The Conservancy is initiating a series of technical “Preservation 101s” on preservation issues that face Wright building stewards. Going more in-depth into issues than our “Tech Tips,” they are intended to provide a basic understanding of each issue so that building stewards can ask educated questions of their architects, contractors and other preservation professionals.

Brick

Brick was a favorite material of Wright’s and he specified it often.  Almost all of his later Usonian houses included masonry units in the form of brick, concrete block or stone as an integral component of the houses’ structure.  Masonry is, typically, fairly durable, but like all building materials, its preservation is not a given without proper care and, if necessary, repair by knowledgeable craftspeople.

Wright’s preferred ways of designing details for masonry construction reflected traditional masonry practices as well as effects specific to his own creative goals. A number of these practices are significantly different from modern construction methods. While these practices can complicate restoration work and at times exacerbate problems, the payoff in terms of the unique architectural qualities of the building should be kept in mind and retained when possible. 

When maintaining and restoring masonry on a Wright-designed structure it is critical to work with an architect or masonry contractor who is both experienced in historic building practices and is willing to investigate restoration techniques that are appropriate for the project at hand.

Masonry units (brick, block and stone) share a common construction element – mortar which binds the units together.  Each masonry unit type has a somewhat similar relationship to the mortar, moisture and other environmental impacts.  However, some differences exist between and within these masonry unit types which are best addressed as exceptions and/or examples at the end of the following discussion of brick in order to maintain focus on the abundant similarities.

The Symptoms

As with most building maintenance matters, water infiltration is a chief cause of masonry deterioration. Visible issues are usually symptoms of a systemic problem, so it is critical to understand the source of a problem before attempting to remedy it. The most typical of these symptoms are:

  • Efflorescence: This is the powdery substance, usually white, that can appear on the surfaces of masonry walls. It is caused when water-soluble salts that may be present in mortar or masonry units are carried to the surface of a structure by moisture moving through a wall. When the moisture evaporates, the powder remains on the surface. In new construction, it is not necessarily a cause for concern. Then it is fairly common and after a cleaning with water and brush it typically disappears. In existing construction, however, efflorescence may be a sign of unwanted moisture in a wall. Additionally, efflorescence can present when the use of de-icing salts are used too closely to brick masonry structures.  It is recommended that an alternative product containing calcium chloride, sand or kitty litter be substituted to provide traction.
  • Spalling: This occurs when the outer face of a brick separates from the main portion of the brick. This is usually the result of moisture or salts in the brick freezing, expanding and pushing off the outer layer. Spalling may also occur if the mortar used is stronger (has a higher compressive strength) than the brick.  Through thermal expansion of materials, the stronger mortar can damage the brick.  See below for additional discussion regarding mortar.  This can be more than just unsightly. The faces of brick are typically more durable than the inner material.  Once the face is removed, deterioration of the brick can be accelerated.
  • Crumbling or otherwise deteriorating brick: Deteriorating brick can be a sign of long-term moisture settled in a location. It may also be a sign of structural instability that should be investigated in order to determine the exact cause and corrected prior to repairing.

Common Causes for Moisture Infiltration:

Moisture will always take the path of least resistance when entering a structure. That path is not always the most obvious one, though. For example, the source of a roof leak can be some distance from where it manifests itself inside a building.  For this reason, it may be necessary to look beyond the immediate area where moisture is exhibiting itself to understand its origin.

It is important to understand the construction of your wall: is it solid (monolithic) without an airspace? To determine this you may want to review the drawings for the house that are available through the Avery Library [link]. You may also be able to understand the wall through non-destructive investigation. The top of a wall may be visible above a soffit or at the back of cupboards or cabinets.

Ideally, the way a wall is designed should minimize the likelihood of moisture entering a wall and if it does, the design must allow for ways for moisture to exit the wall. Modern masonry walls are typically veneers that provide a durable and attractive face for the layers of insulation and load bearing construction behind them. An airspace between the brick and back up wall provides a location for moisture to safely exit a veneer wall through weep holes at the bottom of the wall construction.

Contrary to this type of construction, Wright typically sought monolithic construction, with minimal layers of material between the inside an outside of a building. For this reason, his masonry walls tend to be either solid masonry or two layers (known as “wythes”) of masonry with an airspace in between.

When Wright’s walls can “breath,” so that moisture has the time and the air movement needed for it to exit the wall system, then there are unlikely to be any of the problems described above.  Unfortunately, moisture is not always able to escape a wall. Reasons for this include:

  • Masonry is in a high-water- intensity area: At locations such as by a downspout or near grade, moisture does not always have a chance to evaporate. If these areas are shaded from sunlight by eaves, vegetation or their orientation to the compass, the condition can be intensified.
  • Lack of flashing or coping: Free-standing walls, such as privacy walls or parapets are particularly prone to this condition. In standard construction, such walls are typically topped with a stone cap that covers sheet-metal flashing. Flashing provides a waterproof barrier at the top of the wall. Standard walls may have a visible sheet-metal coping which covers the top of the wall.

