What Is Roof Flashing?

The main purpose of a roof structure is to protect a home or commercial building against outside weather conditions.

By far, roofers would agree that the most difficult element to manage is water, whether it comes in the form of rain, snow, sleet, or hail. After a roof has been installed properly based on the specifications listed in the previous chapter, roof flashing is the final defense barrier against water leakage.

In its most basic form, roof flashing is sheet metal that is placed over walls and roof joints to keep outside water from seeping into a building.

Roof flashing is manufactured as a thin piece of sheet metal, normally made from galvanized steel or aluminum. It will be installed to resist water at a vulnerable point in a roof structure, like a joint or angle.

Flashing is a beneficial addition to a roof since it makes it difficult, if not impossible, for water to penetrate a roof joint; in order do so, water must move against gravity to breach roofing materials underneath or over metal flashing.

Upon installation, flashing on a roof can be either concealed or exposed. Flashing may serve a number of different purposes, like:

  • Flashing placed around protruding objects on a roof, like chimneys and pipes.
  • Flashing embedded into a wall to direct leaking water outside of a structure.
  • Flashing concealed underneath doors and windowsills to prevent water entry.
  • Flashing constructed in a wall base to keep water from escaping.
  • Flashing to prevent roof penetration by waterproofing supports, pipes, cables, and other protrusions.

Although it may seem simple, flashing may be the most difficult and most technical aspect of roof installation.

Examples of roof flashing can be seen below:11


Flashing will be installed at intersecting walls, parapets, and roofs. Roof flashing will be made up of two different components used to discourage water accumulation:

  1. Base Flashing
  2. Cap/Counter Flashing

Base flashing is installed directly onto a roof structure in an L shape, with one portion of the flashing that extends underneath roofing materials by a minimum of 6 inches, if not more. The other portion of the L flashing will extend to protect the vertical adjoining roof surface.

Cap flashing will be attached directly to a wall or projection that intersects with a roof. Cap flashing is intended to overlap base flashing by minimum of 6 inches so that water can’t penetrate exposed joints in a roof structure.

While base and cap flashing are most commonly used independent of one another to allow for the full reinforcement of roof and wall structures, the two may bind on occasion, often related to ice damage or improper installation.

Since roofing materials are known to expand and contract with changes in season, flashing will provide a leak-proof barrier to support a roof’s integrity.