How Does Insidious Damage In A Historic Brick Wall Look Like?

In an article two weeks ago, we looked at the issues of structural rigidity in a historic brick building.  This week we are looking at the same topic but focused on how the small types of deterioration without proper restoration, such as repointing, lead to detrimental failure.

We start by looking at the obvious signs of the surface of historic brick masonry, such as biocolonization.  

Biocolonization, related to historic brick masonry preservation and restoration, refers to the process by which living organisms, such as algae, fungi, lichen, and plants, establish themselves and grow on the surface, and eventually deeper, of historic brick and mortar structures. This colonization can occur due to the porous nature of the historic mortar and brick units and the availability of moisture, especially in older masonry where mortar deterioration or gaps exist. While it might not seem immediately harmful, biocolonization poses a threat destabilization through the the long-term deterioration of the integrity of historic brickwork.

The presence of living organisms on historic bricks can lead to several detrimental effects. First, their root systems can penetrate and dislodge the small elements, ay first, of bricks and mortar joints, causing physical damage and compromising structural stability. Second, the biological activity can accelerate the deterioration of mortar, exacerbating the weakening of the masonry. Biocolonization requires careful attention and management in preservation efforts to prevent further decay and structural compromise while preserving the historical and architectural significance of the masonry.

The picture below shows an example of a type of publicization on a tree trunk, very similar to the same type of biocolonization or moss /  algae growth on a brick wall, together in the collage these images are combined. The brick wall happened to be only a few feet away from the tree.  You can see you, because of the shaded area, both mediums, tree bark and historic brick wall, provide an area where algae can grow.  The brick wall must be repointed to maintain the building.  It is a required part of historic preservation of masonry.

Biocolonization

Biocolonization can cause slow but critically damaging destruction to historic brick masonry structures.  Several causal factors contribute to this phenomenon: 

• Surface Erosion: Algae, lichens, cyanobacteria, and fungi, which are common biocolonizers, produce metabolic byproducts like organic acids. These acids react with the minerals in the brick, leading to surface erosion over time. This erosion gradually weakens the structural integrity of the bricks.  While historic brick masonry is highly resistant to the deleterious effects of UV exposure and fire (elements which commonly ruin other building materials) historic brick and mortar are particularly susceptible to breakdown and deterioration when exposed to acids.  Even in cleaning, care must be used when working with acids on historic masonry surfaces.
• Moisture Retention: Biofilms, and lichens, like plant growth can form a barrier which retains and traps moisture in masonry and brick and mortar. Biofilms, created by microorganisms, trap moisture against the brick or mortar surface. This retained moisture not only promotes the growth of more microorganisms but also facilitates the freeze-thaw cycles that can physically stress and crack the bricks.   As masonry becomes hydrated from condensation of precipitation, it will naturally dry out over time with exposure to warm and dry elements of weather, but when covered with plant or mirobioogical life, drying is deterred.  Trapped moisture leads to accelerated deterioration in historic masonry.

The picture below shows a portion of the base of a brick wall, at a header joint sandwiched between courses of rowlock brick in a common bond historic brick wall. The surface of the wall is covered in a growth of algae which will lead to the growth of moss. This area of the wall, when hydrated or wet from precipitation and rain, stays wet longer than it would if it were not covered with this biological growth.

The next picture below shows an exterior wall, exterior on both sides like a fence between properties.  Often exterior walls, exterior on both sides, especially between properties or used like a fence, can be referred to as garden walls or in some cases can be called screen walls or pony walls. Screen walls often have perforation or absences between bricks to allow air or light to pass through.  Pony walls will often be short like a kneewall.  Unlike a typical kneewall though, pony walls are generally not built to or expected to have a superimposed load.

This particular garden wall is built as two separate walls, one on each side of the property line and built double wythe each so in total this would be a quadruple wythe wall.  Without a particular coping, over the many decades since the original construction, the mortar and masonry at the top of the historic brick wall has become deteriorated and is now allowing the Virginia Creeper vines to grow into the mortar and exacerbate or accelerate deterioration.  The deterioration is not just accelerating but accelerating at an accelerating rate.  The difference is that this type of deterioration is nonlinear, it is exponentially increasing.  Deterioration of this type should be prioritized for mitigation through repointing and restoration.

• Physical Damage: Plant roots, particularly when mortar joints are deteriorated and not repointed in a timely manner, can penetrate the cracks and crevices in brickwork and mortar. As these roots grow, they exert pressure on the bricks and mortar, potentially causing cracking and even displacing bricks.  This type of deterioration is a quintessential example of damage causing more damage.  The onset of this type of deterioration begets more, but at a yet faster rate which leads to yet more and more deterioration.

The next picture below shows a similar wall but with more significant damage. in this case a tree root has grown into the brick and caused the brick to separate and bend out of plane, unlike soft and pliable or flexible materials though, bricks do not actually bend, the mortar joints in this case have become broken and cracked and the brick is dislodged, resulting in overall lateral deflection of the face of the wall. lateral deflection is a serious type of structural masonry failure. once lateral deflection takes place, larger sections of the wall have to be rebuilt or structurally reconfigured

You can see signs of the serious damage done to this wall. a stair step crack has formed and the brick has been pushed out of plane, over two inches away from its original vertical shape. the wall is now out of plumb and will continue to deteriorating until a portion of this wall is rebuilt. in most cases, tuck pointing or brick repointing can stop damage or susceptibility of the mortar joints before damage can it be incurred, when conditions of deteriorated are not maintained they can allow areas of historic brick masonry to become significantly deteriorated to the point that more serious damage cannot be easily mitigated.

