Restoring the Breath of the Building: The Life-Saving Science Behind Historic Masonry Repairs

Words: Courtney DeRusha, DeRusha Masonry
Photos: DeRusha Masonry

When I first set out to become a historic preservation and restoration mason, I imagined that most of my trade would involve repairing the effects of old age. Instead, 99 percent of my work is attempting to stabilize and reverse damage caused by recent and even minor repairs that employed the wrong materials and methods for the structure.

Using incompatible materials to repoint, parge, clean, waterproof, or aesthetically freshen the masonry parts of any historic structure is the number one cause of damage to original -and often irreplaceable – masonry materials and craftsmanship. And that ensuing damage can happen incredibly fast.

Following is an overview of the guiding principles that inform my work, and the science behind them.

It can be used by any practitioner who is not usually involved with historic masonry, but I share it mainly as a way to educate clients, including homeowners, building stewards, non-profit boards of directors, architects, and engineers.

Providing a thorough explanation for my proposed materials and methods has helped my business breed trust and compliance.

Most importantly, educating clients turns clients into educators.

There is already a vast chasm between the number of restoration masons and the amount of restorative work to be done. We cannot afford to allow damage caused by ignorance to outpace us even more. When our clients know enough to guide other stewards and tradespeople toward compatible repairs, they help us slow the tide of preventable damage and extend the life of our historic built environment.

HOW TO APPROACH MASONRY REPAIRS LIKE A RESTORATION PRO

Invite Water and Historic Masonry to Act Naturally, For You Cannot Make Them Do Otherwise.
Modern construction is built on the principle of exclusion, using contemporary drainage systems, high-tech membranes, and sealants to keep water out entirely.

Historic masonry operates on a different philosophy: transpiration. Older buildings were designed to take on a certain amount of moisture and safely release it.

When we approach restoration without understanding this fundamental difference in function, we often do more harm than good. To preserve both the structural integrity and aesthetic beauty of historic brick and stone, restoration must always prioritize compatibility of methods and materials with the building ecosystem we are treating.

Understand How Water Moves Through Masonry
Trapped water is the number one natural cause of masonry deterioration. Meanwhile, masonry units and mortar act like sponges, absorbing and holding many gallons of liquid water and water vapor at any time. This water is often absorbed from the ground up, but can also saturate walls after a soaking rain or driving snow.

This water moves through masonry primarily through a network of microscopic pores and channels that act like tiny straws, in a process called capillary action.

In masonry, water molecules are more strongly attracted to the mineral surfaces of brick, stone, or concrete (adhesion) than they are to each other (cohesion). This attraction creates a wicking effect that draws moisture against gravity and deep into a wall system.

The density of masonry plays a critical role in determining the speed and direction of this movement. Lower-density materials possess larger, more interconnected pores, which allow water to travel more quickly but often over shorter vertical distances because the "straws" are too wide to maintain strong capillary tension.

Conversely, high-density materials with tighter, smaller pores can draw water significantly higher - a process often called rising damp.

Because water naturally seeks the path of least resistance, it will move from areas of high saturation toward drier, less dense areas of the masonry. Our goal is to help water migrate to exterior surfaces where evaporation can occur.

If our least dense and most porous material resides behind coatings, or in the interior of our masonry assemblies, or in masonry units that are surrounded by mortar less breathable than the units themselves, we do not just trap water in our structures: we can actually cause ingress of water from the exterior to the interior. There, ice and mineral jacking can quickly deteriorate the heart of a structure.

For masonry to shed water, mortars, parge coats, stains, sealers, paints, and any other materials that surround or sit on the surface of masonry units must be less dense and more porous* than the masonry units themselves.

In historic structures, this often means using specialty materials that are considered “soft” or “weak” by commercial standards. These materials become longer lasting, more structurally sound, and more preservative than modern materials when used in historic settings.

Investigate the Historic Building Envelope
Historic buildings are delicate ecosystems in which materials and methods work together to allow for both insulation and transpiration, and the historic building envelope changes dramatically through time. Some walls are mass masonry (double or triple wythe brick, stone, early cast stone, early cement, etc.) while others are combinations of materials (structural clay tile with brick veneer, natural cement stucco over wood lathe, parged stone rubble, etc.).

