioration. [1]
• A crown needs reinforcement. BIA says adequate reinforcement should be placed in the cap to help control cracking due to shrinkage and thermal movements, and that additional reinforcement may be necessary in the overhanging portion. [1]
• A crown needs to be able to move independently of the brick. BIA calls for a bond break between the brickwork and the setting bed so the cap can respond to differential movement without distressing the brickwork. [1]
• The gap around the flue is a detail, not an afterthought. BIA says the cap should be thoroughly primed, backed and sealed at the cap and flue liner interface, and that the flue liner should extend a minimum of 2 in. (50 mm) above the top of the cap. [1]
• If the crown does not overhang, the brick has to do the job instead. BIA says the last two courses should be corbeled out to form a drip to reduce the water running down the face of the chimney. [1]
• Chimneys wear faster than walls. BIA says sills, parapets, chimneys and copings that experience more severe exposures may require repairs at shorter intervals than its general estimates. [2]
• Fixing the crown without fixing what feeds it is wasted money. NPS says the root cause of deterioration should always be dealt with first, or the mortar deterioration will continue and the repointing will have been a waste of time and money. [3]

One liner: The crown is the smallest piece of masonry on your building and the only one holding water off all the rest of it.

What is a chimney crown, and is it the same thing as a chimney cap?

In short: The crown is the slab across the top of the masonry. In BIA language that slab is the “chimney cap,” and the metal thing over the flue is a separate component called a rain cap.

This is the single most confusing piece of vocabulary in chimney work, and it costs homeowners money because two different products get quoted under one word. In the trade and in most Chicago estimates, “crown” means the sloped slab of concrete or mortar poured across the top of the chimney, spanning from the outer edge of the brick in to the flue liner. “Cap” usually means the metal rain hood bolted over the flue opening.

The Brick Industry Association uses the word differently, and since BIA is the document your mason’s specification is ultimately derived from, it is worth knowing. In Technical Notes 19B, the slab is the chimney cap, and it gets its own materials section and its own construction section. [1] The metal hood is the rain cap, and it gets a separate treatment: BIA notes that rain caps vary from sophisticated turbine type metal caps to simple slabs set above the termination point of the flue liner, that manufacturer information on the cap’s effect on gas flow through the chimney should be obtained, and that if the cap is metal it should be corrosion-resistant. [1]

So when you read “cap replacement” on a quote, find out which one is being replaced. A rain cap is a bolt-on part. A crown is a demolition-and-pour job at the top of a chimney, usually with staging, and the two are not in the same price range. They also solve different problems. The rain cap keeps rain and animals out of the flue. The crown keeps water off the brickwork, out of the joints, and out of the gap around the flue liner. A perfect rain cap on a cracked crown solves almost nothing.

Both terms appear on this page. Where the source says “cap” and means the slab, that is noted. Where we use “crown,” we mean the slab.

Why does the crown fail before the rest of the chimney?

In short: Because it is the most exposed masonry on the building, it is usually the thinnest, and it is the one element that has to move against something else.

Start with exposure. BIA observes plainly that the chimney, by the nature of its function, is at least partially exposed to weathering, which is why it specifies brick conforming to ASTM C216 Grade SW or ASTM C62 Grade SW for chimneys, to assure sufficient durability. [1] Grade SW is the severe weathering grade. BIA is telling you, in the materials section, that a chimney is not a wall.

Technical Note 46 makes the same point from the maintenance side. Its estimates for how long building materials last before repairs become necessary are explicitly based on brickwork in vertical applications exposed to normal weathering conditions in the United States, and BIA adds directly that sills, parapets, chimneys and copings that experience more severe exposures may require repairs at shorter intervals. [2] A chimney crown is horizontal, it is at the highest point on the building, it is wetted from above and from all four sides, and nothing shades it. Every assumption behind the general service-life numbers is wrong at the top of a chimney.

Then there is geometry. BIA’s prescription is a cap with thickened sides and overhangs, and it says directly that these are what reduce the potential for water penetration. [1] The version that got built on most Chicago chimneys is the opposite: a thin wash of mortar troweled from the flue out to the brick edge, tapering to nothing at the perimeter. BIA names that specific failure. Feathering the cap to the edge should be avoided, it says, since this substantially reduces the thickness at the edge and therefore the potential for deterioration is increased. [1] The thinnest part of the slab is the part sitting directly over the outside face of the brick, which is exactly where you do not want it to break.