Wright often designed his walls to be topped with a course of “row lock” bricks, which are placed on their side and connect the two faces of a wall. While this provided an aesthetic effect wanted by Wright, the porous brick and mortar cap allows moisture into a wall that would otherwise be protected by a coping of stone or metal.

Wall Copings:

Common Wright-designed condition (left), typical non-Wright coping and flashing detail (right).

Similar conditions can occur at the top of chimneys. At one house, only a slight concrete wash protected a fireplace mass that is 16 feet by 4 feet. As that was neglected, water infiltrated alarmingly.

  • Recessed mortar joints: Recessed horizontal mortar joints are a signature design element on many Wright buildings, emphasizing the buildings’ horizontality. The ledges at the top of each brick course also provide convenient locations for water to enter brick and mortar. When moisture is able to escape the wall, this is typically not a problem, when it is not, this detail can exacerbate problems.

Mortar Joints:

Common Wright-designed recessed joints (left), typical non-Wright concave joint aligned with face of brick (right).

Recessed mortar joints at Martin House in Buffalo

  • Too-strong mortar: Because of the damage moisture can do to brick, it is preferable to have it exit a wall through mortar instead of brick. Mortar should be the “sacrificial” material in a wall. If something goes wrong, it should be the mortar that is damaged, not the masonry. Replacing the mortar in the wall, known as “repointing,” is much easier than replacing brick or stone. For this reason, mortar should always be less strong than the brick it surrounds.

Historic mortars typically had a higher proportion of lime to Portland cement.  Portland cement makes mortars strong and durable, but it also makes them rigid and much less permeable to moisture. If repointing is needed, the new mortar should match the original mortar in color, texture, joint profile, and compressive strength (as long as the compressive strength of the existing is less than the brick).

The National Park Service has a very helpful brief on mortar and re-pointing.

  • Inappropriate sealer: Because of a masonry wall’s need to breath, sealers of any type must be chosen with great, great care. Sealers typically fall into two categories: waterproof and water-repellent. While waterproof coatings may keep water and water vapor from entering a wall from the exterior, if water enters the wall from any other source, such as vapor from the interior, a waterproof exterior coating may accentuate a problem by trapping and not allowing the vapor to exit the wall. While water-repellant coatings theoretically allow water vapor to exit a wall while repelling liquid water, water can change from vapor to liquid within a wall and therefore be trapped, causing the same sort of problem found with a waterproof coating.

The National Park Service brief on cleaning and coating of masonry reviews the pros and cons of each system.

Possible next steps:

As noted above, proper masonry care is based on minimizing the amount of moisture that enters a wall, and if moisture does enter the wall, providing a means for the moisture to leave the wall.

Remember that each restoration/maintenance/repair situation involving a Wright building requires analysis and research to identify the correct approach. This information should be used to evaluate your own specific situation and as you consider these and other approaches. The Conservancy strongly recommends that owners consult with a design professional and an experienced contractor to determine which method is best for their specific projects.

  • Minimize plantings around the building: As a first step make sure you have air movement around your building. While shade provided by trees and other plantings provides natural cooling benefits, care should be taken so that such plantings do not create micro-climates that discourage the evaporation of moisture in adjacent walls.
  • Movement of water around the building: Make sure water is not able to pool around your building for extended periods of time. Drainage can be improved by perimeter grading that leads water way from the building.
  • Help the wall breath: If your wall has a cavity in the middle of it that is not filled with insulation, there may be ways to create enough air movement in the wall to allow moisture to evaporate. This may be done though opening up ventilation wholes at the top and inserting weeps at the bottom of the wall.  This step should be taken with great care, ideally with the involvement of an experience preservation architect. Consider experimenting in an inconspicuous location to see if the technique is effective and does not have unexpected side effects. Also, make sure that any openings do not create places of entry for vermin.
  • Repointing: proper repointing is an important part of building maintenance, but it can often be detrimental when it is done with too strong a mortar. Also, the common practice of repointing with mortar flush to the surface of the brick is counter to Wright’s common use of recessed mortar joints to emphasize a building’s horizontality. See the National Park Service brief referenced above.

Sealers: Sealers should be used with great care. If the source of a problem is not understood, they can cause more damage than the original problem. For walls coped with brick, sealing the top only with a water-repellant sealer may be considered. This would significantly reduce the amount of liquid water that enters a wall, but by leaving the vertical walls unsealed, water vapor that changes to liquid within the wall would still be allowed to escape. See the National Park Service brief referenced above.

