Preventing and treating distress in brick veneer cavity walls [AIA course]

Masonry construction is one of the oldest methods of creating built structures. However, the brick wall has changed considerably since the first fired clay bricks were laid nearly 8,000 years ago. To address problems and considerations with modern brick masonry, it’s important to understand the components of the wall assembly, how they behave, and what can go wrong.

Today’s complex brick cavity walls share little in common with the solid masonry construction of Romanesque church towers or early load-bearing high-rises, with their thick, stout walls. With multiple layers, or wythes, of brick bonded together, these historic masonry walls relied on the unified structural capacity of the wall thickness to withstand horizontal and vertical forces and to provide weather protection.

LEARNING OBJECTIVES

After reading this article, you should be able to:

+ Identify the basic components of a brick veneer cavity wall and explain their function as part of the larger assembly.

+ Develop an appropriate inspection and maintenance program for brick masonry façades, to promote the longevity and integrity of the wall assembly.

+ Describe applicable codes and standards governing the design and construction of brick veneer cavity walls.

+ Recognize the observable symptoms of common types of masonry distress and accurately attribute the outward signs of trouble to the underlying problem

In contrast, modern brick veneer cavity walls anchor a single wythe of face brick across an air space, typically two to four inches wide, to a backup material. Part of a drainage system that includes flashings, drip edges, and weep holes, the air space creates a pathway for moisture to exit the wall assembly, while a steel, concrete, or masonry backup provides structural support.

What may seem on the outside to be a wall composed of a single material—clay brick—is actually a composite wall system, with materials ranging from steel and concrete to flexible flashings and sealant, in addition to traditional brick-and-mortar elements. How and where these materials come together, and why they behave the way they do, is of critical importance to the weather protection, integrity, and longevity of the wall assembly.

As modern masonry construction has grown increasingly complex, so too has the design, detailing, and installation of brick cavity walls become more demanding. Where once a skilled mason was all it took to achieve a durable exterior wall, proper construction now demands the efforts of numerous tradespeople working in collaboration with the design professional to achieve a watertight, structurally stable, aesthetically appealing masonry exterior.

Prevention of water infiltration, structural failure, and other woes by means of appropriate design and meticulous workmanship is the ideal, but for existing buildings, the mission becomes timely and accurate identification of emerging problems, with repair strategies that provide lasting solutions.

The Basics of Brick Wall Design

As reinforced concrete and steel framing have eliminated the need for load-bearing masonry, building design has evolved new approaches to waterproofing protection. Early brick walls relied on the mass and depth of the masonry to absorb rainwater and ambient moisture, and to release it back into the atmosphere. Given the nominal thickness of modern brick veneers, the mass of masonry is insufficient to absorb and release environmental moisture without allowing water penetration into the building interior. Therefore, cavity wall design provides for a space between the back of the brick veneer and the face of the backup material, so that water that breaches the brick exterior can drain out of the wall system without reaching the building interior.

Flashing system and waterproofing. Where transitions must exist between the brick veneer and the backup, such as at shelf angles, lintels, and the base of the wall, flashing—a flexible, impermeable material—is used to collect water and drain it to the exterior. Serving both to direct water and protect the brick masonry from moisture damage, the flashing is in turn protected by counter-flashings, which are attached to or directly laid into the backup. At the face of the wall, drip edges, or downward bends in rigid flashings, encourage water to form droplets that fall away from the wall surface, rather than travel back up under the flashing and into the wall assembly.

Copper, lead-coated copper, and stainless steel are the traditional materials for flashings, and they remain the most durable and reliable options. However, these materials are expensive, so flashing tends to be composed of flexible plastics, fabric, and composite metals. Given the projected lifespan of a modern wall assembly, it’s best to avoid materials that degrade quickly, such as polyvinylchloride (PVC), which may last as little as five years.

Water collected by the flashings is drained to the exterior by means of weeps. While open head joints in the brick course above the flashings provide a simple, effective weep system, vents, screens, and other inserts may be used to disguise the open joints and prevent insect ingress.

Lateral support. Masonry anchors secure the wall assembly to the building structure, while masonry ties connect multiple wythes of masonry together or join a masonry veneer to a backup wall composed of another material, such as concrete masonry units (CMU) or metal studs with sheathing. All of the metal accessories for a masonry cavity wall should be stainless steel and spaced at appropriate intervals, as determined by building codes, industry standards, and the design professional.

To control shrinkage cracks in masonry, as well as to tie multiple wythes of masonry together and to anchor masonry veneers, horizontal joint reinforcement is incorporated into the exterior wall system. Two or more longitudinal wires with perpendicular (ladder type) or angled (truss type) cross wires are laid in the mortar joint, with the longitudinal wires parallel to the face of the wall.

