Crumbling Mortar Joints on a Chimney

Why the joints go before the brick

A well-built brick wall is designed with the mortar as the weaker partner. The National Park Service's guidance on repointing explains that mortar should be softer or more permeable than the bricks it holds, so that movement and moisture are relieved through the joints rather than through the brick. Joints can be renewed. Bricks are much harder to replace well.

That design means crumbling joints are partly the system working as intended. The question is pace. A chimney above the roof takes rain on every face and goes through more freeze and thaw cycles than a wall under the eaves, and BIA's maintenance guidance says chimneys, parapets and sills under severe exposure may need repairs at shorter intervals than the rest of a building. Mortar that wears out in a handful of winters, or only in one spot, is reacting to something.

Four places to look, safest first

You can learn a lot without setting foot on the roof. Work through these in order and take dated photos at each.

1. From the yardUse binoculars or a phone zoom on each face of the chimney. Look for joints that read as dark shadow lines rather than flat bands, bricks that look outlined, plants or moss growing from joints, and patches where a newer, different-colored mortar was smeared over the old.
2. At the roof edge and belowCheck gutters and the ground under the chimney for sand, grit and small mortar crumbs after a storm. A gutter that collects sand below a chimney is evidence that something above it is eroding.
3. In the atticIf there is solid flooring to stand on, find the chimney where it rises through the attic. Loose mortar on the attic floor, sandy joints on the chimney itself or water marks on the masonry are worth photographing.
4. In the basement and fireboxLook at the chimney base, the area around the cleanout door, and the joints inside a cold firebox. Sand or bits of masonry collecting at the bottom of a flue, which DuraVent lists among the signs of condensation damage from gas appliances, means the deterioration may be inside the chimney rather than on its face.

Stay off the roof. Crumbling joints near the top often mean loose bricks, and a chimney is the worst possible handhold.

The scrape and depth check, where you can reach

If any part of the chimney is within reach from the ground or the basement, you can test the mortar directly. BIA describes light scraping with a metal tool, together with a visual check, as the common way to find joints that need repointing. A key or a small screwdriver is enough.

• Drag the tip lightly along a joint. Sound mortar resists and leaves a faint line. Deteriorated mortar sands away or breaks out in crumbs under light pressure.
• Measure how far the mortar sits back from the brick face. Lay a straightedge across the brick and measure to the mortar. BIA uses erosion of more than 1/4 inch as one trigger for repointing.
• Look at the edges. A hairline gap between the mortar and the brick, even where the mortar itself is hard, is also on BIA's list.
• Compare. Test a sheltered spot and an exposed one. A big difference between them is a clue about where the water is going.

Scrape gently and stop there. Digging out joints is part of the repair itself and has to be done to a consistent depth with compatible mortar to follow. The results from reachable joints also do not tell you about joints thirty feet up.

Reading where the loss is concentrated

Mortar that fails evenly over a very old chimney is often simply worn out. Mortar that fails in one zone usually has a water source above or beside it. Some patterns that are worth describing to whoever inspects the chimney:

• The top few courses, just below the crown. Water from a cracked or flat crown, or from rain falling around an uncapped flue, soaks this area first. See crown vs. cap.
• One face only. The side that takes the prevailing storms wears faster. That face may also be the one with the worst flashing.
• A band just above the roofline. Counterflashing set into joints there can work loose, and water running off the flashing can keep that band wet.
• Near the base of an outside chimney. The NPS repointing brief shows an example of mortar leached away by water from a leaking downspout, and it names leaking gutters and rising damp among the root causes that must be dealt with before repointing.
• Inside the firebox or at the flue base. These have their own causes, covered below.

A pattern is a lead, not a diagnosis. Two chimneys with the same eroded band can have different sources, which is why the parts above the damage get checked first.

Joints you cannot see from outside

A chimney has mortar joints that never see weather. They fail for different reasons and matter for different reasons.

Flue liner joints. The 2003 International Residential Code specified medium-duty refractory mortar for setting clay flue liners, with joints kept tight and smooth on the flue side. An open or eroded liner joint lets heat and flue gases reach the masonry around the liner. These joints can only be seen with a camera run through the flue. See what cracked flue tiles mean.

Firebox joints. The same code text limits joints between firebricks to 1/4 inch and calls for refractory mortar there. Eroded firebox joints let heat into the masonry behind the firebrick. See inside the firebox.

Joints attacked from the inside. Burning gas or oil produces water vapor along with the other exhaust. When that vapor condenses inside an oversized or cold flue, the result is acidic and can break down mortar from within, sometimes showing on the outside as crumbling near the top of the chimney. See chimney condensation.

When crumbling becomes a structural question

Missing mortar is a maintenance item until the bricks it holds start to move. BIA's maintenance note lists several conditions that warrant investigation by a professional engineer, including cracks wider than 0.075 inch (about 2 mm), cracks running through several bricks, and cracks that follow a stepped or diagonal pattern.

On a chimney, also treat these as reasons to act quickly:

• Bricks that are loose, missing or visibly shifted, especially near the top.
• A lean, or daylight showing between the chimney and the house wall. See leaning or separating chimney.
• Pieces of brick or mortar landing on the roof, deck or walkway.

Keep people, pets and cars out of the area below a chimney that is shedding masonry, and stop using the fireplace and any connected appliance until it has been evaluated.

How quickly it tends to progress

There is no reliable timetable, but the mechanism speeds itself up. A joint that sits back from the brick face holds water on its ledge instead of shedding it. Wetter mortar weakens faster in freeze and thaw, and wetter brick beside it is more likely to spall. Once mortar is gone, water reaches the inside of the wall and the back of the flue liner more easily.

The practical step is to compare. Photograph the same faces from the same spots now and again after the next hard freeze. A change you can see between two photos taken months apart is useful evidence. Lime-rich mortars also have some ability to heal fine cracks: BIA's mortar note explains that water and carbon dioxide react with the lime in the joint to form calcium carbonate, which can seal hairline cracks. That is a slow process for fine cracks, not a fix for open joints.

What the repair depends on

Three decisions shape a repair, and each comes from the inspection rather than from the crumbling itself.

• Stop the water first. NPS guidance is clear that the cause of deterioration should be dealt with before repointing begins. On a chimney, that usually means the crown, cap or flashing.
• Choose a compatible mortar. BIA's selection guidance favors the weakest mortar type that still does the job, pointing to Type O for repointing older brick and Type N as a possible fit for newer work. See chimney mortar types and repointing vs. tuckpointing.
• Decide between repointing and rebuilding. When bricks are loose or damaged through their thickness, rebuilding the affected section may make more sense than repointing around them. See repair or rebuild.

When we assess crumbling joints, we look at the crown, cap and flashing above the damage, check the masonry from the roof when it is safe, scan the flue with a camera when it allows and check the attic side where it is accessible. Photos of each finding reach you before any repair is booked. The work itself is described on our masonry repair and repointing page.

Questions homeowners ask

Sources

1. Brick Industry Association, Technical Note 46: Maintenance of Brick Masonry (PDF)
2. National Park Service, Preservation Brief 2: Repointing Mortar Joints in Historic Masonry Buildings (PDF)
3. Brick Industry Association, Technical Note 8: Mortars for Brickwork (PDF)
4. Brick Industry Association, Technical Note 8B: Mortars for Brickwork, Selection and Quality Assurance (PDF)
5. 2003 International Residential Code, Chapter 10, Chimneys and Fireplaces (text reproduced by Buckley Rumford Fireplaces)
6. DuraVent: Relining Chimneys for Gas Appliances (technical guide, PDF)

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