Then there is movement. A chimney crown is a concrete element sitting on a brick element, above a flue liner that gets hot. Those three things expand and contract at different rates and at different times. BIA handles this two ways. For prefabricated caps, which are set in place on a mortar bed, it calls for a bond break between the brickwork and the setting bed to allow the cap to respond to the differential movement it will encounter without distressing the brickwork. [1] For cast-in-place caps, it says flashing is highly recommended, and that the flashing may also be considered as the bond break material. [1] If your crown was poured directly onto the brick with no bond break and no flashing, it is rigidly tied to something that moves differently than it does. It will crack, and the crack is a consequence of the detail, not of bad luck.

And there is shrinkage, which BIA calls out as a certainty rather than a risk. Waterproofing requirements for a cast-in-place cap are different from a prefabricated one, it says, since shrinkage of the concrete as it cures is a certainty. [1] That is why BIA asks for adequate reinforcement to be placed in the cap to help control cracking due to shrinkage and thermal movements, and notes that additional reinforcement may be necessary in the portion of the cap that overhangs the face of the chimney. [1] An unreinforced pour is going to crack as it cures. The only question is whether there is steel in it to keep those cracks tight.

Put those four together and the failure is overdetermined. The most exposed element on the building was poured thin at the edge, unreinforced, bonded rigidly to masonry that moves differently, in a material that shrinks as it cures. Chicago then runs it through freeze and thaw every winter. The crown is not failing early. It is failing on schedule for how it was built.

What does a failing crown actually do to the chimney below it?

In short: It stops being a roof. Water that should have run off the edge goes into the joints, into the gap around the flue, and down inside the stack.

A crown has exactly one job: take the water that lands on the top of the chimney and put it on the ground, not in the masonry. It does that by being impermeable, by sloping, by overhanging far enough that the runoff is thrown clear of the brick face, and by being sealed where it meets the flue liner. Break any one of those and the water goes somewhere else.

The first destination is the crack itself. A crack in a crown is not a cosmetic line, it is a funnel sitting on the top course of brick with several square feet of collection area draining into it. Whatever lands on that slab now arrives at one point in the masonry rather than being shed evenly off the edge.

The second destination is the flue interface. BIA is specific that the cap, whichever type, should be thoroughly primed, backed and sealed at the cap and flue liner interface to reduce the potential for water penetration. [1] That joint is between two materials that move differently and it is the one part of the crown that is guaranteed to be in tension every time a fire is lit. When the sealant there fails, water runs down the outside of the flue liner, inside the chimney, where nothing is designed to shed it and nobody can see it.

The third destination is the face of the brick. This is why the overhang exists. BIA is explicit about the fallback: when using a chimney cap that does not overhang the face of the chimney, the last two courses of the chimney brickwork should be corbeled out to form a drip to help reduce the amount of water allowed to run down the face of the chimney. [1] Note what that sentence assumes. BIA takes it as given that water running down the face of the chimney is a problem worth two courses of corbeled brick to prevent. A crown with no overhang and no corbel means every rainfall washes the full height of the stack.

And once water is in the masonry, you get the conditions that fill BIA’s maintenance checklist. Technical Note 46 lists, under caps, copings and sills, the conditions to look for: inadequate slope, cracked units, hairline cracks in mortar, loose units, open joints, out-of-plumb, and drips needed. [2] Every one of those is either a cause or a consequence of water sitting where it should have been shed. BIA also states the consequence in general terms: neglecting maintenance of components such as caps, copings, sills, lintels and sealant joints may lead to deterioration of other elements in the wall. [2]

There is also a mechanism worth understanding, because it explains why a wet chimney loses brick faces rather than just looking damp. NPS describes it in Preservation Brief 2: when moisture evaporates from masonry it deposits soluble salts either on the surface as efflorescence or below the surface as subflorescence, and while salts on the surface are usually relatively harmless, salt crystallization within a masonry unit creates pressure that can cause parts of the outer surface to spall off or delaminate. [3] Repeated wetting is not neutral. It moves salt into the brick and then breaks the brick from the inside. That is the same mechanism behind spalling brick faces elsewhere on a building, running on a chimney that gets wetted from five directions instead of one.

Finally, the flue. BIA requires flue liners conforming to ASTM C315 and says they should be thoroughly inspected just prior to installation for cracks or other damage that might contribute to smoke and flue gas leakage. [1] Water arriving at the outside of a liner through a failed crown joint, freezing there, and cycling all winter is not a condition the liner was specified for. A crown problem becomes a liner problem, and a liner problem is a combustion safety problem, not a masonry one.

How do you inspect a chimney crown without getting on the roof?

In short: BIA tells you to use binoculars from the ground and from adjacent roofs and balconies, do it each season, and write down what you see.

You are not going to see the top surface of a crown from the sidewalk. You can see a surprising amount of what matters anyway, and BIA sets out the method in Technical Note 46 rather than leaving it to guesswork.

