Crumbling mortar and flaking brick: what actually needs doing

Repointing

Removing deteriorated mortar from the joints and packing new mortar in. This is the correct name for what nearly every homeowner with a crumbling chimney is asking about.

Tuckpointing

A decorative finish: a flush joint colored to match the brick, with a thin contrasting strip laid over it to imitate a very fine joint. The masonry industry's glossary says the word is used incorrectly as a synonym for repointing.

Head joint

The vertical joint between bricks, as opposed to the horizontal bed joint. Head joints are the hard ones to cut back properly, and they are where shortcut work shows first.

Spall

A fragment broken off the face of a masonry unit by a blow or by weather. A spalled brick has lost its outer wearing surface.

Mortar joints are the sacrificial part of the masonry by design — they give way before the brick does — so some weathering is aging, not failure. The Brick Industry Association draws the line at four conditions, found by looking closely and scraping lightly with a metal tool to reach sound mortar.

• Erosion past a quarter inch. The joint has receded more than about 1/4 inch behind the brick face. A maintenance trigger, not a safety threshold.
• Crumbling or voided mortar. Mortar that breaks up under light pressure, or open pockets where mortar is simply gone.
• Hairline cracks in the mortar. Fine cracks running along the joint itself.
• Separation at the brick. A gap where the mortar has lost its grip on the brick edge. Water runs in there even though the joint looks full.

Masonry is always moving a little, and those stresses have to be relieved somewhere. In correctly built masonry the mortar relieves them, because the mortar is the weaker material. A mortar harder than the brick will not give, so the stress goes into the units instead, producing cracking and spalling that cannot be repointed out. Hard, high-cement mortar is also less vapor-permeable than most older brick, so moisture that should leave through the joints leaves through the brick instead, depositing salts inside the unit until the crystals pop the face off.

• The selection rule. Choose the mortar type with the lowest compressive strength that meets the requirements. The Brick Industry Association says strength is the property most often over-emphasized in mortar selection.
• The repointing rule. Repointing mortar must be equal to or lower in compressive strength than the mortar it replaces, and softer and more vapor-permeable than the brick.
• The type ladder. ASTM types run from Type M at roughly 2,500 psi down through S, N and O to Type K at roughly 75 psi. Type N suits much newer brickwork, Type O is common for repointing older brickwork, Type K is generally reserved for historic masonry. Where the original cannot be judged by eye, samples can go to a laboratory first.

Brick and mortar are networks of microscopic capillaries that pull water in by suction and dry themselves by evaporation. Damage starts when the masonry stops fully drying between wettings: saturated brick can crack, craze, spall or disintegrate under repeated freeze-thaw cycling, as water inside expands on freezing and levers thin plates off the face.

The Puget Sound version is not a deep freeze — it is the overlap. NOAA's 1991–2020 normals for Sea-Tac put December and January among the wettest months of the year while they also carry the highest counts of freezing nights, and because nearly every freezing night thaws by day, the normals imply roughly two dozen crossings of the freezing point in a typical winter. A chimney is the worst-exposed masonry on the house, and the Brick Industry Association notes that chimneys, parapets and copings need repair on shorter intervals than the wall below them.

1. 1

   Find the water source first

   Repointing will not fix a chimney being fed water by failed flashing, a cracked crown, or an overflowing gutter. The National Park Service is blunt: without eliminating the source, deterioration continues and the repointing was wasted.

2. 2

   Cut back to a uniform depth

   Deteriorated mortar comes out to a uniform depth of at least twice the joint width — generally around 3/4 inch on typical brickwork — or deeper where sound mortar has not been reached. Inadequate depth is what makes new mortar pop back out.

3. 3

   Chisel the head joints by hand

   Power tools have a limited, supervised place on hard mortar in wide horizontal joints. Neither the National Park Service nor the Brick Industry Association allows them on vertical head joints, where a slip cuts into the brick above or below.

4. 4

   Dampen, then pack in thin lifts

   The brick is dampened and allowed to absorb the surface water first. Mortar goes back in layers of about a quarter inch, each packed tight and left to reach thumbprint hardness. Filling a deep joint in one pass invites shrinkage cracks down the middle of the new work.

5. 5

   Tool the finish to match

   The last layer is tooled at thumbprint hardness, which presses the mortar against the brick and consolidates it. The profile matches the original; where the profile is genuinely open, concave and V joints resist water penetration more effectively than flat or raked ones.

6. 6

   Replace spalled brick individually

   Broken or heavily spalled units are cut out and replaced, matched to the originals in physical properties as well as color, texture and size, with brick that will be wet and freezing meeting the severe weathering grade. An exact visual match to an old brick is often not obtainable.

Repointing restores the weather surface of the masonry. It does not add structural capacity, straighten anything, or replace brick in bulk. Three things change the scope.

• The proportion tips. Spot repointing is preferable where the rest is sound, but once roughly a quarter to a half of an area needs work, doing the whole area is often more economical — especially where access, not mortar, is the expensive part.
• The damage is in the brick. Widespread spalled brick, cracks through multiple units, stepped or diagonal cracking, or a lean out of plane are the Brick Industry Association's markers for investigation. At that point the conversation is about rebuilding a section.
• It is unreinforced masonry and the work goes past the joints. In Seattle, alteration and repair of unreinforced masonry chimneys is governed by SDCI Director's Rule 07-2025, effective January 8, 2026, superseding Director's Rule 5-2004. It requires the work to be built in reinforced masonry, or the chimney replaced with a factory-built metal chimney — a like-for-like unreinforced brick rebuild is not a compliance path.