• Biogenic Stains: Some microorganisms produce pigments that result in unsightly staining on the brick surface. These stains not only affect the aesthetic appeal but can also trap additional moisture, exacerbating the deterioration process.  Often the same conditions that create biocolonization and the lack of timely restoration and repointing is coincident with deterioration and oxidation of associated steel or iron embedded anchors or hardware.  The resultant oxidation also leads to staining of the facade and adjacent hardscape.  

What Leads To Biocolonization

Causal contributing factors can lead to biocolonization in masonry.  Some examples follow:

1. High Humidity: Regions with high humidity levels provide favorable conditions for the growth of microorganisms. Damp environments, such as shaded areas or those near bodies of water, are particularly susceptible.  The rear of Capitol Hill row homes, for example, often have rear ells, small reduced passageways which do not allow much direct sunlight or air to enter and flow.  These areas have significant exposure to moisture and unfortunately dry out very slowly.
2. Nutrient Availability: Microorganisms require nutrients to grow, and historic brick surfaces can accumulate organic matter like dust, pollutants, and airborne particles. These materials serve as a nutrient source for colonizing organisms.
3. Shaded Areas: Shaded portions of historic structures are more susceptible to biocolonization because they offer the ideal conditions of moisture and reduced sunlight that many microorganisms require.  As well, areas where there is significant shading, based on the context or layout of the footprint of the building or adjacent buildings, deterioration overall increases or happens at a increased rate. The components of historic brick masonry are simply susceptible to moisture and when moisture lingers longer, deterioration increases.
4. Air Pollution: Airborne pollutants, including sulfur dioxide and nitrogen oxides, can create an acidic environment. Some microorganisms thrive in such conditions.  In our individual experience, we see areas of historic deterioration equivalently between densely populated urban areas such as the historic neighborhoods of Washington DC and country rural areas. We do not see a specific increase in deterioration in areas with higher air pollution, but we understand that from an engineering and scientific perspective there may be a correlation with pollution and historic masonry deterioration.
5. Neglected Maintenance: The lack of regular cleaning and maintenance allows biocolonization to flourish unchecked. Over time, it becomes more challenging to remove established biofilms.  We see this all the time in Washington, DC. Problems and deterioration are easy to manage and handle, but over time the damage becomes exponentially worse.

In this photo below, you can see a wet mortar joint during a rainstorm, but this mortar holds and retains moisture at every rain event, every time there’s precipitation.  Algae is growing in the recessed mortar joint, as the mortar deteriorates, another acute example where repointing is needed.

 The next photo shows the same mortar joint from a direct angle.

Mitigation and Prevention:

To mitigate the slow, damaging effects of biocolonization on historic brick masonry structures, preservation efforts typically include:

Gentle Cleaning: Employing cleaning techniques that safely remove biofilms without harming the bricks, such as low-pressure washing or chemical cleaning.  This type of approach would often work well where historic mortar is not yet deteriorated.  Often though, in modern times, over 100 years after the original construction cleaning without repointing is not actually effective.

The picture below shows a historic brick wall with three types of vine leafs growing on the surface of the wall.

The vines, in this case, as shown in the photo below have grown into the mortar joints and also into susceptible areas behind the fascia boards and the gutter at the eave of the roof.  It is an example of one of the many beautiful properties, built with historic brick, that have been neglected and unmaintained in recent years.

• Biocides: Application of biocides can prevent the growth of algae, lichens, and mosses but are not really necessary.  In many cases, old fashion elbow grease (hard work and manual labor) is actually more effective.  In other cases though, of poison ivy, for example, we recommend wearing appropriate protective clothing and gloves and considering biocides to kill large root systems at main tap roots.
• Improved Drainage: Proper drainage systems can help reduce moisture buildup on brick surfaces.  In cases with low exposure to sunlight, such as rear ell areas or in backyards close to fences or garden walls, increased drainage may not be a plausible option.
• Regular Inspection: Routine inspections can identify early signs of biocolonization, enabling timely intervention to prevent further damage.

Conclusion

In conclusion, biocolonization poses a genuine threat to the longevity and aesthetics of historic brick masonry structures. Understanding the processes involved and addressing the causal factors are essential steps in preserving these valuable architectural assets.  The best approach, in most cases, may be working with a proactive contractor to preserve the masonry of a historic building.

Picture below shows a wall with mortar missing at several locations between the historic brick. this wall needs repointing and you can see actual plant growth between some of the mortar joints where stems and leaves are growing from the brickwork to the outside of the building.

The next picture below shows a closer view of that wall, but in this picture you can see the Deep recesses clearly. Those recesses are over one inch deep. This wall was originally built with a flush struck mortar joint. The mortar joint originally came out to the edge of the brick. The brick mortar that now remains deep within the recess of the former mortar joints is often also soft and deteriorated, largely just the constituent sand without the line based cementitious binder still intact.

These elements of deterioration lead to structural destabilization.  In the upcoming week, we will look at additional examples and discuss how repointing, while it seems like a restoration and maintenance type of requirement, can lead to reduction of deterioration and destruction of historic masonry.