What is Thru-Wall Flashing?

Thru-wall flashing is a waterproof barrier built into a wall to direct water out of the wall and prevent moisture from damaging the building.

Definition

Thru-wall flashing is a waterproofing material installed across the thickness of a wall, usually where moisture could collect or enter the building, to catch water that gets past the exterior surface and direct it safely outside.

It is typically made from materials such as sheet metal, rubberized asphalt, or flexible waterproof membranes. It is installed at key points in the wall, including the base of masonry walls, above openings, and where different building materials meet.

Purpose

Its main purpose is to keep water from getting trapped inside a wall assembly, since even well-built masonry walls can allow some moisture to seep through joints or absorb into the material. Flashing collects this water and directs it toward the exterior through drainage openings called weep holes.

By managing moisture, thru-wall flashing helps prevent water damage, mold growth, deterioration of masonry, and corrosion of metal components. It also helps protect the building's structural elements and extends the life of the wall assembly.

Examples of Use

Thru-wall flashing is used in several areas where water needs a clear path out of the wall.

At the base of masonry walls: Installed near the bottom of a brick veneer wall to direct moisture out through weep holes before it reaches the building's interior.
Above windows and doors: Used above masonry openings to collect water and direct it away from the frames and surrounding wall assembly.
At shelf angles: Installed where masonry is supported by steel shelf angles to prevent water from collecting around the structural supports.
At roof-to-wall intersections: Used where a roof meets a masonry wall to help manage water and prevent it from entering the wall assembly.
At changes in wall materials: Installed where different materials or sections of a wall meet to maintain a continuous moisture-management system.

Related Terms

Balustrade

A balustrade is a row of vertical supports topped by a railing that's used along stairs, balconies, decks, and other elevated areas.

Barrel Shell

A barrel shell is a curved roof or structural surface shaped like a half-cylinder, designed to span large spaces and distribute loads across its structure.

Cantilever

A cantilever is a structural element that extends horizontally beyond its support, with one end fixed and the other free.

Notes

Thru-wall flashing is an important part of a building's moisture-management system.
It should be installed with the correct slope and termination details so water drains toward the exterior.
Proper overlaps, sealed joints, and end dams help prevent water from leaking behind the flashing.
The material and installation method should be suitable for the wall assembly and compatible with adjacent building materials.
Poorly installed or damaged flashing can allow moisture to build up inside the wall, potentially leading to costly repairs.

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