Possible use of sealer:

Sealer may be considered in locations where they do not inhibit water movement out of a wall.

  • Introduction of through-wall flashing beneath rowlock copings: At rowlock copings that are in need of repointing or resetting of the rowlock bricks due to deterioration, the insertion of a flashing in the mortar bed beneath the rowlock will prevent water from penetrating further down into the wall.
  • Introduction of a coping flashing atop a rowlock coping: At locations where the top of the wall is not visible from the ground or principal views of the building, consideration may be given to the introduction of a copper, zinc coated copper, or stainless steel flashing atop the wall. These are suitable metals to use with masonry and are compatible with mortar – note: as natural copper patinas could leave a green stain on masonry surfaces where there is moisture run-off. The edge of the metal must have a slight pitch to drain to one side or the other of the wall and extend a fraction of an inch over the coping edge to drip free of the wall. This subtle “hat” can provide protection from moisture infiltration through the top of a wall where removal of the rowlock for through-wall flashing is not practical or desired.

The steps suggested here for rowlock courses may also be considered for chimney tops.

  • Brick replacement: Bricks that are damaged may require replacement. Matching brick is tricky job, not only are color and texture important, but older brick was often fired differently from modern brick. This could give the brick you’re trying to match a subtle difference in finish that may only shows up in certain light. It’s important to compare actual samples of proposed brick to be used on site with the brick that is being replaced.

Salvage brick may be an option to consider.  This can take time and effort, but you may be able to find appropriate brick this way. An experience restoration mason may have connections that can lead to salvage brick, but there is no guarantee that the right brick will be available at a given time. You may need to consider temporary repairs while making your search. Depending on the type of deterioration, type of brick and its face texture, it is possible the unit could be removed, cleaned and reversed with the backside exposed.

Masonry Addendum: Limestone

This addendum builds on the preceding information on brick masonry. Masonry information from above applies except as noted.

Limestone has been a preferred building material since the time of pyramids and the sphinx.  Over the sweep of time, it has proven to be versatile, durable and cost efficient.  Its properties yield density for strength but is also soft enough to be shaped.  Limestone’s advantages for residential construction feature temperature control, low maintenance and provide an elegant, warming, natural feeling that only improves with age.  Wright employed it for Taliesin, his own home, and specified its use for many of his designs.

Limestone is a sedimentation formation and, therefore, exhibits variation in size, density and color.  For example, Indiana Limestone is soft while Wisconsin Dolomite Limestone is hard.  As the forgoing study for brick has informed, this difference must be considered when selecting the appropriate mortar.  Color can vary from shades of white, yellow, pink, and orange.

Construction methods for Wright’s limestone reveal significant differences to that of brick.  While brick surfaces are generally uniform in plane, limestone is irregular in shape and may be laid with random stone projecting beyond the plane of the walls.  This patterning is a distinctive feature in Wright buildings and provides greater surface texture.

Below are examples of a typical brick pattern (left) and a limestone pattern (right). Limestone photos by Ted Muntz

Differences in limestone used in construction are also noteworthy.  The rock may be rough cut on all sides or fabricated smooth on top and bottom for more uniform placement.  In either application, however, the stone must be laid so that the rock face is facing slightly down in order to shed moisture away from the recessed mortar.  This can be particularly difficult for projected, rough-cut limestone which presents more opportunities for moisture retention.

Below are images of limestone that is Rough Cut on all sides (left) and Machined on the top and bottom (right)

Maintenance for limestone and mortar are similar as for brick – keep surfaces clear of organic build-up and prevent moisture from contact with mortar.  Periodic pressure washing will remove organic build-up but may be an indication that further remediation is necessary.  Pressure washing limestone should not exceed 500 psi.  Chemical cleaning is also an option.  “Wet and Forget” is one such product which is commercially available in both interior and exterior formulations.

Replacement of spalling limestone and deteriorated mortar, when necessary, should be performed by knowledgeable contractors experienced in historical building practices.  Photographs should be taken prior to the work being done and stones marked and reinstalled in their original places and any replacements made to a similar size, shape and color.  All stone should be laid with stone faces facing down.  Stones that exhibit a potential for the same problem but were not removed, may be slightly and inconspicuously machined to the same effect, as illustrated below.

In addition, the appropriate mortar should be selected, color matched to the existing mortar, raked and finished with a rounding jointing tool to a slightly concave surface when the mortar is thumbprint hard.  This will more effectively shed moisture from the mortar cavity.

Below are images of  a raked mortar joint (left) and a concave mortar joint (right)

Please Note: The provision of this information or mention of a specific product or products does not constitute endorsement, recommendation, preference or approval by the Conservancy.


Posted April 18, 2022

Posted April 18, 2022