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Repointing Granite Stone Walls – Part III

Masonry restoration is a very interesting job, but the facet of historic brick repointing is even more fastidious and nuanced. Stone masonry restoration happens to have some overlapping similarities to historic brick tuckpointing, but it also involves some different technical aspects.

There are two significant differences between the restoration of historic brick vs. the restorationof historic stone masonry:

  1. Since the mortar joints of rubble walls lack linearity, it’s a more tedious process to rake and remove the old mortar of rubble stone walls. Even in the case of ashlar stone, when ashlar stones are of different sizes and shapes used in combination, it’s also more difficult to rake the mortar joints than it is in comparison to restoration of brick masonry. The ratio of mortar joints to the surface face area of masonry units might, in some cases, be higher in brick walls. Overall though, depending on the size of the stones used in stone masonry, there’s generally overall a higher amount of linear length of mortar joint in brick masonry tuckpointing than in stone masonry tuckpointing but it’s often much more complicated to rake the joints of stone masonry than brick masonry, especially when the stone is built in rubble formThe second, and possibly most important big difference between brick pointing and stone repointing is that stone joints are often very voluminous.  Because of the common near perfectly planar surfaces of brick walls, deterioration in the joints at the face of the wall are relatively limited, in most cases. We’ve seen extreme examples where brick wall mortar has deteriorated to the point that the walls are literally falling apart from one brick to the next, but in most cases the depth of significant deterioration is limited to about 1″ of depth.  Even if the overall comparative surface area of the brick joints are larger or the brick joint linear length is longer, stone walls generally have a much greater volume of mortar to be removed and replaced in the joint restoration and tuck pointing or repointing process.

Also for reference and comparison, in the picture below you can see an example of a split faced ashlar masonry wall. Looking at this example in comparison and contrast with the examples above, you can see there are many different forms and shapes within which stone masonry can be built and assembled.  The split-face has a more rustic aesthetic, as would a rubble wall, but in this case the wall happens to be built with ashlar stone.

split-faced-ashlar-masonry-wall

In the picture below you can see the large size of voids between the stone units in this masonry wall.   Some of those voids go several inches deep into the masonry wall and are even much larger beyond the face where they are visible on the surface.   Like deep caves and cavernous, these voids can actually open up larger on the inside than they appear on the surface.

masonry-wall-voids

A few select Lessons Learned and tips for Rubble stone masonry restoration:

