Preventing and treating distress in brick veneer cavity walls
asonry 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.
The design, detailing, and installation of brick cavity walls become more demanding as masonry construction becomes more complex.
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.
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.