Our website has tons of information about brick restoration, brick tuck pointing, brick repointing and historic brick facade upkeep, maintenance and restoration, with a focus on preservation. Last week, we took a closer look at a similar but difficult area of masonry restoration: granite wall repointing, specifically pointing building walls built with stone. This week, we look even closer into granite stone repointing and restoration. It’s a process very similar to brick masonry restoration and repointing but there are some significant points of contrast. It’s a process very similar to brick masonry restoration and repointing but there are some significant points of contrast.
The picture below shows one of many examples of a granite wall mortar with voids in the joint. The original setting of the stone likely included a full joint that filled the interstitial space between masonry stone units. This open void allows water to enter the space between stones and should be filled. In this particular example, this is the exterior side of an exterior wall. Hence there’s greater importance to protect the wall from water entry and greater importance to preserve those joints. The interior side of the wall also matters, it’s important to keep that interior side of the wall in good condition and maintained and even restore the mortar joints at the interior side of the wall from time to time but the interior side of the wall is of arguably slightly lesser importance because it doesn’t face the harsh exterior elements such as precipitation, wind, and ice.
This week we also focus on some of the key differences between tuck pointing of granite stone walls and the typical more common restoration of historic brick masonry. Brick restoration involves several areas of care, focused around repointing and preservation. The work is also tedious and difficult work that requires extensive training and experience. We will show some of the points of comparison because using brick masonry restoration, such as tuck pointing and repointing as a baseline, is a good context to understand the stone masonry restoration process.
In the image below, you can see an example from one of our recent projects. This granite foundation wall was repointed and restored from the inside side of the foundation. This basement has been excavated and even dug out further than originally built, in the years after the original construction. The basement suffered from water entry through the granite stone foundation wall. There are multiple ways to treat water entry and foundation hydrostatic pressure mitigation, one of the best methods is a positive side system approach where the wall is sealed and treated from the exterior side but work of that type generally requires extensive excavation. As you’ll see through further images below, we found voids and recesses in the mortar at the granite wall that were so deep and extensive that it was required to fill those areas with mortar as part of a repointing process.
Filling deep voids of this type is very difficult. It’s hard to get tools deep into those recessed areas, but a typical repointing tool is useful.
If you look closely in the image, you can see signs of file colonization. There are thin fibers of root hair that had grown into the mortar joint. This type of deterioration and bioconization can happen, not just in stone masonry, but also in historic brick masonry. In very old historic buildings, where restoration and brick pointing has not been conducted at proper schedules, it’s very common to find that historic brick mortar joints turn back to almost nothing more than sand, similar to a loose sand originally used and mixed in the historic lime mortar. Traces of the lime may be still found, but the effective characteristic of the lime binder is completely nullified.
The side by side comparison, below, shows the area of plant growth, in the mortar joint, up close. In deteriorated mortars, both in brick masonry and in stone masonry, alike, when the mortar joints are deteriorated to the point and there’s nothing more left than basically sand, roots can grow freely, in a pervasive way. At the time of original construction, the lime in the mortar worked as a characteristic. binder. It gives the mortar strength by holding the aggregate in position, similar to other types of cement, like Portland, for example. But unlike Portland, lime mortar has a lower compressive strength, higher modulus of elasticity, higher permeability and susceptibility to water. As shown in the pictures below, lime mortar will break down and deteriorate faster and more extensively than Portland mortar. For historic brick construction, it’s essential and important to use lime mortar because the bricks of the historic time are not strong enough themselves to work with Portland mortar, but as shown here, lime mortar also is not does not perform as well as Portland mortar in particular ways.
The phenomenon of accelerated building deterioration takes place in all shapes and forms throughout buildings, over time. Buildings are major assemblies of multiple different types of materials and they often have this element in common. As buildings deteriorate and as particular areas of the buildings experience deterioration and disintegration, that deterioration happens at increasing rates. In other words, once a building system starts to break down, that breakdown happens faster and faster as time goes by. The picture above and the picture below show the issue of bioconalization and plant growth in mortar joints, up close. As the mortar joints deteriorate, that deterioration allows the roots to grow and that root growth also causes increasing damage and deterioration at an increasing rate happening faster and faster. The roots actually push and make way through the areas of mortar joints and create new more extensive channels which water can enter. As the roots grow and more water enters, it becomes increasingly easier for the roots to continue growing and for water to continue entering. We describe the issue of accelerated deterioration as being non-linear because when modeled in graphic form, the curve or amount of deterioration is expressed in a non-linear curve. By comparison, a linear curve would depict deterioration continuing at a predictable continuous rate, in a straight line using continuous units of measurement. Here though that rate increases and increases, faster and faster, as it goes. The same phenomenon occurs both in brick masonry and stone masonry.
