Does My Chimney Need Repointing?

Why mortar wears out before brick

Mortar does two jobs in a chimney: it bonds the bricks into one structure and it seals the gaps between them against water. In a well-built wall it is also meant to be the weaker, softer partner. When water, frost and movement work on the masonry, the mortar takes the damage and erodes slowly, so the brick does not have to. Repointing replaces that worn layer before the bricks themselves start to suffer.

That design only works if the mortar is replaced when it wears. Once joints have eroded deep enough, water gets into the body of the chimney, the bricks lose their support at the edges, and freeze-thaw starts breaking the brick faces. At that point repointing alone is no longer enough.

Signs you can see from the ground

Stand back where you can see the whole chimney, ideally with binoculars, on a dry day with the sun across it. Healthy joints sit nearly flush with the brick face and read as a clean grid. Worn joints look dark and shadowed, because the mortar has receded.

• Joints set back from the brick face, leaving a visible shadow line
• Gaps or open joints, especially in the top few courses
• Hairline cracks along joints, or a stair-step crack across several courses
• Moss, weeds or small plants growing out of the joints
• White powder or streaks on the brick, showing water moving through it
• Bricks that look out of line, loose or missing

Signs inside the house

Worn mortar lets rain into the chimney body, and the water shows up indoors. In the attic, where the chimney passes through, look for damp brick, a musty smell or stained rafters and sheathing next to it. In the rooms below, watch for peeling paint, bubbled plaster or brown stains on the chimney breast after storms. In the cellar, sand or bits of brick collecting where the chimney meets the cellar floor can mean the joints inside are breaking down.

The same clues can also come from a cracked crown, a missing cap or failed flashing, so they are worth tracing before deciding the joints are to blame. Often more than one is involved.

What wears chimney mortar in older homes

On pre-war chimneys in this region, these are the usual causes, roughly from most to least common:

• Freeze-thaw on the exposed stack above the roof, which takes weather from every side
• A split crown or an open, uncapped flue that lets water into the top of the chimney
• Acidic condensation from oil and gas boilers, which softens joints from inside the flue
• Earlier repairs with hard cement mortar on soft old brick, which trap water and crack edges
• Ivy and moss, whose roots and moisture work into weak joints
• Plain age: lime mortar laid generations ago that has slowly weathered away

What cracks tell you

A fine crack along a single joint is usually shrinkage or age, and repointing deals with it. A crack that runs in a stair-step pattern across many courses, a crack that passes through the bricks themselves, or a chimney that leans or bulges is a sign of movement. That is a structural question: the cause might be a failing footing, rusted steel in the masonry, or the stack shifting against the roof framing, and repointing will not fix it.

Location helps too. Cracks near the top tend to come from water and frost. Cracks at the roofline can come from the chimney and the house moving at different rates, especially where roof framing was built tight against the brick.

Why the mortar has to match the brick

Many brownstones, rowhouses and pre-war homes were built with relatively soft brick laid in lime-rich mortar. When those chimneys are later repointed with hard, cement-heavy mortar, the new joints end up stronger and less porous than the brick around them. Water that used to escape through the mortar now leaves through the brick faces instead, and in freezing weather those faces pop off. The spalled bricks next to hard, bright joints are a familiar sight on old chimneys.

A proper repointing matches the mortar to the brick's strength and to the original joint profile and color as closely as practical. On a landmarked or historic house, the local preservation office may have its own expectations, so check before work starts.

Repoint, partly rebuild, or rebuild

Repointing fits when the bricks are sound and the problem is worn joints. If a band of bricks near the top is loose, cracked through or badly spalled, the usual answer is to take the chimney down to sound masonry, often to the roofline, and rebuild that section with a new crown, then repoint below.

A chimney that leans, bulges, or is cracked through its body needs its structure assessed before any decision about mortar. Repointing a moving chimney only hides the cracks for a season or two.

What the assessment looks at

A technician judges the joints up close rather than from the street. Scratching a joint with a key or a pick shows whether the mortar is still hard or has turned to sand, and the depth of erosion decides how much has to be cut out. The bricks are checked for cracks, spalled faces and looseness, especially in the top courses and around the flashing.

The crown, cap and flashing are examined at the same time, because a repointing job done under a cracked crown starts failing again from the top. Inside, the attic and the chimney breast are checked for damp, and on a boiler flue the cleanout is opened to see whether mortar sand and tile flakes are collecting at the base. That tells you whether the flue needs attention along with the outside of the stack.

How proper repointing is done

The failed mortar is cut out to a consistent depth, down to sound material, without chipping the brick edges. The joints are brushed out and dampened so the old brick does not pull water out of the new mortar too fast. New mortar suited to the brick is packed in layers, then tooled to match the original joint shape. Broken or spalled bricks are cut out and replaced as the work goes.

Mortar needs to cure without freezing, so this is work for weather above freezing. Repointing pairs naturally with a crown and cap check, because the joints last far longer once the top of the chimney stops letting water in. Before any of it starts, the crew walks you through what they found and puts the scope and price in writing.