Modern exterior cladding is designed to protect buildings from the elements, but no cladding system is completely waterproof. Wind-driven rain, changing weather conditions, and normal building movement can allow small amounts of water to penetrate behind siding, brick, stone, or other façade materials.
This reality is why today’s building envelope systems are designed around water management rather than water exclusion. One of the most important components of that strategy is the drainage plane.
What Is a Drainage Plane?
A drainage plane is the pathway that allows incidental water to move safely down and out of a wall assembly before it can damage the structure.
Instead of assuming water will never get behind the cladding, modern building science accepts that some moisture intrusion is inevitable. The goal is to provide a continuous path for that water to escape.
Properly designed drainage planes help protect wall assemblies by directing moisture toward flashing, weep systems, or other designated exit points.
Why Cladding Alone Isn’t Enough
Exterior cladding serves as the building’s first line of defense, shedding the majority of rainwater before it reaches the wall assembly.
However, joints, fasteners, penetrations, and transitions can all create opportunities for moisture to bypass the exterior surface.
Without a functioning drainage plane, incidental water may remain trapped within the wall assembly, increasing the risk of material deterioration, mold growth, and reduced insulation performance over time.
The Role of a Weather-Resistive Barrier
A weather-resistive barrier (WRB) serves as the secondary line of defense by helping prevent liquid water from reaching the structural wall components.
When paired with proper flashing and detailing, the WRB directs water downward where it can safely exit the building envelope.
Many modern WRBs are also vapor permeable, allowing moisture vapor to escape while continuing to resist liquid water intrusion.
Why Drainable WRBs Add Another Layer of Protection
Some wall assemblies incorporate drainable weather-resistive barriers, which include an integrated drainage space behind the cladding.
This small separation creates a pathway that encourages water to drain while also promoting airflow that can assist with drying. By reducing prolonged moisture contact between the cladding and the WRB, drainable systems can support long-term wall performance in many building applications.
Where ALTA® Fits
ALTA® 360 is engineered as a drainable weather-resistive barrier designed to support moisture management behind exterior cladding. By combining a vapor-permeable WRB with an integrated drainage layer, ALTA helps create a continuous drainage path while allowing the wall assembly to maintain drying potential.
Rather than relying on a single component, ALTA is designed to work alongside flashing, air control layers, insulation, and exterior cladding as part of a complete building envelope system.
Looking Ahead
A drainage plane is one of the building envelope’s most important—but often least visible—features. While homeowners may never see it, the ability to safely manage incidental moisture can have a significant impact on the long-term durability of a building.
In Part 4 of our Building Envelope Basics series, we’ll explore vapor permeability and explain why allowing wall assemblies to dry is just as important as keeping water out.