Why is my chimney brick flaking?

What spalling looks like

Spalling is the loss of the outer face of a brick or stone. You might see:

• Brick faces that have popped off in flat chips or shallow scoops.
• Flaking or peeling layers, sometimes called delamination.
• Crumbling corners and edges.
• Brick fragments on the roof, in gutters or on the ground near the chimney.
• Mortar joints that stick out past the brick because the brick face has eroded back.

Spalling often concentrates near the top of the chimney, just below the crown, and on the sides that take the most wind-driven rain. On the inside, flaking firebox brick or bits of tile in the fireplace can have different causes, such as heat or flue damage, and need their own evaluation.

How freeze-thaw breaks brick

Brick and mortar are porous. They absorb water. When that water freezes, it expands. If the brick is saturated and the temperature swings above and below freezing many times in a winter, the repeated expansion breaks the outer layer loose. New Jersey winters bring plenty of those cycles.

A chimney is especially exposed. It stands above the roof, catches rain on all sides and on top, and has no overhang to protect it. The Brick Industry Association notes in its maintenance guidance that chimneys see more severe exposure than ordinary walls and may need repairs at shorter intervals.

The key point is that dry brick does not spall from freezing. Spalling tells you the masonry is staying wet. Finding out why is the most important part of the repair.

Other causes of flaking brick

• Salt crystallization. When dissolved salts crystallize just under the surface instead of on it, they can push the face off. The Brick Industry Association describes this as subflorescence or cryptoflorescence.
• Hard mortar in soft brick. When a chimney is repointed with mortar that is harder and less permeable than the brick, water and stress move into the brick instead of the joint. The brick edges then break down first.
• Flue condensation. In unlined or damaged flues serving gas or oil appliances, acidic condensate can soak the masonry from the inside. DuraVent's relining guidance lists spalling among the visible results.
• Pressure washing and harsh cleaning. These can strip the fired outer skin of brick and leave a softer interior exposed.
• Underfired or salvaged brick. Some brick is softer and more absorbent, and it weathers faster.

Why some sealers make spalling worse

It seems logical to coat a flaking chimney so water cannot get in. The problem is that water also gets in from the top, the inside and through joints and cracks. If a coating stops water vapor from escaping through the face, the moisture stays in the brick and freezes there.

The National Park Service's Preservation Brief 1 explains the difference between water-repellent coatings, which are designed to let vapor pass, and waterproof coatings, which block both liquid and vapor. It warns that water trapped in a wall behind a coating can freeze and cause spalling, and that water problems usually come from poor or deferred maintenance rather than from porous masonry. The Brick Industry Association likewise says film-forming coatings should not be applied to exterior brickwork.

More on this in Do chimney sealers work?

How serious is it?

This is general guidance. The actual condition depends on how deep the damage goes, which you cannot judge from the ground.

Repair options

The repair depends on the extent of the damage and its cause.

• Brick replacement. Damaged bricks are cut out and replaced with matching, compatible units.
• Repointing. Deteriorated joints are removed to a consistent depth and refilled with mortar suited to the brick. See masonry repair and repointing.
• Crown and cap work. Stopping water at the top of the chimney. See crown repair and caps.
• Partial or full rebuild. When spalling is extensive, the top courses are loose, or the structure is compromised, rebuilding from a sound point is often more durable than patching. See chimney rebuilding and Repair or rebuild a chimney?
• Liner work if flue condensation is part of the cause.

What you can do now

While you arrange an inspection, a few steps limit further damage and risk:

• Keep people and pets away from the area below the chimney if pieces are falling. Do not park directly under it.
• Do not climb onto the roof to inspect it yourself. Spalled brick and loose crowns make poor handholds.
• Clean out gutters and downspouts near the chimney so water drains away from it.
• Photograph the damage from the ground so changes are easy to spot later.
• If brick or tile fragments are showing up inside the firebox or at the base of the flue, stop using the fireplace or connected appliance until the flue is inspected.
• Hold off on coatings, paint or caulk until the cause is known.

None of these fix the problem. They buy time and help the person inspecting the chimney understand what has been happening.

What affects the scope of the repair

Two spalling chimneys that look similar from the driveway can need very different work. The scope depends on:

• How many bricks are affected and whether the damage is shallow or runs deep into the unit.
• Where the damage is. Damage confined to the top few courses is often handled differently from damage spread down the whole stack.
• The condition of the mortar. Soft, eroded joints throughout usually mean repointing alongside brick replacement.
• The crown and cap. If water is entering from the top, the top has to be corrected or the repair will not last.
• Matching. Older or unusual brick can be harder to match in size, color and hardness.
• Access. Roof pitch, height and the roofing material affect how the work is staged.
• What is happening inside the flue. Condensation damage may add liner work.

For a broader view, see What affects chimney repair cost.

How we assess a spalling chimney

We start with an assessment of the exterior masonry, crown, cap and flashing, from the roof when it is safe. We check the flue with a camera when the flue allows and look at the attic side when it is accessible. You get findings with photos and a clear explanation of whether the damage is local or widespread before any repair is scheduled. If the condition suggests a structural problem, we will say so. Request service to get started.

Questions homeowners ask

Sources

1. Brick Industry Association: Technical Note 46, Maintenance of Brick Masonry
2. Brick Industry Association: Technical Note 23A, Efflorescence: Causes and Prevention
3. National Park Service: Preservation Brief 1, Assessing Cleaning and Water-Repellent Treatments for Historic Masonry Buildings
4. DuraVent: Relining Chimneys for Gas Appliances (Ventinox technical guide, PDF)

This guide is general information. It can't diagnose a specific chimney; an inspection can.