Work on the interior side of the building is extremely more complicated than work on the exterior of a building, in most cases. Containment of dust is almost always part of typical work in brick restoration or repointing, but in this particular case since it is only interior, dust containment is even more important. In all cases though, no matter how much effort is put into containment, debris and dust can never be 100% contained. It was lucky though in this unique circumstance because the basement happened to be unfinished, which allowed for a useful, cost-reducing latitude and freedom to work expeditiously without extensive areas of containment.
The picture below shows not water at the base of the wall, podredumbre out upon the floor and concrete base retaining curb. That water entered the foundation from hydrostatic pressure on the exterior of the wall. Even this little bit of water, shown here on the bare concrete floor, is problematic on the interior of a building because water is harmful to other building materials. However, by comparison if we had used water to clean the raked mortar joints, the amount of water would have been many multiples of the small amount of water shown, through permeation of exterior hydrostatic pressure. Therefore, using water to clean the mortar joints after the raking process, is not a feasible option on the building interior.
Stone and brick masonry pointing seem similar in many ways, they use a lot of the same tools and a lot of the same skills. There are important similarities that transfer between the slightly distinct types of work, but in some ways the seemingly slight distinction between these types of works can actually be surprisingly massive.
Ashlar masonry is similar to standardized or modular masonry restoration, yet there are significant differences in repointing and restoration. The picture below shows an example of a granite masonry stone wall next to a brick masonry wall. Both of these walls are actually ashlar. In the case of the stone, the stone pieces are individually all cut to different sizes but they are cut in rectilinear form. The face of the stone is relatively rough but they are not split faced units. Each face is even cut in a proximate rectilinear form.
By comparison, rubble stone masonry is made of individual stones which are not cut and only roughly formed before being installed in the masonry wall. Rubble stone walls are extremely common throughout the world. Rubble stone masonry lacks the inherent stability of ashlar masonry. Ashlar masonry is cut so each successive course can neatly sit on the horizontal flat surface of its predecessor course below. Flat surfaces are rare in rubble stonework. However, this does not mean that rubble stone walls must lack stability, it just requires a significant amount of sorting, culling and setting in a jigsaw type of fashion. Good stonemasons can build a rubble stone wall so that it is self-supporting even in a dry stack type assembly without the use of stone mortar. It’s obviously more difficult, in many ways, to build a self supporting rubble wall than it is to build a self supporting ashlar wall. For example, it takes a significantly higher amount of time to select and set stone so that it is self supporting in rubble walls than it does in ashlar walls. It’s a little bit like a game of Tetris, in both examples, but much more complicated in the example of a rubble stone wall.
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
- Cantilever
- Cementitious siding
- Cheek wall, masonry — Draft
- Chemical testing
- Code, building — Draft
- Cold joint
- Cold weather masonry work — Draft
- Damp proof course
- Downspout
- Electrical distribution panel — Draft
- Fenestration
- Ferrous metals
- Great Chicago Fire
- Green bricks
- Gutter, roof
- Lime mortar
- Lintel
- Load path
- Oriel window
- Oxidation
- Parapet coping
- Plug, clay
- Pressed bricks
- Raking, of mortar joints
- Raggle, aka reglet
- Rectilinear
- Roman bricks
- Roman arches
- Roof eave
- Roof termination
- Row buildings and row homes
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- Rubbed bricks
- Rubble stone masonry
- Sand, Builder’s
- Sedimentary rock
- Scratch coat
- Sprung arch
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- Squint bricks,
- Strike, or striking of mortar
- Tapestry bricks
- Tooth-in, interlocking masonry connections
- Vitreous
- Water diversion systems
- Zipper-joint
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.