BIA’s general inspection recommendations are: perform periodic inspections, preferably each season; use binoculars, adjacent roof areas and balconies to permit close-range observation of conditions at upper floors; use its Brick Masonry Inspection Checklist to document the condition of the brick masonry; and supplement the checklist with photographs, sketches and notes on floor plans or elevation drawings. [2] It adds that seasonal inspection periods are recommended so that the behavior of building materials in various weather conditions can be observed, and that interior surveys should note stains or damage to finishes that may indicate potential water ingress. [2]

That last item is the one homeowners skip. A crown failure very often shows up first as a stain on an interior wall or ceiling near the chimney chase, months before anyone looks at the top of the chimney. BIA’s Table 3 of possible causes of masonry distress lists moisture-related stains and damaged interior finishes among the observed conditions to work backwards from. [2]

BIA also wants the records kept. Inspection records, including conditions and comments, should be kept to identify changes in materials, any performance issues and necessary repairs, and when maintenance or repairs are undertaken the documentation should include before-and-after photographs, so that both sets of records can be referenced during future inspections to gauge when repairs were last completed and when repairs will become necessary. [2] For a chimney crown this matters more than it sounds. A hairline crack photographed this spring and rephotographed next spring tells you whether you are looking at a stable shrinkage crack or an opening one.

What to actually look at, using BIA’s own checklist line for caps, copings and sills: [2]

• Slope. BIA lists inadequate slope as a checklist condition. A crown should visibly shed away from the flue toward the edge. Standing water or a dark drying pattern on the slab is a slope problem.
• Cracked units and hairline cracks in mortar. On a crown, look at the perimeter first, because that is where BIA says a feathered edge is thinnest and most prone to deterioration. [1]
• Loose units. Any brick in the top two or three courses that looks displaced relative to its neighbours.
• Open joints. Gaps in the mortar joints of the top courses, which is where crown runoff lands.
• Out-of-plumb. Sight the stack against a vertical edge of the building.
• Drips needed. This is BIA telling you to check whether the water has any way to leave the edge of the slab without running back onto the brick.

Then look at the two things next to the crown. The flue liner should be visibly proud of the slab; BIA says it should extend a minimum of 2 in. (50 mm) above the top of the cap. [1] And the joint between the slab and the liner should be an intact sealant joint, not an open gap, because that interface is the one BIA specifically requires to be primed, backed and sealed. [1]

If the chimney is visible from a neighbouring roof or an upper window, that is the shot worth taking. BIA specifically names adjacent balconies or roof areas as a way to observe portions of the facade that are difficult to see from the ground. [2]

Is a cracked crown repaired, resurfaced, or rebuilt?

In short: It depends on whether the slab still has the shape BIA asks for. If it was feathered, flush and unreinforced, you are replacing geometry, and a coating over the old shape does not create geometry.

There are three things a contractor can do to a failing crown, and they are not interchangeable.

Seal the cracks. For genuinely hairline cracking in otherwise sound mortar, BIA does describe face grouting as an option: if mortar joints develop small hairline cracks, surface grouting may be an effective measure to fill them, using a portland cement, hydrated lime and fine sand grout with the sand passing a No. 30 sieve, applied to dampened joints with a stiff fiber brush to force the grout into the cracks, with two coats usually required to effectively reduce moisture penetration. [2] BIA is careful about what this is for. It is a treatment for hairline cracks, and BIA notes that repointing is generally recommended and performed more often because it is better suited to correct various types and severities of mortar joint deterioration compared with face grouting. [2] Sealing a crack does nothing about slope, overhang, thickness or reinforcement.

Resurface the slab. A flexible crown coating troweled over the existing slab is the most commonly sold crown repair in Chicago, and on the right crown it is a defensible maintenance step. On the wrong crown it is money spent on a shape that is the actual problem. Re-read what BIA asks for: thickened sides, overhangs, reinforcement in the slab and additional reinforcement in the overhanging portion, a bond break under it, and flashing highly recommended for cast-in-place work. [1] A coating adds none of those. If the existing slab is feathered to nothing at the edge, coating it produces a coated feathered edge, and BIA’s objection to a feathered edge was never that it was uncoated.

