While many joints are partially or completely filled (closed) with one or more materials, some others are left as (open) voids without any filler, such as the aforementioned breaks between pieces of roof or facade cladding or open gaps between panels in rainscreen cladding. Open joints are no less important to the performance of building enclosures, as there are reasons for their use. When choosing to fill a joint or not, it is essential to understand its intended purpose within an assembly, and the properties the joint should embody to fulfill that purpose.
Joint responsibilities: Movement
One of the most important roles that joints serve is providing accommodations for movement. As noted, buildings are subject to dynamic interactions, including cyclical processes such as thermal expansion and contraction. When enclosures are heated from daytime solar radiation and hot outside air during warmer seasons, they become larger. Inversely, they become smaller during evenings and cooler seasons. Wetting and drying is another cyclical process, as porous exterior materials such as wood and stone can retain moisture when exposed to precipitation and high humidity, causing temporary swelling.
Cumulative movement is also a factor, such as the enlargement of clay masonry from gradual reabsorption of moisture, or the shrinking of concrete materials as they cure. Differential settlement is a cumulative movement that occurs when adjacent parts of an enclosure sink at different rates or to different degrees. This can be due to variations in soil conditions, foundation configuration, or the weight of the enclosure itself. Everchanging gravity loading from snow, people, and furniture, and lateral loads such as wind pressure and seismic activity, are also factors that cause structural deflection and building movement.
In all these situations, thoughtfully designed joints facilitate overall or localized flexibility or rigidity of an enclosure where it is determined a building should sway with or brace against forces. Closed filled joints absorb the stress of stretched, compressed, or sliding adjacent enclosure parts. Open joints provide designated spaces for adjacent enclosure parts to move into or away from as they expand, contract, settle, or shear. For closed joints, selecting materials that provide necessary softness and elasticity is essential; otherwise, movement stress could rip the joint material apart and result in cohesive failure (cracking or tearing of the joint).
Movement stress can also cause adhesive failure if the joint infill is not properly bonded to its adjacent enclosure parts, causing the material to separate from its substrate. Although the entire enclosure may be subject to multiple forms of movement, some joints are more responsible than others for absorbing and mitigating these forces, depending on assembly type. Expansion joints, for example, typically installed at certain intervals along the length of an enclosure system, are specifically designated to help relieve cyclical thermal stress. Similarly, control joints may be installed at certain intervals in concrete systems to accommodate cumulative shrinking.
