Chimney masonry repair in Murray Hill by local masons

Pre-hydrating mortar for repointing

Repointing mortar is mixed differently from laying mortar. BIA's note describes the steps: the dry materials are mixed, then just enough water is added to make a damp mix that holds its shape when squeezed into a ball. The mix stands for a period, and then more water is added to reach a working consistency that is still drier than the mortar used to lay brick. The stiffer mix packs into the joint more firmly and shrinks less as it cures.

The result is joints that fill all the way back, with less chance of a fine crack opening along their edges. The mix is used within its working time, and anything that has started to set is thrown out rather than retempered.

A cured sample before the color is set

Wet mortar looks darker than it will once it cures, so judging a color match from the mixing tub leads to repairs that stand out. Sand, cement and pigment each pull the shade one way or another. Before a large repair, a trial batch is pressed into a test joint or onto a board and left to harden beside the brick.

Once dry, it is compared in daylight against the old joints from a few steps back, as the wall is normally seen, and the recipe is adjusted if needed. The approved proportions are written down, so every batch mixed after that matches the one you saw.

Repairs in the attic and the air gap to framing

Inside the attic and between floors, the code calls for an air space between the chimney masonry and any combustible framing, and that space may only be filled with fireblocking. Masonry repairs made from inside the attic, such as repointing an interior face, have to leave that gap clear. Mortar droppings, patching material and debris that fall into it are removed before we leave. If we find the gap already filled with insulation packed against the brick, or framing added too close, it is recorded as a clearance issue to correct, separate from the masonry work itself.