Structural securement of enclosure parts can be achieved with other joint materials. In many sheet-metal cladding systems, panels are anchored at their overlapping seams. The hemmed edges of adjoining metal parts interlock with one another, in a series of folded metal cleats concealed by the seams, which are mechanically fastened to the substrate. These joints can take on various configurations, including standing seams (interlocking projecting flanges with concealed anchor clips), batten seams (a historic system with sheet-metal-capped wood battens as the primary points of connection), and flat-lock seams (which have no projections and interlock directly). Since exposed sheet metal can be especially prone to thermal expansion, these joints also play an essential role in accommodating movement. Neglecting to regularly space joints at intervals appropriate to the type and thickness of the metal cladding can cause buckling and connection failure as it places excessive stress on the system. Providing sufficiently spaced cleats is also critical for these joints, as inadequate securement can result in loose and broken connections from wind uplift. For cladding that involves two or more metals, sealant or foam pad infill may be required at joints to separate the dissimilar metals, which would otherwise succumb to corrosion from galvanic action.
Joint responsibilities: Weatherproofing
Another important responsibility of joints is controlling the passage of water, vapor (water in gas form), and air. There are several ways water can move through enclosure joints. Bulk water enters primarily by gravity, as precipitation washes down enclosure surfaces and falls through openings, as well as through hydrostatic pressure, when accumulated water pushes through the surface. Wind and air pressure differentials between the two sides of an enclosure can also drive precipitation into openings. Smaller amounts of moisture can enter via capillary action (water working through microscopic pores) and surface tension (water adhering to internal surfaces and traveling along them). In general, air does not pass through most filled (closed) enclosure joints unless the joints have open cells or vents that intentionally facilitate air movement. A common misconception is that joints always should be filled with materials that completely seal against all moisture intrusion, to avoid leaks and reduce energy loss. However, there are scenarios where maintaining breathability is essential. Understanding the intended behavior of different enclosure assemblies is key to knowing if and how associated joints should be open or closed.
While dabs of roofing cement or mortar may be used for adhesion, many steep-slope roof systems, including metal, asphalt, slate, and clay shingles, have unsealed open joints. This is because they rely primarily on hydro-kinetic activity (the shedding of water down a steep pitch) to move bulk water off the roof before it accumulates and penetrates. Open joints also facilitate air and vapor movement through the roofing system to reduce condensation and mold in attic spaces and prevent decay of wood substrates. Many wall cladding and siding systems operate similarly, as boards or panels can be positively lapped to protect the top joints, but allow for incidental moisture to weep out bottom joints.
Although open joints in many rainscreen wall cladding systems are not lapped, unless designed in a louver or shingle configuration, they still shield against bulk water from precipitation. Moisture can work its way through the open joints by wind and surface tension, but the air cavity behind the cladding facilitates moisture evaporation through ventilation. Some rainscreen systems are pressure equalized, vented in a way to allow air flow inside the cavity to equal outside air pressure, reducing the likelihood of moisture penetration from wind-driven rain. In addition, anchor straps, which secure the cladding to the substrate, often have profiles that facilitate water drainage through the cavity. Most open-joint systems incorporate underlayments or membranes attached to the substrate to further protect from moisture, vapor, and air infiltration.
Unlike steep-slope roofing, low-slope assemblies contain sealed, closed joints. This is because their lack of significant pitch requires them to deal with hydrostatic pressure, so the entire system must be watertight. For flat-seam sheet metal roofing, this is achieved with solder, a tin-lead mixture melted into the joints with an iron. For membrane roofing, joints are closed by positively lapping the membrane sheets and then sealing laps together. This can be accomplished in various ways, including cold-applied roofing adhesive, heat welding (which fuses modified bitumen membranes together with torch application), seam tape (common for single-ply synthetic rubber and plastic membranes), or liquid-applied membrane sealing (frequently used with modified bitumen roofing).
