Chimney Services

Chimney mortar repair in Bertha, in detail

Repointing mortar no stronger than the old

Stronger mortar is not better mortar for repointing. Technical Note 46 requires the repointing mortar to be no higher in compressive strength than the original, since a harder joint pushes stress onto the edges of the brick and can spall its face.

In practice the mix is chosen for the chimney in front of us. Our specialists look at the brick, the existing joints and any earlier repairs before recommending a mortar type, and the estimate names it. If a past repair used a hard mix that is already damaging the brick, we show you where.

Chimney mortar types under ASTM C270

ASTM C270, the standard specification for mortar for unit masonry, sorts mortar into Types M, S, N and O. The Brick Industry Association describes Type N as a general all-purpose mortar, Type S as a stronger all-purpose mortar and Type O as a low-strength mortar used mostly for interior work and restoration.

For chimney brickwork, BIA Technical Note 19B recommends Type N portland cement-lime mortar and accepts Type S where higher lateral loads call for it. Repointing adds the compatibility rule on top of that, so an older chimney may take a softer type than new construction would. We explain the choice for your chimney in plain terms.

Undoing an earlier repair made with hard mortar

A patch made with hard, cement-rich mortar can look solid while it harms the brick around it. The Park Service brief says repointing mortar should be softer or more permeable than the brick and no harder than the historic mortar, and it shows a wall where moisture trapped behind hard joints eventually pushed the faces off soft brick.

If your chimney shows that pattern, our specialists point out the joints involved and explain the fix, which is careful removal of the hard mortar and repointing with a compatible mix. Where brick faces have already been lost, that brick work is priced as its own line.

Chimney cracks that point to movement

Some cracks come from the chimney moving rather than the mortar wearing. A crack that steps diagonally through several courses, runs through brick as well as joints or keeps widening is a different problem, and Technical Note 46 places structural cracks outside ordinary maintenance, calling for professional investigation of the cause.

Filling those cracks with new mortar would hide the symptom for a while and leave the cause in place. When our specialists see signs of movement on a chimney in Bertha, we explain what we found, and the options may include rebuilding part of the stack. Tuckpointing is recommended only where the masonry is stable.