How salt and water push the face off chimney brick

What spalling looks like from the yard

Spalling chimney brick usually shows itself in pieces before anyone looks up. Look for thin flakes or curved chips lying on the roof, collecting in the gutter or scattered at the foot of the chimney. On the chimney itself, spalled bricks have a rough, pitted or cratered face where the smooth outer skin has come away, and the damage often sits in the top courses or on one side.

Flaking chimney brick is different from the white powder of efflorescence, although the two often appear together. Efflorescence is a deposit on top of the brick. Spalling is the loss of the brick's own surface. Our guide to white stains on chimney brick covers the deposit. If pieces are falling, keep people, pets and cars clear of the area below until the chimney has been checked.

How sea salt ends up inside a chimney

Salt reaches masonry in more than one way. The Brick Industry Association's efflorescence note lists rainwater among the sources of soluble salts in masonry and names sodium chloride, with seawater as its source, among the salts it finds. The International Molybdenum Association's guidance on coastal metal adds that how far inland sea salt travels depends on the local wind, which is why exposure can vary from one street to the next.

On a chimney within reach of salt spray, the likely sequence runs like this. Spray leaves a salty film on the brick and mortar. Rain dissolves it and carries it into the pores. The chimney then dries in sun and wind, and the salt is left behind somewhere in the brick. Repeated wetting and drying can keep adding to it.

Crystals that grow just under the face

Where the salt ends up decides whether it does harm. The National Park Service's guidance on repointing explains that when moisture evaporates from masonry, it leaves soluble salts either on the surface, where they show as efflorescence, or below the surface as subflorescence. Surface salts are usually harmless. Salt crystallizing inside a brick creates pressure that can make parts of the outer surface spall off or delaminate.

The Brick Industry Association uses the term cryptoflorescence for the same hidden deposit and describes the pressure it develops as tremendous. That is the mechanism behind salt spalling. The face does not wear away from outside; it is pushed off from just behind. When people ask what causes spalling brick near the coast, this is the first mechanism to rule in or out.

Telling salt damage from water, mortar and coatings

Other causes produce spalling too, and the repair depends on which one is at work. The pattern usually gives it away, and it is worth describing what you see before anyone proposes a fix.

• Salt spalling tends to follow the faces that stay wet longest and dry in the sun, often near the top and on the side that takes wind-driven rain, and it often comes with white deposits nearby.
• Water alone, entering through open joints, a cracked crown or failed flashing, keeps a zone of brick saturated. The Brick Industry Association notes that moisture then penetrates further through any broken or heavily spalled units.
• Mortar harder than the brick shows a different pattern. The National Park Service explains that mortar harder than the brick cannot flex, so the stress is relieved through the brick itself, and the brick cracks and spalls. Look for damage lining up with joints that were repointed in a hard, gray mortar.
• A coating on the brick shows as a surface film peeling in sheets or broad, shallow flaking across a whole face, sometimes with blisters where moisture pushed from behind.

Why a sealed or painted chimney spalls faster

A coating looks like protection, so it is often the first thing suggested. The National Park Service separates two kinds. Waterproof coatings, which include some elastomeric paints, seal the surface against both liquid water and water vapor. Water repellents let vapor pass but still reduce how freely the masonry can breathe. Its guidance warns that subflorescence may cause the surface to spall, particularly where a water repellent has been applied, because the coating slows the moisture on its way out.

In its efflorescence note, the Brick Industry Association reaches the same conclusion. Salts carried toward a coated face stop at the inner edge of the coating, where the water escapes as vapor and the salt is left inside the brick. On a chimney already spalling, the National Park Service adds that a coating can form a crust that later pulls off even more of the surface. And coatings can be very difficult to remove once they fail. On a salty, damp chimney, a film is the wrong first step.

Replacing brick so the problem does not move next door

Repair follows an order. The National Park Service stresses that the root cause of deterioration has to be dealt with before the masonry is repaired, and for salt spalling the cause is the water that keeps carrying salt in. That means the crown, flashing and joints come first, followed by the brick.

Brick replacement itself is laid out in the Brick Industry Association's maintenance note. Damaged units are cut out without harming the bricks around them, the new brick is chosen to match the appearance of the old using sample panels, and the new mortar should have physical properties similar to the existing mortar with a compressive strength no higher than the original. The note warns that the wrong mortar can cause spalling and cracking in the surrounding masonry. A hard patch set beside soft old brick simply moves the damage next door.

The same note singles out chimneys, along with sills, parapets and copings, as places whose harsher exposure can bring repairs around sooner. That is not a reason to wait. Broken or heavily spalled brick lets more water in, which feeds more salt into the bricks beside it.

How urgent a spalled chimney is

Urgency depends on what is coming loose. A few shallow flakes on one face are a reason to plan repairs before the next wet season, not an emergency. Bricks that have shifted out of line, joints open deep enough to see into, pieces landing on a walkway or damage near the top above a door or driveway call for a prompt look, and the area below should be kept clear in the meantime.

Damage inside the firebox is a separate matter, because that brick contains the fire. Leave the fireplace off until spalled or cracked firebox brick has been checked, whatever the outside of the chimney looks like.

What to note before anyone climbs up

• Where the flakes are landing, and roughly how much appears after a storm.
• Which side of the chimney is damaged, and whether the damage is near the top, spread across a face or next to particular joints.
• Any white deposits, dark wet patches or stains near the damaged area.
• Whether the chimney was ever painted, sealed or repointed, and roughly when.
• Any damage inside the firebox or on the chimney breast indoors.

Stopping the water before the brick work

At a free inspection we look at the damaged brick and at the water behind it, meaning the crown, the flashing, the joints and any coating on the face. The estimate we write separates the work that stops the water from the spalling brick repair itself, and from wider chimney masonry repair where joints need attention, with each part priced upfront. While the chimney is open, it is covered when each work day ends, and the debris leaves with us.

Sources

• National Park Service Preservation Brief 2, Repointing Mortar Joints in Historic Masonry Buildings: Subflorescence and salt crystallization spalling the surface, mortar harder than the brick, and fixing the root cause first.
• National Park Service Preservation Brief 1, Cleaning and Water-Repellent Treatments: Waterproof coatings versus water repellents, and spalling behind coatings.
• Brick Industry Association Technical Note 23A, Efflorescence, Causes and Prevention: Rainwater and seawater as salt sources, and cryptoflorescence pressure spalling brick.
• Brick Industry Association Technical Note 46, Maintenance of Brick Masonry: Chimney exposure, replacing damaged brick, matching new units and choosing compatible mortar.
• Brick Industry Association Technical Note 7, Water Penetration Resistance, Design and Detailing: The severe wind-driven rain rating given to the Eastern Seaboard and Gulf Coast.
• International Molybdenum Association, stainless steel selection for exterior applications: Local wind patterns deciding how far sea salt travels inland.