See here why repointing granite stone walls so common in Washington DC
Our website has tons of information about brick restoration, brick tuck pointing, brick repointing, and historic brick facade upkeep and maintenance, with a focus on preservation. Today we take a closer look at a similar but difficult area of masonry restoration, granite wall repointing, specifically pointing building walls built with stone. Granite stone is a common material used in the construction of yard or site retaining walls. We see them all over the city, in Washington DC’s historic neighborhoods such as Capitol Hill, Dupont Circle, and Georgetown. But in some fewer limited cases, granite stone was also used in the historic construction of building foundations. Those foundations actually, in most cases, predate the granite stone retaining walls. In fact, some architecturally focused historians argue that the granite stone retaining walls that are so common in Washington DC were built decades after the original construction of The adjacent buildings.
The adjacent image shows a somewhat typical granite stone foundation wall. This particular wall happens to be a rubble type stone wall. Rubble stone walls use individual stone units that are randomly shaped (to an extent) and dressed or shaped in a rectilinear shape. Ashlar stone masonry, by comparison, has stones that are shaped in square or rectangular type of shapes. Ashlar stone can still have a split face at the outer surface (and angles or slight anomalies at other angles and faces and still be classified as “ashlar”. Split faced stone masonry has an esthetic that is more rustic or natural then typical rectangular masonry unit.
Rubble stone is not dressed or shaped in a rectilinear form (but in the image above, you might notice that the vertical exposed face of the rubble stone is actually very close to flat, especially compared to a split faced stone which is actually generally tapered towards an apex at the center of each stone’s vertical face). Ashlar stone masonry, by comparison, has individual stone units which are shaped in a rectilinear or somewhat square or rectangular shape. Ashlar stones can have a split face and still be classified as ashlar. Split faced stones have an aesthetic that is more rustic or natural than a perfectly rectilinear shaped stone.
The historic church in the picture below has a terrific exterior wall layout which creates an incredible architectural facade variety with bays and alternating roof styles. This building is also built with a granite stone, but the stone used here happens to be an ashlar stone with a split face.
When you look closer at the At the individual masonry units, of the Stonewall, you can see that there is actually not as much plainer variation in the vertical face of the wall as you would find in a typical rubble stone wall. Even though this is considered a split face stone.
The particular masonry joints, at this building, are lined with a red accent joint, similar to a grapevine or toothpaste joint found in a typical Capitol Hill row home front yard retaining wall. These types of buildings always look timeless, like they could last forever, and it is true that they can last for centuries, if well-maintained. However, similar to the ubiquitous historic brick masonry of Washington DC, stone built walls also need to be restored from time to time. The common brick pointing that we talk about so often is similar to historic stone restoration, but there are key differences.
A quick summary of some of the most important differences follow:
- Brick joint raking
- Non-linearity of masonry joints
- Depth of mortar recesses (and deterioration) between stone units
- Type of mortar used for stone vs. historic brick pointing
Brick joint raking
Brick joint ranking is a tedious and labor intensive process. It requires a lot of toil and effort. It is tiring, dusty, and dirty work. However, in the case of raking stone mortar joints, especially in rubble stone wall construction, it’s a very different process. Plug chisels can be useful for brick joint raking, especially in historic brick restoration, in the repointing process, but mechanical tools cannot really be used effectively in stone pointing. The most impactful difference of all though is that in rubble masonry construction the masonry joints cannot be raked in a continuous depth or linear manner as they can with historic brick joints.
Non-linearity of masonry joints
Typically, in a standard historic brick wall there are 2 main types of joints: bed joints and perpend joints. There are several different types of brick bonds which are used in wall construction (the common bond and flemish bonds are prolific in Washington, DC, for example), even in historic times. The perpend joints usually are more tedious to rake and point, because of the short length between interruption, but the bed joints make up twice as much mortar joint length and are normally near-perfectly flat and largely uninterrupted.
The picture below shows the interior side of an exterior foundation wall of a building built over one hundred years ago. This building is built with rubble granite stone. The upper walls of this building are built directly on top of that granite stone foundation wall. This wall has been sloppily repaired in the past, but only in a very superficial scam pointing type of way. This wall has been sloppily repaired in the past, but only in a very superficial scam pointing type of way. Because the previous repair work, years ago, was not done properly, the mortar that was applied removes easily and in this particular case has not caused long term detrimental damage to the underlying stone masonry.
Depth of mortar recesses (and deterioration) between stone units
In typical common joint brick restoration, we will rig the joints, removing deteriorative mortar to a depth of over ¾ of an inch. The amount or depth of raking required depends on several factors.
Factors influencing the depth of joint raking:
- The extent of deterioration, in other words the amount to which the joint is deteriorated, not in terms of physical size, but in terms of scale of degradation. For example, one type of mortar joint, depending on exposure to elements and the type of original mortar used, and several other factors, could be deteriorated so much that it is pure dust, even at the interior parts beyond the surface, to an extent. On the other hand another type of mortar joint might still be relatively hard but have cracking and deterioration just at the surface. Both joints would need to be restored, but the extent of raking required may be very different.
