Usually when you see recessed mortar joints on a wall, it means the mortar has eroded due to weathering or water infiltration. But in the early and mid-twentieth century, recessed mortar joints were part of the design of some buildings. Take this wall, for example. The mortar is recessed about a quarter inch or 3/8-inch back from the face of the brick.
Recessed joints, sometimes called raked joints, were most popular in the middle of the twentieth century, though sometimes you can find them on early twentieth century buildings. (I worked on a 1908 building in Manhattan that originally had recessed joints at the brick cladding.) You almost never see recessed mortar joints on stone or terra cotta buildings; they were exclusively used on brick masonry walls.
An article in Popular Mechanics from 1958 explains how to create recessed (raked) joints:
‘Raked’ joints produce dark shadows that accent the pattern of the masonry…. To make this joint, the extruded mortar is first cut off flush with the brick, using a trowel. After a lapse of a few minutes, when the mortar begins to congeal, the mortar joint is raked out to the desired depth. The vertical joints are raked out first, followed by the horizontal joints. … To complete this type of a joint, a square tool of the same width as that of the joint is run over the mortar to compress it and fill all voids. (W. B. Eagan, “Eight Types of Mortar Joints and How to Make Them”, Popular Mechanics, July 1958, p. 176-178)
Recessed mortar joints were mainly an aesthetic choice by the mason or the architect. As noted above, recessed mortar provides a dark shadow line and draws attention to the brick rather than the mortar. Sometimes masons used dark mortar to accentuate the shadow, as in the photo below, though light-colored recessed mortar was successfully used in the wall in the photo above. Recessed mortar joints also hide flaws in the masonry. The wall below has uneven horizontal joints; although the irregularity is apparent, the deep shadows created by the recessed mortar make you wonder whether the shadow creates the irregular line rather than the masonry.
One of the biggest challenges of recessed mortar joints is repointing them once the mortar begins to fail. Many times, unskilled masons will erroneously think the recess is caused by eroded mortar. The masons will then fill the recess with new mortar, which changes the composition of a façade from one of prominent brick to one of prominent mortar. In the photo below, the original recessed mortar joints are visible at the center and right side of the photo. At the far left, a mason “repointed” the mortar joints by filling in the original recess, thus changing the appearance of the wall.
It is best to leave repointing of recessed mortar joints to skilled masons who have experience working on brick walls with recessed mortar. They will have the necessary skill and tools to accurately repoint the wall without changing the building’s original aesthetic.