Regardless of the system, most historic masonry was designed to act as a physical separator between the conditioned interior and the unconditioned exterior. Unlike modern cavity wall assemblies with vapor barriers and drainage-enabled air spaces, historic mass masonry and masonry veneer assemblies were constructed to allow for absorbed water, its evaporation, and any resulting expansion and contraction of adjoining structural members or finishes.

Begin With Wholistic Forensics
No masonry repair will be effective long-term if the damage was merely a symptom of a greater problem. We determine best practices by looking for the following:

  • Failures: Are there stepped cracks, shear cracks, or other signs of settlement or forceful movement? Is there localized spalling? Have metal lintels, fasteners or supports increased in size due to rust and shifted the masonry units around them? Is efflorescence rampant or returning soon after removal? Has masonry been aggressively cleaned, or has masonry trapped behind paint or Portland cement mortar turned to damp piles of sand?

Rust is a growth process. Here, a ¼” ferrous metal lintel has grown to more than an inch in height, lifting the building stones above it significantly out of level.

  • Sources of Held Water: Most masonry issues are drainage issues. Before we blame historic bricks, stones, natural hydraulic cement products, or mortar joints for repeated water infiltration, we look for other sources, including:
  1. Placement, size, and health of gutters and downspouts
  2. The health of roof systems, chimneys, sealants, and flashing
  3. The grade (pitch) of the surrounding soil and pavement, and the location of plantings that grow against or overhang the structure
  4. The health of below-grade features like window wells, French drains, cisterns or dry wells, and signs of rising damp in basements and foundations
  • Compatibility: Finally, we look most closely at the type of masonry being repaired and its compatibility with anything that surrounds it.
  1. How soft are the historic masonry units? Do we find soft-fired clay brick or hard glazed units? Sedimentary stone or weathered marble vs. crystalline rock like gneiss or granite? Is early cast stone, architectural terra cotta, mold-formed cement block, true cinder block, or pre-1950s concrete present?
  2. Is mortar likely original (visible sand and stone particles of different sizes and colors, visible pockets of undissolved lime, and/or an overall color other than cement grey) or has it been replaced with cement mortar (hard, uniform, sitting proud of deteriorated masonry, and/or a different color or profile than aged-looking mortar joints found elsewhere)
  3. Have changes been made to the building system that could compromise historic masonry? Any of the following can disrupt a structure’s equilibrium or cause new areas of decay:
  • New, high-efficiency heating systems can vent more moisture and acidity into brick chimney stacks than they were designed to handle.
  • Insulation installed against a stone foundation wall or at the sill, or behind an exterior masonry wall, can disrupt the assembly’s ability to manage water.
  • Any new feature attached to an old masonry exterior without consideration for expansion, contraction, flashing, and drainage
  • Any cleaning, painting, sealing, or repointing
Mitigate Water At its Source
To prevent excess water infiltration, we must ensure that a building's drainage systems are functioning as designed.

  • Keep Weep Holes Clear: If the building was designed with weep holes, we must ensure they have not been removed or plugged with mortar or debris.

  • Move Water Away From Foundations: The root systems and canopies of close plantings, piles of dirt and mulch, plowed snow piles, low spots in grade, raised soil levels that sit above damp-proofed courses, clogged or missing gutters, short downspouts, and runoff from paved surfaces are common sources of water being held in and against masonry for prolonged periods.
Use Compatible Mortar
The most common mistake in masonry restoration is using modern Portland cement on historic structures. While Portland cement is strong and fast-setting, it is also incredibly hard and impermeable compared to most historic masonry materials.

Masonry structures built before the mid-20th century typically employed mortars using forms of lime and/or natural cements, including slaked lime putty or hot lime, natural hydraulic lime (NHL), natural hydraulic cements, and other pozzolans. There are two critical reasons to match original mortars as closely as possible:

  • Compressive Strength: Mortar should always be weaker than the surrounding masonry. As a building expands and contracts with the seasons, mortar joints act as sacrificial stress absorbers. All mortar must eventually be repointed, but we want the mortar to take the stress of movement, not the original bricks and stones.

