Every exterior wall assembly faces one relentless enemy: water driven sideways by wind. Rain screen detailing is the architectural strategy for managing that threat, and on the PDD exam, you need to know how to resolve these assemblies at the detail level.

A rain screen wall separates the outer cladding from the structural backup with a deliberate air cavity. That cavity serves three purposes: it drains water that penetrates the cladding, it allows airflow to dry residual moisture, and when properly compartmentalized, it equalizes air pressure across the cladding so wind can't push water through joints.

Pressure equalization is the key concept here. When the air pressure inside the cavity matches the wind pressure outside the cladding, the driving force behind water infiltration drops to nearly zero. Achieving this requires compartmentalizing the cavity into small zones with vent openings sized to allow rapid pressure response.

Veneer anchoring ties the outer skin back to the structural wall while allowing differential movement. Brick ties, stone anchors, and metal panel clips all need to accommodate thermal expansion, structural deflection, and moisture-related dimensional changes without creating thermal bridges or water paths.

This topic sits squarely in PDD territory because it's about detailing and documenting these systems, not selecting them. You're resolving how the drainage plane stays continuous at window heads, how flashing turns corners, where weep holes go, and how the air barrier connects across transitions.