Rebuild the crown. This is removing the existing slab and building the element BIA describes. Two options exist, and BIA has a preference. A prefabricated chimney cap should be used, it says, because this type provides better durability and is more easily made water-resistant than a cast-in-place cap; in the construction section it repeats that prefabricated caps generally provide superior performance as compared to the cast-in-place type. [1] Prefabricated caps are set in place on a mortar bed, with a bond break between the brickwork and the setting bed. [1] If a cast-in-place cap is used instead, BIA says it should conform to the shape and minimum dimensions of the prefabricated detail, with flashing highly recommended, adequate reinforcement to control shrinkage and thermal cracking, and no feathered edge. [1]

One honest limitation. BIA gives the controlling dimensions of a correct cap in a figure rather than in text, and a figure cannot be reproduced reliably from the document as text. So this page does not give you a crown thickness or an overhang projection in inches, because we cannot cite one we actually read. What we can tell you is the full set of requirements BIA states in words: thickened sides, an overhang, no feathering at the edge, reinforcement including extra reinforcement in the overhang, a bond break below, flashing highly recommended on cast-in-place work, a sealed and backed joint at the flue liner, and a flue liner projecting a minimum of 2 in. (50 mm) above the top of the slab. [1] Ask your contractor for the dimensions from Technical Notes 19B Figure 3, and if they cannot name the document, that tells you something.

There is a fourth option people reach for, which is corbelled brick and a mortar wash instead of a slab. BIA addresses the mortar wash in the context of racking the chimney back, and is lukewarm about it: preferred construction consists of a setting bed over the racked face with uncored or paving brick set to provide a weather resistant surface, and mortar washes may also be used, but they may not be as durable, and when using a mortar wash it should not bridge over the rack but should fill each step individually. [1] “May not be as durable” is BIA being polite. A mortar wash is the thing a lot of failed Chicago crowns already are.

If the crown is rebuilt, what does a correct one have?

In short: Shape, steel, a bond break, a sealed flue joint and a way for water to leave the edge. Those are the five things BIA names.

Shape. Thickened sides and overhangs, because BIA says these are what reduce the potential for water penetration, and no feathering to the edge, because BIA says feathering substantially reduces thickness at the edge and increases the potential for deterioration. [1] If the crown cannot be made to overhang, BIA’s substitute is structural rather than cosmetic: the last two courses of chimney brickwork should be corbeled out to form a drip to help reduce the amount of water allowed to run down the face of the chimney. [1]

Steel. Adequate reinforcement placed in the cap to help control cracking due to shrinkage and thermal movements, with additional reinforcement where the cap overhangs the face of the chimney. [1] BIA is not hedging here; it treats shrinkage cracking as a certainty in a cast-in-place pour, which makes reinforcement the thing that decides whether the cracks stay tight or open into channels. [1] For the record, BIA’s general chimney reinforcement specification calls for steel conforming to ASTM A185 welded wire, ASTM A615, A616 or A617 steel bar, or ASTM A82 wire. [1]

A bond break. Between the brickwork and the setting bed for prefabricated caps, so the cap can respond to the differential movement it will encounter without distressing the brickwork; for cast-in-place caps, flashing is highly recommended and may also be considered as the bond break material. [1] This is the detail most often missing, and the one that explains crowns that crack again two years after being rebuilt. A slab poured hard onto the brick has nowhere to go.

A sealed flue interface. Thoroughly primed, backed and sealed at the cap and flue liner interface. [1] “Backed” is doing real work in that sentence: BIA says elsewhere that regardless of the sealant used, proper priming and backing rope are a must. [1] A bead of caulk smeared into an unprimed, unbacked gap is not the detail.

The right sealant. BIA recommends a good grade polysulfide, butyl or silicone rubber sealant, and says oil-based sealants should not be used. [1] It also has a warning about attitude that is worth quoting on any chimney job: caulking is frequently considered a means of correcting or hiding poor workmanship, rather than as an integral part of construction, and it should be detailed and installed with the same care as the other elements of the structure. [1] Technical Note 46 gives the compatible generic families for brickwork as polyurethanes, silicones and polysulfides. [2] It also sets the expectation for how long that joint lasts: BIA’s estimated time to repair for sealant joints is 5 to 20 years, against 100 to 150+ years for brick walls. [2] The sealant at the flue is a maintenance item with a service life, not a permanent part of the chimney.

Flue projection. The flue liner should extend a minimum of 2 in. (50 mm) above the top of the cap. [1] If your flue is flush with the slab or buried in it, water that lands on the crown can run straight into the flue, and the interface BIA wants sealed has no reveal to seal against.

The crown is fixed. What else at the top of that chimney needs looking at?

In short: Flashing at the roofline, the mortar joints in the stack, and whether the chimney meets the height and clearance rules it was supposed to be built to.

