Architectural expansion joints are necessary, predetermined gaps in large structures that are designed to absorb environmental movement in buildings. When done correctly, they tend to be integrated with their construction such that they blend in with a design and almost disappear. Hence, it is easy to overlook the fact that they can be a potential source of water and moisture infiltration and damage. This infiltration could be problematic for the expansion joint itself, or it could cause problems for other building materials or occupants too. Either way, when using expansion joints that need to cut across exterior surfaces, their ability to resist water needs to be factored in along with the other requirements for the joints.

All images courtesy of Inpro

Expansion joints are used in larger buildings to allow movement to occur, but at the same time, they need to provide protection from water intrusion.

This course will look at the ways that expansion joint filler systems can be designed and specified to provide the needed performance characteristics and still remain water resistive. In the process, it will examine the types of moisture and bulk water concerns that need to be addressed, the other performance needs of expansion joints, and the various types of solutions available.

Water and Building Envelopes

Data from the United States Environmental Protection Agency (EPA) indicates that most buildings are likely to experience some form of impact from unwanted or excessive moisture accumulation. Those conditions can lead to serious problems, such as the degradation, deterioration, or even failure of building materials, development of mold and mildew, and possible risks to human health and safety. Repairing any of these conditions after the building is constructed and occupied typically involves opening up construction assemblies, which is disruptive, time-consuming, and costly. Hence, it is no wonder that there is great interest in understanding how moisture can be controlled in buildings to avoid any or all of these potential problems and risks.

The Whole Building Design Guide (WBDG), a program of the National Institute of Building Sciences, provides some of the best, objective, state-of-the-art thinking on this topic. It identifies three main causes of moisture movement: 1) water impingement or leakage (as in a roof, wall, or floor system), 2) movement of moist air (through gaps or openings in roofs, walls, or floors), and 3) vapor diffusion through materials that can occur slowly over time but saturate and damage materials nonetheless. The WBDG points out that solutions cover the gamut of design and construction activities, stating, “Preventive and remedial measures include rainwater tight detail design; prevention of uncontrolled air movement; reduction of indoor air moisture content; reduction of water vapor diffusion into walls and roofs; selection of building materials with appropriate water transmission characteristics; and proper field workmanship quality control.” Listed all together, this may sound like a tall order, but in essence, it means that everyone involved in a building project has a role to play in managing moisture in buildings, starting with the design team.

The potential effects of water and moisture in buildings on human health should not be overlooked either. Wet materials that are organic (e.g., wood, paper, cellulosic, etc.) can become perfect breeding grounds for mold and mildew to form. Many people will react negatively to the release of those spores into the air, causing respiratory ailments or exacerbating existing ones, such as asthma. When those symptoms are found to be connected to the building conditions, issues of professional liability and risk management become top of mind.

Expansion Joints in Buildings

With an understanding of the water issue, let us focus now on an overview of what expansion joints really are and how they are designed into buildings. Expansion joints are fundamentally defined as predetermined gaps in building structures designed to allow for environmental movement. The location, size, and movement requirements for all such expansion joints are project specific and appropriately established by the structural engineer of record.

Recognizing that an open joint in a building needs to be addressed, it is then typically up to the architect to select the means to cover or seal the joint. Common ways to do this can involve caulking or sealant for narrow joints, compressible fillers, or metal covers that are exposed or concealed. The typical traits of an architectural joint cover system include the ability to absorb building movement, support a given load, maintain safe egress where applicable, and be compatible with adjacent surface finishes.

Before looking closer at those filler or architectural cover systems, the following are some things to be aware of regarding expansion joint design in general.

Nominal Joint Size

The design width of an expansion joint at an average air temperature is referred to as the nominal joint size. The selection of any type of joint filler or cover system starts with understanding this nominal joint size and the range of movement between the minimum fully contracted size and the maximum fully expanded size. The expansion joint system selected needs to accommodate this full movement range.

Identifying the nominal joint width and the anticipated movement type that a joint is subjected to is the first step in designing an appropriate expansion joint system.

Type of Movement

Building sections can move due to several common reasons. Thermal movements are most typical and caused by daily environmental temperature changes in and around the structure. Thermal movement is primarily one-directional in nature and is the result of the expansion and contraction of structural elements as affected by heat, cold, and humidity levels. The amount of thermal movement is typically approximately 10–25 percent of the nominal joint size. This means the minimum contracted size (during hot temperatures) should be 10–25 percent less than the nominal joint size, and the maximum expanded size (during cold temperatures) should be 10–25 percent more than the nominal joint size.

Seismic activity can also be a source of movement that may be horizontal, vertical, in shear, or a combination of all three. Seismic joint widths may need to increase with higher floor levels to protect a structure during earthquakes or other seismic events. These joints must have the capacity for movement of approximately 50–100 percent of the nominal joint size.

Finally, wind-load-induced movement, created by high winds, can cause a structure to sway back and forth. Such wind-load-induced movement can be perpendicular or parallel to the joint.