Provisions for movement. Building materials expand or contract when exposed to external stresses, such as changes in temperature (thermal movement), moisture/humidity (moisture movement), dead and live loads and external lateral forces (elastic deformation/creep). As brick draws in moisture from its environment, it will increase in size over an extended period of time. Most of the expansion typically occurs in the first few months after the brick is fired, but continues at a lower rate over the following years.

Another main contributor to movement of brick is thermal expansion and contraction. Because both thermal and moisture volume changes are related to the height of the wall assembly, their cumulative effect can be significant, particularly over tall sections of veneer.

Just as brick expands over time, concrete tends to shrink. As a composite wall system that often incorporates both of these materials, a brick masonry cavity wall must accommodate these tendencies. Typically, provision for movement is achieved through horizontal and vertical expansion joints and shelf angles, along with adjustable veneer anchors that allow the two materials to move differentially within their planes, while still providing anchorage to resist out-of-plane forces, including wind and seismic pressures.

To accommodate vertical movement, steel shelf angles (also known as relieving angles) may be installed at intervals along the wall elevation, typically at each floor, to support the masonry above a horizontal expansion joint, allowing for vertical expansion of the brickwork. Use of shelf angles depends upon the type of structure, height of the building, size and location of windows, type of masonry anchorage, and a variety of other factors, including building code requirements.

Along with shelf angles, vertical and horizontal expansion joints are used to separate the masonry wall into segments and thereby prevent cracking. As dissimilar materials in the assembly change in response to temperature, moisture expansion, elastic deformation, settlement, and creep, each will move according to its own tendencies. Horizontal expansion joints are particularly important for masonry veneers attached to a reinforced concrete frame, as the concrete backup tends to shrink, while the brick tends to expand, a phenomenon known as frame shortening. Composed of flexible materials, expansion joints can close and stretch as building components shrink or expand.

Spacing of vertical expansion joints is determined by considering anticipated wall movement, size of the joint, and compressibility of the joint materials. Typically, expansion joints are placed near corners, where stress may be greatest, as well as at or near window and door openings, as appropriate.

EDITOR’S NOTE

Additional reading is required for this course. To earn 1.0 AIA CES learning units, study the article carefully and take the exam.

Coursing and dimensions. The appearance of brick masonry walls is characterized by the type and dimensions of the brick units, the mortar type and tooling profile of the joints, and the coursing, or pattern, of the brick layout. In contemporary brick cavity walls, the brick veneer is secured with metal wall ties to the backup material. However, façade designs may incorporate false headers in the veneer to simulate traditional coursing patterns.

Brick size is standardized according to “nominal dimensions,” which account not only for the size of the brick unit itself, but for the completed assembly in the wall system, including mortar joints. Bear in mind that openings, corners, and wall heights must take into account the dimensions and coursing using brick module, or increments based on the size of the brick.

Mortar joints. Regardless of the type of masonry construction, all brick units are held together using mortar, a mixture of cement, aggregate, and water that is buttered, or spread, between bricks during stacking. There is no single best mortar type for all structures and situations, but a good maxim is to select the weakest mortar that will do the job. Mortar that is too hard does not permit movement of adjacent brick and can cause cracks and spalls, in which pieces of the brick face are forced off.

Different joint types provide different weathering capabilities, with concave, rodded, or V-shaped joints providing the best durability and water penetration resistance. Struck, raked, beaded, or extruded tooling profiles should be used with caution, as they tend to provide poor weather protection and degrade more quickly.

Code requirements. The detailing and structural design of masonry is dictated by the governing building code, which often references ACI 530 – Building Code Requirements for Masonry Structures, a consensus standard from the American Concrete Institute (ACI), the American Society of Civil Engineers (ASCE), and the Masonry Society (TMS). In addition to the standards set forth in these codes, technical publications from the Brick Industry Association (known as Tech Notes) are typically used when designing and detailing masonry structures.

Building codes and standards include prescriptive requirements for the attachment of the veneer to a variety of backup materials, such as wood, metal framing, and CMU. Wind and seismic design requirements for each type of construction are typically based on ASCE 7 – Minimum Design Loads for Buildings and Other Structures, or on the governing building code, which often references ASCE 7.

Periodic Inspection and Maintenance for Brick Masonry Walls: If properly designed and constructed, brick masonry can be highly durable and tends to demand little in the way of routine repairs. However, it is still important to conduct regular inspection of the building façade to identify emerging problems and to plan for replacement of materials approaching the end of their lifespan. For tips on inspection and maintenance, visit www.BDCnetwork.com/BrickTips.