  1. Go overboard in preparation.   There is a reasonable limit in terms of what is optimal and even if you spend all of the money in the world and all of the time just in preparation, you will still never be 100% perfectly prepared. Preparation includes setting up tarps, decontainment, masking, and protection. All of these things are important, useful, and worthwhile, but when doing brick and stone masonry restoration and/or mortar removal, such as in the tuck pointing processes, and on the interior of a building, you can never protect at a level of 100% nor perfect because brick mortar dust becomes airborne and travels in a way which cannot be fully controlled.  Nonetheless, take this step very seriously and invest in proper preparation.   There’s an old adage in the construction industry that says that an ounce of prevention is worth of pound of cure. We found that in preparation for masonry pointing work, this old adage often holds true.
  2. Use air to clean.  Where interior space will not allow for application of significant water, such as with a hose, be prepared to use compressed air to clean the recesses between mortar joints.    Using compressed air isn’t perfect either though, this process comes with its own drawbacks. For example, using air to clean mortar in the form of dust will make that dust airborne and the dust will therein spread significantly. It’s very important to use a good mask for the purpose of preventing inhalation of silica dust, and the worker’s clothes will become very dirty. It can be smart to use coverall suits. These extra PPE measures are cumbersome and expensive but required when using air to clean mortar joints. Also, as explained above,  Containment becomes extremely difficult and technically impossible to be done perfectly when using compressed air to clean mortar joints.   Again, in this particular facet of restoration, the application or context is very similar between both brick masonry and stone masonry, however the volume of mortar to be repointed is significantly higher in stone masonry when the mortar joints and associated voids are much larger.
  3. Wet the remaining mortar.  Even when you are not using a hose and an extremely high amount or voluminous amount of water to clean the mortar joints, still always wet the mortar joints before application of new mortar.   This type of process would apply to both brick masonry restoration and stone restoration, but in the case of stonewalls, the existing remaining joint mortar might be even older and drier and have a much greater thickness. A typical triple wythe brick wall is only about 12″ thick. Whereas a stone foundation wall can often be over 2′ thick.
  4. Clean stone facings early.   In modern masonry construction, muriatic acid is often used to clean the face of masonry after mortar has been applied and even after the mortar has cured to the point that the acid will not eat away too quickly at the mortar joints themselves. That muriatic acid can clean excess mortar from the face of the bricks. In contrast though, pure muriatic acid, even diluted with water to a 5% ratio cannot really be used with historic bricks because historic bricks have greater porosity and are more susceptible to acid such as muriatic acid which can eat away at the face of the brick to the point that the brick can become damaged by the acid.
  5. Use a variety of pointing tools.   We use a massive amount of different tools in pointing, in some of our small toolboxes will have over 100 different pointing tools,   Different tools have different purposes. We use a variety of tools, just to rake and remove the mortar and a variety of different trials. Some of our preferred tools for stone pointing are the 2-handle hook for raking, the 3″ Philadelphia trowel for holding mortar like a small scale hawk-tool or hod, and the ½” pointing tool.
  6. Push with force in all directions.   Generally, brick masonry repointing is only done to a depth of 1.25″. Joints in brick masonry can be pointed deeper, and in select spot locations a deeper raking of the joint is often necessary. In fact, sometimes in brick masonry restoration, we will find that the existing joints have deteriorated much deeper, sometimes all the way through a 8.5″ or 9″ double wythe brick wall.   Normally though, the majority of brick masonry repointing should not be raked more than 1.25″ depth.  Raking is the process of removal of old mortar. In other words, brick joints should not have masonry mortar removed more than 1.25″ in a wholesale approach. Stone masonry, by comparison though, is very different. Often stone masonry will have large voids that work left open and unfilled from the original time of construction.  Particularly in foundation construction or restoration, it’s very important to fill as much of those open joints as possible because they create or become cavernous channels that run through the wall and can be a pathway for bioconization, plant roots, air movement, and even water entry. It’s not the easiest method to apply, but the best process is to push mortar deep into the recesses of the wall. Several tools can be used to help, but often the best is a simple brick pointing tool, one a trowel-type handle.  However, as described in item #5 above a wider than typical brick pointing tool might be better for stone.
  7. Burn bio-colonization or cut it out completely.  (It’s important for us to explain here that all of our information provided is informational only and we don’t recommend any homeowners do any work on their own. This is the type of work that only professional contractors should be doing because it is work that requires extensive experience and requires great care.  Tuckpointing and brick and stone masonry construction is hard, heavy, and dangerous work.  We would never recommend a homeowner used heat or fire or any type of combustion to do any work on their home. It’s only safe to leave that type of work to professionals.)

Where bio-colonization is found in existing remaining old mortar joints, one of the potential ways to remove root growth from bio-colonization, might be burning the roots with a torch. As explained above, extreme care must be used in this process and always have the work done by a professional licensed contractor.

Alternative methods such as herbicides will potentially damage the mortar and cause issues of decoupling and / or toxification and contamination of the mortar, particularly where new mortar is installed.   This type of issue applies pretty much across-the-board both in tuckpointing and restoration of brick masonry and stone masonry.

The picture below shows an example of the completed pointing work at the granite stone wall.   You can see the mortar joints are very wide and large but they have been struck, after tuck pointing, smooth with a recessed but flat mortar joint.

completed-pointing-workgranite-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

Mold

  • 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
  • Sedimentary rock
  • Scratch coat
  • Sprung arch
    • 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.