Recessed Mortar

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.

Sloppy Mortar

Let’s look a little more closely at mortar – a key component of masonry construction.  Most of the time when a brick wall is laid up, mortar is installed flush to the face of the brick, or perhaps it has a concave or an incised appearance.  But sometimes you see sloppy mortar that projects out past the masonry, such as in the photo below.


Usually when you see sloppy mortar on a wall, it is a clue that the wall was never meant to be visible.  When he was erecting the building, the mason of this wall probably took care to create nice, smooth mortar joints at the interior of the building.  But for the exterior, the mason probably could not clean up the mortar joints because there was another wall laid up against it.  His sloppy mortar work was exposed only when the adjacent building was demolished to create this garden.  It was entirely unintentional.

Sometimes, however, sloppy mortar was an intentional choice of the architect.  There is a style of mortar in the Front Range that I have never noticed back east: ‘weeping mortar’.  From what I can tell, it seems to have been popular between the 1930s and 1950s – perhaps even slightly later – and it entailed allowing the mortar to ooze out of the masonry while the wall was being constructed.  I saw this Tudor-Revival style building with weeping mortar joints on the University of Colorado campus a few weeks ago.


It was built in 1931 as the Pi Kappa Alpha fraternity house, and was designed by celebrated Boulder architect Glen H. Huntington [pdf] (not to be confused with his father, Glen W. Huntington, an architect who practiced primarily in Denver).  The house was purchased by CU in 1970 and currently serves as the University Administration Building.  It was made using beautiful red and brown flashed brick, whose colors are created through a deliberate manipulation of the chemicals and temperatures in the kiln.  You can see the gray mortar ‘weeping’ out of the joints in the photo below.


Usually a mason scrapes the mortar off the brick when he adds a new course of brick, but for this building the mortar spills were left on the brick intentionally.  The same technique was used for the small adjacent building – the University Administration Annex at 924 Broadway.


Weeping mortar gives the masonry an interesting texture, to say the least.  I have seen the same treatment on a concrete block hotel building on East Colfax in Denver, in several mid-century neighborhoods, and on a few other buildings in the Denver suburbs.

UPDATE: I have been asked several times by homeowners whether weeping mortar can be removed. The short answer is, sure, all mortar can be removed (in theory). But I suppose the real answer depends on a the strength of the mortar, and the workmanship of the mason who will be removing it.  Weeping mortar would need to be carefully chipped off with a chisel.  An errant chisel blow to the mortar could chip the face of the brick.  Once the weeping mortar was removed, the mortar between the bricks would need to be cut back at least 1/2″ behind the face of the brick to allow the mason to repoint the mortar with a flush or concave tooling.  Weeping mortar joints are typically wider than the narrow late-19th century “butter” joints, but it would not difficult for a mason who was removing mortar to cut into the brick using a grinder.  Great care would need to be taken to prevent damage to the brick when removing weeping mortar.  In addition, the stronger the weeping mortar, the harder it will be to remove.  Weeping mortar was used on Depression-era and mid-century structures, so I would guess the mortar contained more cement than earlier mortar mixes, giving it strength and durability.  So, yes, weeping mortar can be removed from brick, but as with most repair and rehabilitation projects, it is important to hire a skilled contractor with experience working on older structures.

The Trouble with Strong Mortar

I was riding my bike on the Cherry Creek Trail the other day and noticed a honeycomb texture to the wall.  It looks kind of cool, but it’s always a bad sign for the wall.  This happens when the masonry wall is built with strong mortar.  As you can see in the photo below, this stone wall was erected on top of a concrete retaining wall.  There are plants or grass on the other side of the wall, allowing moisture to easily get into the wall.


Moisture is inherently lazy: it seeks the path of least resistance.  When water enters a wall, it will seep out of cracks or open mortar joints.  But if it becomes trapped, it will migrate into the masonry or mortar, whichever is the weaker material.  As the moisture builds up just beneath the exposed surface of the masonry, some moisture will evaporate causing little damage to its host.  In wintertime, however, the moisture freezes and thaws within the pores of the host masonry or mortar, and because ice has greater volume than liquid water, the ice microscopically breaks apart its host.  Repeated freeze-thaw cycles cause the surface of the wall to erode.

In this case, the stone is weaker than the strong mortar, and the stone has eroded deeply in many places.  Notice in the photo below that the mortar is the pink and has pink, black and white speckles.  Did you also notice that’s the same color and texture as the concrete retaining wall?  It is likely that the mortar contains only cement, which is too strong for a sandstone wall.


To prevent this honeycomb deterioration, most masons use mortar that is softer and more porous than the masonry.  They do this for two simple reasons: is far easier and far less expensive to replace eroded mortar than it is to cut new stone and rebuild the wall.

For those of you who are curious about appropriate mortar mixes, here’s a very quick overview: The American Society for Testing and Materials (ASTM) categorizes mortars based on overall strength.  Using every other letter from the phrase “mason work”, ASTM standardized each type of mortar and its components (all by volume), from strongest to softest.

  • Type M mortar has 1 part cement, 1/4 part lime, and 3 to 3 3/4 parts sand.
  • Type S mortar has 1 part cement, 1/2 part lime, and 4 to 4 1/2 parts sand.
  • Type N mortar has 1 part cement, 1 part lime and 5 to 6 parts sand.
  • Type O mortar has 1 part cement, 2 parts lime and 8 to 9 part sand.
  • Type K mortar has 1 part cement, 3 parts lime and 1o to 12 parts sand.
  • There is also a non-standard “Type L” mortar, which contains no cement, 1 part lime and 2 1/4 to 3 parts sand.

Historically, masons did not use cement in their mortar or only used a small amount of cement or cement-like material as a strengthening agent.  Therefore, it is rare to find Type M or Type S mortars used on historic buildings.  In fact, the earliest buildings in our country would not have had any cement in their mortar as Portland cement was not manufactured in the US until 1872, and natural cement was not discovered until the construction of the Erie Canal in the early nineteenth century.  (Before cements came on the market, masons often used additives like pozzolans and ash to increase the strength and setting time of lime-rich mortars.)

Most buildings built with soft stone or low-fired brick manufactured before the industrial revolution would have had a predominantly lime-based mortar (Type O, K or “L”).  Buildings with higher-fired brick from the late-nineteenth and early-twentieth centuries, or buildings with stronger stone could withstand equal amounts of lime and cement (Type N).  Keep in mind this is a vast generalization, as building materials and climates differ.  For more detailed information about selecting mortar for use on historic buildings, check out the National Park Service’s Preservation Brief 2: Repointing Mortar Joints in Historic Masonry Buildings.