Photograph of a brick chimney cracked exterior, possibly due to heat. Masonry Chimney Crack Inspection & Diagnosis
     

  • CHIMNEY CRACK DETECTION & DIAGNOSIS - CONTENTS: Cracks and spits in brick chimney sides - what is their cause, significance, danger? - How to diagnose the cause of cracks in brick, concrete block and stone chimneys - How to evaluate the importance of cracks in masonry chimneys
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Diagnose cracks in chimneys:

This article catalogs the types of chimney cracks and movement that may be found in brick, stone, or concrete block chimneys; we describe the inspection and and diagnosis of the cause of each type of chimney cracking and we suggest the probable severity, safety concerns, and chimney repairs that may be necessary.

We include links to additional detailed articles about each type of chimney cracking or movement.

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Cracked Brick Masonry Chimney Sides

This article series on chimneys, chimney construction, and chimney safety provide detailed suggestions describing how to perform a thorough visual inspection of chimneys for safety and other defects. Chimney inspection methods and chimney repair methods are also discussed.

Guide to Diagnosing & Evaluating Cracks in Brick Chimneys

Photograph of a brick chimney cracked exterior, possibly due to heat. Photograph of a brick chimney cracked exterior, possibly due to heat.

Our brick chimney photographs just above illustrate a common (and dangerous) crack pattern found in corbeled (stair-stepped) chimneys where a brick chimney passes through an attic floor and is angled over to exit at the chimney ridge.

At the chimney in our photo at above left, look very closely at the masonry joint where the chimney begins its transition from vertical to angled.

To prevent cracks in a leaned-brick chimney such as this one, the chimney depends on absolutely stable support by the roof framing structure where it passes through the roof to outside. Unless the brick chimney was adequately supported and constructed it may lean, causing the crack pattern we show at above right.

Watch out: often the crack in a leaning brick chimney occurs at the attic floor where the chimney begins its transition from vertical to angled - a spot where the crack may be hard to spot.

See CHIMNEY INSPECTION INDOORS for a discussion of chimney movement that opens a hard-to-find crack where a corbeled brick chimney passes through an attic floor.

List of Typical Causes of Cracks in Brick Masonry Chimneys & Flues

Photograph of a damaged masonry chimney.Cracks in a brick masonry chimney such as shown in these photographs may be caused by improper original chimney construction. This damage also appears on concrete block constructed chimneys.

  • Improper construction: failure to leave air space between flue line and masonry chimney sides - if the mason does not leave an expansion gap surrounding the clay flue liners as they are set into the chimney during construction, as the flue heats up during use, the expanding flue may crack the surrounding brick.
  • Water leaks: at the chimney top (missing rain cap, faulty chimney cap top seal), or at the chimney sides (defective flashing, wind-blown rain, open mortar joints) can send water into the chimney structure where in freezing climates frost can lead to cracks to the chimney itself or to its flue liner.

    We illustrate water and frost damage to a brick chimney in this article, below.

    Also see CHIMNEY CAP & CROWN

    and CHIMNEY FLASHING Mistakes & Leaks. And since water and frost can also cause surface spalling of brick or concrete block chimneys,

    also see Chimney Spalling, Exterior.
  • Chimney movement: a chimney which is set on a defective footing or foundation (more improper chimney construction) or a chimney which was not properly secured to the building may lean, bend, or curl, leading to cracks that usually appear in the mortar joints.
    See BRACING for MASONRY CHIMNEYS
  • Thermal expansion cracks in chimneys: a masonry chimney may crack from thermal expansion, or its internal flue may crack from thermal expansion, if the chimney was not properly constructed, failing to leave space for movement as the chimney interior heats up when in use. We illustrate thermal cracking in a brick chimney in this article, just below.

    Also see THERMAL EXPANSION of MATERIALS for a table of the coefficient of expansion of common building materials.
  • Mechanical damaged chimney cracking: a masonry chimney may crack due to stresses from a chimney-mounted antenna, or from earthquake or nearby site blasting stresses.
    See EARTHQUAKE CHIMNEY COLLAPSE DANGERS

The second cracked chimney at above right is a bit more suspect because we see what might be traces of soot or creosote having washed out through the cracks to the chimney exterior. If this proves to be the case this flue is certainly seriously damaged.

Frost Cracking in Brick Chimneys & Flues - outdoor & indoor evidence of brick chimney damage

Frost cracked brick chimney viewed in attic (C) DanieL Friedman


At left we show a very common crack pattern found in brick masonry chimneys & flues - water and frost cracking at the chimney top.

Considering that there is a nice thick concrete chimney cap. why do we have this brick movement and mortar-joint cracking?

Perhaps the chimney cap is cracked, flat, not draining, or it was not sealed around the flue, or a rain cap was missing.