Common Problems and Their Causes

Like all building assemblies, brick masonry is not without problems inherent to the material, the type of construction, and shortcomings in the design and construction of the wall assembly. Astute observation of the early warning signs of masonry distress enables prompt remediation of the problem, which often provides a cost savings over the long term. Emerging problems are best addressed well before they become emergencies.

Of all the problems associated with brick masonry construction, those resulting from water penetration are the most common.

Efflorescence. Observable as white stains or salty streaks on the surface of masonry walls, efflorescence occurs when moisture within the wall assembly leaches water-soluble salts from the mortar or masonry. The main cause of efflorescence is water infiltration, whether from poor mortar joints, cracked brick, or other sources. Also, sloppy workmanship can clog the wall cavity with excess mortar and prevent water from reaching flashings and exiting the wall. Forced to find another means of escape, trapped moisture travels through the brick, resulting in efflorescence.

Once the problem has been identified and remediated, it may be difficult or even impossible to clean away salt deposits that have bonded to the masonry surface. If moisture has reached the building interior, water damage and mold may also need to be addressed.

Freeze-thaw damage. In temperate climates, water that penetrates masonry walls may lead to structural damage, as low temperatures cause trapped moisture to freeze and expand, applying pressure to the surrounding materials. Typical causes of excess water infiltration in masonry structures include mortar profiles that trap water and direct it into the veneer, poor flashing details around openings or penetrations that allow moisture into the wall system, and roof leaks that travel down into the wall assembly.

Condensation. In some climates and conditions, differences in humidity and air pressure between interior and exterior can drive water vapor into the wall, where it may condense and lead to moisture-related problems. Design provisions to reduce air infiltration, vapor permeability, and thermal bridging can help to control condensation.

Flashing problems. Flashing detailing is particularly important at intersections and terminations. Where the segments of continuous flashing intersect, they should be lapped and sealed using a method appropriate to the material. Discontinuous flashing, as at a window or door opening, should extend beyond the end of the lintel, with the ends turned up to prevent water from running back into the wall from the edge of the flashing.

Plastics, composites, and thin metal cannot be formed into a drip edge. Instead, these materials should be terminated within the veneer to prevent degradation or drooling, in which heat and ultraviolet radiation cause rubberized flashing to soften and exude from the joint.

Restricted movement. When expansion joints are too narrow or spaced too far apart, there is insufficient accommodation for brick expansion, forcing sealant out of the joint and eventually leading to cracks and failure of the surrounding masonry. Failure to include expansion joints at building corners is a common cause of distress. As the masonry expands along the plane of the wall, the brick on each side of the corner comes together, leading to long vertical cracks or brick displacement at building corners.

Structural distress in brick masonry cavity walls is often related to the corrosion of embedded steel elements, which can lead to cracks and displacement. As water penetration causes steel lintels, reinforcement, anchors, ties, and accessories to corrode, they expand, exerting tremendous pressure on the surrounding façade, which can sometimes fail dramatically as a result. Insufficient or failed anchorage of the veneer to the backup can lead to bowing or lateral displacement of the masonry.

Readily identifiable by their tapering shape, deflection cracks may occur at steel shelf angles attached to spandrel beams that deflect. Uneven settlement in the foundation due to unstable soil conditions may also be the cause of stress cracking in masonry walls, as one portion of the structure settles more than another.

Preserving Modern Masonry Façades

Popular for their superior resistance to rain penetration, sound transmission, fire, and heat transfer—and for their cost-effectiveness—masonry cavity walls are ubiquitous across all building styles and types. Although cavity walls did not even feature in building codes until the late 1930s, the extensive testing, research, and field performance data that have emerged since then have refined brick cavity walls to the degree that the Brick Industry Association calls them “the premier masonry wall system.”

Like all building assemblies, even the dependable masonry cavity wall can succumb to leaks, cracks, stains, and deterioration if not correctly designed, built, and maintained. To treat persistent problems, and, better, to prevent new ones from emerging, it is important to understand the basics of brick cavity wall construction and to recognize the symptoms of distress and failure.

By aggressively addressing incipient design, detailing, workmanship, and age-related problems, Building Teams can prolong the life of the building and avoid the expense and disruption of major rehabilitation.

About the Authors: Erin Kesegi, Senior Architect with Hoffmann Architects, applies her expertise in building envelope remediation to address underlying causes of brick masonry distress. Robert Marsoli, Jr., Project Manager, provides engineering services to resolve structural concerns for a range of building types, including brick masonry construction. Both work from the firm’s main office in Hamden, Conn.