  • Permeability: Mortar must be more porous* than the masonry units. This allows the mortar joints to act as the final wick, drawing moisture from the masonry itself and allowing it to evaporate through a sacrificial material. When denser, less porous cement-based mortars surround soft brick, stone or historic concrete units, water is both driven into the masonry units (ingress) and easily trapped within them during freeze/thaw cycles.


Soft mortar joints allow for movement, including compression. A fairly recent repointing with hard cement mortar eliminated the ability of this architectural terra cotta and hard brick façade to shift seasonally, resulting in cracks through the masonry units.

*PRO TIP: Preservationists always recommend performing a mortar analysis that allows you to exactly match repair mortar to the original. This lab analysis identifies both the original cementitious material(s) and sand type/gradation. Historic mortar producers will use lab results and physical samples to produce a custom-mixed mortar to aesthetically match the original mortar for your job. However, this is a very expensive process for average homeowners or for small groups who often oversee local historic sites with very limited budgets. When an original mortar sample cannot be obtained, or when lab analysis exceeds a client’s means, always play it safe by using a very soft, Portland-free mortar made from sand and weak lime: ideally, aged lime putty. This will be your do-no-harm mix. Mineral pigments can be used to add color, and black walnut juice can darken a mortar repair to visually age it if your set mix is too bright. Some suppliers offer pre-mixed blends that are color-matched to common historic mixes, and internet videos demonstrating how to apply and cure lime mortars abound.

Know the Danger of Cleaners, Paints, and Sealants
It is tempting to freshen up or attempt to protect an old building with a good cleaning, coat of paint, or addition of waterproofing sealant. In the world of restoration, these are common fatal errors.

Most paints, even those marketed as masonry safe, are film-forming, meaning they create a coating or barrier over the surface. While this may help rain roll off the surface, it also prevents internally absorbed moisture from escaping.

The permeability rating of most commercially available masonry paints and sealants exceeds that of typical house paint but is still far lower than the permeability rating of most historic masonry materials. It also cannot bond with some types of stone or brick without etching or priming the surface first – both of which risk damaging the surface of the original masonry.


Structural deterioration of this 1920s cast stone and brick monument accelerated quickly after city workers painted it with a slurry coat of neat cement (Portland cement and water). They were attempting to prevent damage by waterproofing it after eƯlorescence, and hairline surface cracks first appeared.

The result? Water collects behind coatings, saturates masonry, and eventually causes decay. By the time damage shows itself in a paint layer, it is often too late to save the underlying structure. This is especially important to note when it comes to foundations and chimneys, which can experience catastrophic failures if they deteriorate deeply before damage can be seen.

To color masonry, we can research mineral-based pigments, potassium silicate stains, and limewashes, which can bond to many types of stone, brick, or historic concrete while remaining breathable. Still, not all stones are compatible with silicates, and any prior use of sealants or coatings can disrupt the bond.

Power washing is also a common cause of failure. Water under pressure is powerful enough to remove the laminated (naturally weatherproof) surface of many historic masonry units, and it drives water into wall assemblies, often causing repeated efflorescence and hidden damage.

*PRO TIP: Before any cleaner or cleaning method is used, it must be tested on replicated materials or on a small, hidden, and least vulnerable section of the structure. We test starting with the weakest possible methods and solutions until we find the most gentle, effective method that does no harm.


Brick failure with mortar survival is a preservationist’s nightmare. The only section of this soft brick wall that failed is one bound by cement mortar in a former repair. Matching bricks were scavenged from an interior wythe, and the mass masonry wall was rebuilt with a lime putty and sharp sand mix.

Conclusion
Restoring a historic building is an act of stewardship. By respecting the original engineering of the building, allowing it to move, breathe, and shed water naturally, you ensure that the structure will stand for another century. Remember: in masonry, soft is strong, and breathable is durable.


About: Technical Talks

Services

More Resources

Copyright © 2023