Flashing. The second-largest water entry at a chimney is where it passes through the roof, and BIA specifies that junction in detail. Corrosion-resistant sheet metal flashing is required by most building codes, and BIA advises that quality materials should be specified since replacement may be expensive and troublesome. [1] Base flashing is installed first on the faces of the chimney perpendicular to the ridgeline with tabs at each corner, and should extend a minimum of 4 in. (100 mm) up the face of the chimney and along the roof. [1] Counter flashing goes over the base flashing, inserted into a mortar joint for 3/4 to 1 in. (19.1 mm to 25 mm) and mortared solidly into the joint, lapping the base flashing by at least 3 in. (75 mm). [1] Where flashing is installed in sections, the flashing higher up the roofline should lap over the lower flashing a minimum of 2 in. (50 mm), all joints in base and counter flashing should be thoroughly sealed, and the unexposed side of any bends should also be sealed. [1]

That is a checkable specification. Counter flashing that is surface-mounted and caulked to the brick rather than let into a mortar joint is not what BIA describes, and it is one of the most common shortcuts on a chimney. Technical Note 46 adds the maintenance view: flashing that has been omitted, damaged or improperly installed may permit moisture to penetrate to the building interior, and correcting it is invasive, requiring removal of brick, bracing the brick above, installing new flashing and replacing the removed brick units. [2] It also warns that other methods may be used to address water penetration, but these are not necessarily long-term solutions and will not comply with the building code when flashing is missing. [2] Caulk over a flashing failure is that shortcut.

A cricket. On a wide chimney, water coming down the roof has to get around it. BIA says a cricket is usually wanted for chimneys whose dimension parallel to the ridgeline is greater than 30 in. (750 mm) and which do not intersect the ridgeline, that its intersection with the chimney should be flashed and counter flashed in the same manner as a normal chimney roof intersection, and that flashing at the roofline should extend to at least 4 in. (100 mm) under the roofing material. [1] If you have a wide chimney mid-slope with no cricket, there is a pond against the upslope face every time it rains.

The joints in the stack. Crown failure and open joints travel together, because the crown is what was supposed to keep water out of those joints. Technical Note 46 gives the conditions that call for repointing rather than leaving it to judgement: mortar erosion exceeding 1/4 in. (6.4 mm), crumbling mortar, mortar with voids, hairline cracks in the mortar, and cracks between the brick and mortar. [2] It sets the preparation standard too: deteriorated mortar should be removed by toothing chisel or a special pointer’s grinder to a uniform depth of a minimum of twice the joint width, generally 3/4 in. (19 mm), or until sound mortar is reached. [2] NPS puts the same requirement slightly differently, calling for removal to a minimum depth of 2 to 2 1/2 times the width of the joint, which for most brick joints means approximately 1/2 to 1 inch, with any loose or disintegrated mortar beyond that minimum depth also removed. [3] If the naming confuses you, we have written separately on what tuckpointing and repointing actually mean; for a chimney, the work in scope is repointing, and our tuckpointing page covers how we price it.

Mortar selection on a chimney is not a free choice either. BIA recommends Type N portland cement-lime mortar for the chimney to allow for both weathering and thermal considerations, with Type S acceptable and sometimes necessary where the chimney is subjected to high lateral forces such as wind loads in excess of 25 psf (1.2 kPa) or seismic loads, and Type M where the chimney is in contact with earth. [1] For the joints that bed the flue liners, BIA highly recommends fireclay mortars because they need to perform under high temperatures, with Type N portland cement-lime mortar as an acceptable substitute. [1]

And the strength rule that governs all repointing applies here with extra force, because a chimney thermally cycles. BIA states that to avoid irreparable brick damage the compressive strength of the repointing mortar must be equal to or lower than that of the original mortar, and that using a mortar with higher compressive strength may significantly impair the surrounding brickwork by increasing stress concentration at the brick/mortar interface, which can lead to spalling of the brick face. [2] NPS makes the mechanism explicit: a mortar stronger in compressive strength than the masonry units will not “give,” so stresses caused by expansion, contraction, moisture migration or settlement get relieved through the masonry units instead, resulting in permanent damage such as cracking and spalling that cannot be repaired easily. [3]

Height, clearance and corbeling. Worth confirming while there is access, because these are the rules the chimney was supposed to be built to and they are easy to check. BIA lists code requirements accepted nearly everywhere: chimney wall thickness a nominal 4 in. (100 mm) unless no flue liner is used, in which case a nominal 8 in. (200 mm) is required; minimum chimney height for fire safety of the greater of 3 ft (1.0 m) above the highest point where the chimney penetrates the roofline or 2 ft (600 mm) higher than any portion of the structure or adjoining structures within 10 ft (3.0 m); minimum 2 in. (50 mm) clearance from combustible material, except 1 in. (25 mm) where the chimney is located entirely outside the structure; and all exterior spaces between the chimney and adjacent components sealed, most commonly by flashing and caulking. [1] On corbeling, masonry chimneys should not be corbeled more than 6 in. (150 mm) from a wall or foundation, and corbeling may not exceed 1 in. (25 mm) projection for each course of brick. [1] Local Chicago requirements govern; BIA notes that code requirements for chimneys may vary on a local basis. [1]

Should the crown or the chimney be sealed after the repair?