- The height (or thickness) of the mortar joint. Taller or wider mortar joints require a deeper depth of raking.
The picture below shows the same interside of an exterior stone foundation, at the same location as shown in one of the photos above, but in this picture, the wall is shown after the joints have been raped and cleaned of deteriorated mortar. In fact, in this picture our company is applying compressed air to help remove some of that deteriorated mortar joint debris after we have physically removed the hardest or densest part of the mortar joints by manual hand tools.
The next picture below shows the same granite stone masonry wall but from a slightly different angle and a bit closer. Here you can see the great depth of the crevices between the stone where the mortar has been removed. This is an example with an abnormally high amount of raking requirements.
Type of mortar used for stone vs. historic brick pointing
The type of mortar used for different types of masonry construction can vary significantly depending on the characteristics of the wall. There are several details which are part of this analysis. We talk about these details in much greater depth in many of the other articles on our website. Here though, in a very general sense we can simply say that granite stone mortar needs to be much harder, and must have a greater permeability resistance than historic brick restoration or repointing mortar.
The trade and practice of historic masonry preservation is focused on extending the life of the historic architectural fabric of Washington, DC so it lasts for many generations to come. This dedication is a struggle, an uphill battle. The population of Washington DC building owners is mixed in the way that some are acutely aware of The historic architectural value of their building, but the other portion of that population concentrates more on the “business” of their building. Real estate and property management businesses after all focus on business and profit, and educating prospective stakeholders on the architectural value is not easy for people overly focused on profit. As well, the municipal governance has an important job, yet in DC is dysfunctional and inept, unable to create a regulatory framework which simply applies the building code effectively to developers, as developers, in some cases successfully cut every corner. Recognizing, protecting, and preserving historic architecture benefits everyone and the greater good, but the marketplace alone will not sufficiently bolster that value and the municipality is unable to do their job to manage and promote (and protect) those values. It’s important for property owners to be aware of the architectural value of their buildings so they can make decisions that benefit The long term value proposition of their property.
In a future post, we will examine other facets of the specific work required to restore granite stone walls.
To properly maintain, repair, and care for these historic buildings, a knowledge, interest and understanding of historic building principles is required. Here in Washington DC, historic masonry buildings are extremely expensive and the amount of financial loss caused by improper repointing and low quality construction is staggering. However, in addition to the direct financial value of the property, there is also a cultural loss when historic buildings are damaged. By comparison, consider neighboring poor cities, when historic buildings are damaged, it’s not just the loss of value to the property owner, there’s also a loss to all inhabitants and visitors of a city, present and future, who care about architecture, history, and culture.
We encourage all of our clients, and all readers of this article and to our blog in general, to prioritize the historic built environment of Washington DC and neighborhoods such as Capitol Hill, Dupont Circle, and Georgetown and become educated on on the difference between proper historic preservation versus improper work which leads to significant damage to the historic fabric of a building.
From a conservation and preservation perspective, several approaches can be taken to improve conditions related to deteriorated historic brick masonry. Primarily, lime mortar brick joints and low temperature fired soft red clay bricks should be inspected and checked on a routine maintenance schedule, either seasonally or at least annually. If brick masonry is kept in good condition, the life of embedded wood elements can be significantly extended. Hire a professional contractor which specializes, understands and appreciates historic construction elements and buildings.
In this article we talked about the terminology and concepts of historic masonry restoration, follow the links below for more related information from the IDS website:
- Binders in mortars and concrete
- Brick burns
- Butter joint
- Capillary action
- Cementitious siding
- Cheek wall, masonry — Draft
- Chemical testing
- Code, building — Draft
- Cold joint
- Cold weather masonry work — Draft
- Electrical distribution panel — Draft
- Ferrous metals
- Great Chicago Fire
- Green bricks
- Gutter, roof
- Lime mortar
- Load path
- Oriel window
- Parapet coping
- Plug, clay
- Pressed bricks
- Raking, of mortar joints
- Raggle, aka reglet
- Roman bricks
- Roman arches
- Roof eave
- Roof termination
- Row buildings and row homes
- Sedimentary rock
- Scratch coat
- Sprung arch
- Strike, or striking of mortar
- Tapestry bricks
- Tooth-in, interlocking masonry connections
- Water diversion systems
These concepts are part of the fundamentals of historic masonry restoration, tuckpointing, and brick repair.
The links in the list above will take you to other articles with more information on defects, failures, preservation and repair of historic masonry. You can learn a lot more on our blog. Feel free to check it out. If you have questions about the historic masonry of your building in Washington DC, fill out the webform below and drop us a line. We will be in touch if we can help.