See CHIMNEY CAP & CROWN for detailed examples of defects at the chimney top that lead to this type of chimney damage.

Also see Chimney Spalling, Exterior.

At below left we show a very common crack pattern found in brick masonry chimneys & flues - a collection of vertical, diagonal, and even some horizontal chimney cracks that are probably due to a combination of water intrusion and (in freezing climates) frost cracking.

Frost cracked brick chimney viewed in attic (C) DanieL Friedman

Even if you do not immediately notice the chimney cracks themselves you are likely to spot this chimney damage by the creosote stains carried to the chimney surface by water entry into the chimney flue.

Of course had these cracks and stains been present on a hidden side of the chimney, say between the chimney and a close-by gable-end wall, you'd not see these clues from within the attic.

But inspecting this chimney outside, if it has had no proper rain cap and chimney cap you should be extra alert for water and frost damage to the chimney and its flue.

See CHIMNEY CAP & CROWN

and CHIMNEY FLASHING Mistakes & Leaks

A second set of clues - water leak stains, may be visible in a fireplace or at a chimney cleanout lower in the building.

Thermal Cracking in Brick Chimneys & Flues May Produce Thin Vertical Openings

Photograph of a brick chimney cracked exterior, possibly due to heat.

At left we show a very common crack pattern found in brick masonry chimneys & flues - a vertical crack that begins in a mortar joint and extends through individual bricks themselves.

Cracked chimney masonry such as shown in the photo of cracks in a brick chimney exterior (at left), may a safety concern if the flue liner or chimney are not intact and fire/gas safe.

The brick chimney crack type shown here is more often caused by thermal expansion (and improper chimney construction) than by frost - frost cracking is often more visually obvious and is often accompanied by brick spalling.

 


Severe Chimney Cracking - Deteriorated, Collapsing Brick or Masonry Block Chimneys

Split brick chimney in a basement (C) Daniel Friedman
Severe chimney cracks that risk imminent chimney collapse, flue gas leaks, and fire hazards, are discussed and illustrated in detail
at Split Openings in Brick Chimneys & Chimney Collapse Risk

We describe a case of an imminent catastrophic chimney collapse
at Split Open ings in Brick & Chimney Collapse

Also see our discussion of cracking in a masonry block flue and review serious chimney collapse and failure hazards
at CHIMNEY COLLAPSE RISKS, REPAIRS.

WARNING: Cracks in a chimney can be very significant and dangerous, risking fire or chimney collapse. Be sure to review the articles
at CHIMNEY CRACK & COLLAPSE HAZARDS.

 

Cracks in Masonry Block or Concrete Block Chimneys

Cracks in a concrete block chimney (C) Daniel Friedman

Cracks in a masonry chimney, particularly concrete block chimneys are often caused by

  • chimney footing tipping or settlement (inadequate foundation)\
  • frost (poor drainage, missing rain cap, porous masonry, improper construction in freezing climates)
  • thermal expansion of the clay flue liner (improper construction)
  • mechanical damage,
  • or perhaps other events

See CRACKED CHIMNEYS, MASONRY BLOCK for details and additional photographs of concrete block chimney cracking.

Chimney Cracks due to Chimney Movement, Tipping, Leaning

Chimney separation at house (C) D Friedman A Puentes

Chimneys that lean, curve, bulge, tip, or otherwise move due to footing settlement and tipping or due to failure to secure a tall chimney to the building also may produce both visible cracks on the chimney exterior and hidden cracks and damage to the chimney flue.

The risk of an unsafe chimney flue lies behind our advice that a thorough inspection of the entire chimney flue is necessary when there is any evidence of chimney movement.

See CHIMNEY LEANING, SEPARATION, MOVEMENT: OUTDOORS for details of the detection, analysis, and repair of leaning and tipping chimneys and chimneys that have separated from their building.

Curved Brick Masonry Chimneys

Photograph of a damaged unsafe brick chimney viewed from outside.

Especially on older buildings using brick chimneys, and more so where the chimney flue is not lined with a modern clay liner, brick chimneys may be seen to curve in one direction.

Often all of the similar chimneys in a neighborhood curve in the same direction. It's not a coincidence. A brick chimney will often curve away from its most weather-exposed side due to sulphation - expanding brick mortar joints caused by the combination of water and sulphur or other minerals.

See CURVED BRICK CHIMNEYS, SULPHATION for further explanation of the cause, significance, and cure of curved brick chimneys.

 

 

Continue reading at CRACKED CHIMNEYS, MASONRY BLOCK or select a topic from the index of chimney articles listed above.

Or see CHIMNEY COLLAPSE RISKS, REPAIRS

Or see SPLIT OPENINGS in BRICKS, CHIMNEY COLLAPSE

Suggested citation for this web page

CHIMNEY INSPECTION DIAGNOSIS REPAIR at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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