In short: Only after the repairs are done, only with a vapour-permeable repellent, and never instead of the crown, the flashing or the joints.

BIA does allow for it, in narrow terms, and the chimney is one of the specific cases it names. Technical Note 46 says water repellents may be used to correct minor deficiencies that remain after completion of repairs, or to reduce the amount of water absorbed by barrier walls and masonry subject to extreme exposures, such as chimneys, parapets, copings and sills. [2] Your chimney is on that list. But every clause around it matters.

The sequence is not negotiable. BIA says use of external coatings on brick masonry should be considered only after completing repair and replacement of brick, mortar joints and other building elements, and careful consideration of the possible consequences. [2] Sealing a chimney with a cracked crown seals water in on the way out while doing nothing about the water coming in from the top.

The substitution is not allowed either. Water repellents and coatings should not replace or be considered equivalent to essential, code-required details that resist water penetration, such as flashing and weeps. [2] And BIA warns that use of coatings for reasons outside their intended application rarely reduces water penetration and may lead to more serious complications with the brickwork. [2]

The product class is constrained. Only water repellents that permit evaporation and the passage of water vapour, such as siloxanes and silanes, should be used on exterior brickwork, and film-forming coatings should not be applied to exterior brickwork. [2] The reason connects back to the salt mechanism NPS describes: if moisture cannot migrate out of the masonry and evaporate, the result is damage to the masonry units. [3]

And it has a service life. BIA’s estimated time to repair for water repellents on walls is 5 to 10 years, which is the shortest interval on its table other than paint finishes. [2] On a chimney, which BIA has already told you is a more severe exposure than a wall, expect the lower end. [2] A repellent is a maintenance subscription, not a repair.

When is a chimney a structural problem rather than a maintenance one?

In short: BIA draws that line explicitly, and several of the conditions on its structural list show up on neglected chimneys.

Technical Note 46 is a maintenance document and says so. Structural issues are more involved than maintenance and are outside its scope, and BIA says such issues generally warrant an investigation performed by a professional engineer to determine the cause and recommend the appropriate repair method. [2] It then warns that indications of structural issues can sometimes appear similar to conditions that require maintenance, which is the trap. [2]

The list BIA gives of observed conditions that are structural includes, but is not limited to: cracks in brick masonry exceeding 0.075 in. (2 mm) in width; cracks through multiple brick units; cracks following a stepped or diagonal pattern; widespread spalled brick; accumulated rust on lintels or shelf angles; out-of-plane movement of brick masonry or other wall elements; and neutral or negative slope on masonry sills and water tables. [2]

Read that against a chimney that has been taking water through a failed crown for a decade. Widespread spalled brick in the stack is on the list. Out-of-plane movement is on the list, and “out-of-plumb” is the very thing BIA’s own caps and copings checklist row tells you to look for. [2] Cracks through multiple units are on the list. A leaning chimney, a chimney shedding brick faces on the weather side, or a chimney with cracking running through brick rather than along joints, is not a crown job with some repointing attached. It is an engineer’s assessment first.

BIA also gives the order of operations for anything moisture-related, and it is the opposite of how most chimney work gets sold. Once a condition becomes evident, it says, the origin of the problem should be determined and action taken to correct both the cause and the visible effect of the condition. [2] It suggests first visually inspecting for a self-evident source before performing a more extensive investigation, as this may save time and money, and says such a process should always be followed if the condition involves water penetration. [2]

NPS says the same thing about repointing more bluntly, and it is the single most useful sentence a homeowner can carry into a chimney estimate: it is erroneous to assume that repointing alone will solve deficiencies that result from other problems, and the root cause of the deterioration, whether leaking roofs or gutters, differential settlement of the building, capillary action causing rising damp, or extreme weather exposure, should always be dealt with prior to beginning work, because without appropriate repairs to eliminate the source of the problem, mortar deterioration will continue and any repointing will have been a waste of time and money. [3]

A chimney crown is very often that root cause. Repointing a stack under a broken crown is the exact scenario NPS is describing.

Who should do the work, and when in the year?

In short: Repointing is a specialist skill rather than a general masonry one, and wall temperature governs the schedule.

Both documents are unusually direct about competence. BIA says repointing operations should be performed only by qualified and experienced repointing craftspeople, and adds the sentence that should be read out loud at every estimate: an individual who is an excellent mason may not be qualified for repointing, and skills should be tested and evaluated prior to the selection of the contractor. [2] NPS goes further for historic work, recommending that specifications stipulate that masons must have a minimum of five years’ experience with repointing historic masonry buildings to be eligible to bid. [3]

BIA also flags the risk that makes this a specialist job rather than a general one: the potential for power tools to damage the brick surrounding the mortar being removed, and the fact that using a grinder to remove head joint mortar will not remove the full depth of material without damaging adjacent brick, so the extra mortar left in head joints must be removed by chisel to achieve a uniform depth. [2] NPS is harder on power tools still: the traditional method of hand chisels and mash hammers poses the least threat of damage and produces the best final product, while using power saws on walls with thin joints, such as most brick walls, almost always results in damage to the masonry units by breaking the edges and by overcutting the head joints. [3]

NPS also suggests a way to check work before it covers the whole chimney: test panels prepared by the contractor using the same techniques that will be used on the rest of the project, with a 3 foot by 3 foot area usually sufficient for brickwork, establishing an acceptable standard of work and serving as a benchmark for evaluating and accepting subsequent work. [3] On a chimney that is easy to arrange, because one face is naturally the least visible.

On timing, NPS gives the number that governs the season: generally speaking, wall temperatures between 40 and 95 degrees F (8 and 38 degrees C) will prevent freezing or excessive evaporation of the water in the mortar. [3] Note that it says wall temperature, not air temperature, which on an exposed chimney in Chicago is not the same thing. NPS adds that repointing should ideally be done in shade, away from strong sunlight, to slow the drying process, and that the use of antifreeze compounds is not recommended. [3]

BIA’s placement requirements assume the same conditions. Repointing mortar should be prehydrated to reduce excessive shrinkage, mixed to a damp consistency that retains its shape when formed into a ball 1 to 1 1/2 hours before adding water for placement; the joints should be dampened but the brickwork must absorb all surface water before the mortar is placed; and the mortar should be packed tightly into the joints in layers no more than 1/4 in. (6.4 mm) thick and tooled when thumbprint hard, with the last layer tooled to match the original profile. [2] None of that works on a frozen wall.

One scoping note from NPS that applies well to chimneys: it is preferable to repoint only those areas that require work rather than an entire wall, but if 25 to 50 percent or more of a wall needs to be repointed, repointing the entire wall may be more cost effective than spot repointing, and total repointing may also be more sensible when access is difficult and requires the erection of expensive scaffolding. [3] A chimney is the definition of difficult access. Once staging is up for the crown, the economics of doing the whole stack change.

Chimney crown questions Chicago homeowners ask

Is a chimney crown the same as a chimney cap?

Not in trade usage. The crown is the slab across the top of the masonry; the cap is usually the metal rain hood over the flue. BIA uses “chimney cap” for the slab and treats the metal hood separately as a “rain cap,” noting that rain caps range from turbine type metal caps to simple slabs set above the termination point of the flue liner and that a metal one should be corrosion-resistant. [1] Check which one a quote means before comparing prices.

Can a cracked chimney crown just be sealed or coated?

For genuinely hairline cracking, BIA describes face grouting with a portland cement, hydrated lime and fine sand mixture applied with a stiff fiber brush, usually in two coats, as an effective measure to fill small hairline cracks. [2] What a coating cannot do is add the things BIA says make a crown work: thickened sides, an overhang, reinforcement, and a bond break. [1] If the slab was feathered thin at the edge, coating it leaves the condition BIA specifically says to avoid. [1]

Why does my crown keep cracking after it was rebuilt?

The most likely missing detail is the bond break. BIA calls for a bond break between the brickwork and the setting bed so the cap can respond to the differential movement it will encounter without distressing the brickwork, and for cast-in-place caps says flashing is highly recommended and may serve as that bond break material. [1] The second likely cause is reinforcement: BIA treats shrinkage of a cast-in-place pour as a certainty and asks for adequate reinforcement to control cracking due to shrinkage and thermal movements, with additional reinforcement in the overhanging portion. [1]

How far should a chimney crown overhang the brick?

BIA gives the controlling dimensions in a figure rather than in text, so we will not quote a number we have not read. What BIA states in words is that the thickened sides and overhangs are what reduce the potential for water penetration, that a cast-in-place cap should conform to the shape and minimum dimensions of the prefabricated detail, and that where a cap does not overhang, the last two courses of chimney brickwork should be corbeled out to form a drip. [1] Ask your contractor to work to Technical Notes 19B Figure 3.

How high should the flue liner stick up above the crown?

BIA says the flue liner should extend a minimum of 2 in. (50 mm) above the top of the cap. [1] That projection also gives the sealant joint something to seal against, which matters because BIA requires the cap to be thoroughly primed, backed and sealed at the cap and flue liner interface. [1]

How often should I be looking at my chimney?

BIA recommends periodic inspections, preferably each season, using binoculars and adjacent roof areas or balconies for close-range observation of upper floors, documented with photographs, sketches and notes. [2] It also recommends keeping the inspection and repair records to reference during future inspections, so you can tell a stable crack from an opening one. [2]

Does a chimney wear out faster than the rest of my brick?

Yes, and BIA says so directly. Its estimated repair intervals are based on brickwork in vertical applications under normal weathering, and it adds that sills, parapets, chimneys and copings that experience more severe exposures may require repairs at shorter intervals. [2] That is why BIA specifies Grade SW brick for chimneys, the severe weathering grade. [1]

Should I waterproof the chimney after the crown is fixed?

BIA names chimneys as one of the cases where a water repellent may be warranted, to reduce the amount of water absorbed by masonry subject to extreme exposures. [2] But only after completing repair and replacement of brick, mortar joints and other building elements; only with a vapour-permeable product such as a siloxane or silane, never a film-forming coating; and never as a replacement for code-required details like flashing. [2] Expect to redo it: BIA puts water repellents on walls at 5 to 10 years. [2]

My chimney leans and is losing brick faces. Is that still a crown repair?

Probably not on its own. BIA lists out-of-plane movement of brick masonry, widespread spalled brick, cracks through multiple brick units and cracks exceeding 0.075 in. (2 mm) as structural conditions that are outside the scope of maintenance and generally warrant investigation by a professional engineer. [2] Get that assessment before anyone prices masonry.

Can crown work be done in a Chicago winter?

The constraint is wall temperature, not the calendar. NPS gives wall temperatures between 40 and 95 degrees F (8 and 38 degrees C) as the range that prevents freezing or excessive evaporation of the water in the mortar, and says antifreeze compounds are not recommended. [3] On an exposed chimney, wall temperature runs colder than air temperature.

Stats box

Definition bank

Entity cards

Chimney crown

Chimney flashing

Chimney mortar joints

Chimney water repellent

What to ask before anyone prices your crown

In short: Ask which element is being replaced, what shape the new one will be, and where the water goes after it leaves it. Those three questions separate a repair from a recoat.

A chimney crown is the cheapest piece of masonry on your building to get right and one of the most expensive to keep ignoring, because BIA is explicit that neglecting caps, copings, sills, lintels and sealant joints may lead to deterioration of other elements in the wall. [2] Everything below the crown is downstream of it.

So when the quote arrives, look for four things. First, which element: the slab or the rain cap, because BIA treats them as separate components and they are not in the same price range. [1] Second, the shape: thickened sides, a real overhang or corbeled drip courses, and no feathered edge, since BIA says feathering substantially increases the potential for deterioration. [1] Third, what is inside and underneath it: reinforcement to control shrinkage and thermal cracking, and a bond break or flashing so the slab can move without distressing the brick. [1] Fourth, the flue interface: primed, backed and sealed, with the liner standing a minimum of 2 in. above the slab. [1]

And ask the question that decides whether any of it lasts, which is where the water goes. BIA’s order of operations for anything moisture-related is to determine the origin of the problem and correct both the cause and the visible effect. [2] NPS puts it more bluntly: without appropriate repairs to eliminate the source of the problem, mortar deterioration will continue and any repointing will have been a waste of time and money. [3] If a quote repoints the stack and leaves the crown, or coats the crown and leaves the flashing, it has priced the symptom.

If the chimney is leaning, shedding brick faces, or cracking through units rather than along joints, that is BIA’s structural list rather than its maintenance list, and the right first call is an engineer. [2] We will tell you that rather than sell you a crown.

If you want someone to read your chimney and tell you which of these you are actually looking at, see our chimney crown repair page, or get a free quote and we will come look at it.

Sources

1. Brick Industry Association, Technical Notes 19B, “Residential Chimneys: Design and Construction”, rev. June 1980, reissued April 1998. https://www.gobrick.com/media/file/19b-residential-chimneys—design-and-construction.pdf
2. Brick Industry Association, Technical Note 46, “Maintenance of Brick Masonry”, December 2017. https://www.gobrick.com/media/file/46-maintenance-of-brick-masonry.pdf
3. National Park Service, Preservation Brief 2, “Repointing Mortar Joints in Historic Masonry Buildings”, Robert C. Mack and John P. Speweik, rev. 1998. https://www.nps.gov/orgs/1739/upload/preservation-brief-02-repointing.pdf