How Does Roof Flashing Work?

When it comes to the stuff that protects your home from rain, snow, wind and hail, the shingles on the roof often get most of the credit. Sure, shingles are durable and form a shield over your head, but they are not the only piece of a roofing system that play an important role. Another piece—roofing flashing—is an underestimated but very essential hero.

If you understand what roof flashing does, you are better prepared to make roofing decisions, since you won’t overlook this integral part of the roofing system. So let’s get started.

What Is Roof Flashing?

Most flashing is made of a rust-resistant material, such as aluminum, copper, or galvanized steel. Sometimes, it is made out of rubber, plastic, or roofing felt, but that is rare. The most common is galvanized steel, while copper and aluminum are usually customized.

Flashing is also formed or soldered into any number of shapes, depending on where it is going to be placed. The main types of flashing include:

  • Chimney flashing – applied around the base of chimneys; requires continuous flashing and saddle flashing
  • Drip edges – prevents water from getting under rakes and eaves
  • Continuous flashing – protects joints between sloped roofing and vertical walls
  • Step flashing – found where side walls meet dormers, chimneys, and skylights. Requires angled pieces.
  • Skylight flashing – usually, a skylight comes with its own flashing to prevent leaks
  • Valley flashing – found where two planes meet; a W-shaped channel
  • Vent pipe flashing – cone-shaped; fits over pipes and flutes

What Is The Purpose of Roof Flashing?

Flashing is a thin, flat, weatherproof piece of material that, with the assistance of something called underlayment, helps keep moisture from getting under the shingles and ruining your roof. That is why you find flashing in the most critical sections of the roof, such as the places where a roof meets a wall and creates a seam (called a valley), around edges (called eaves and rakes), or protrusions, such as skylights and bathroom or kitchen vents.

How Does Roof Flashing Work?

As mentioned above, roof flashing is meant to keep moisture from seeping beneath the shingles and getting into the internal structure of your home. You don’t want this to happen, because that invites all sorts of problems, like dry rot, mold and mildew.

But how does flashing do this? Think back to those walls and valleys. You mainly see intersections at sidewalls and front walls. A sidewall is a vertical wall near the sloping edge of a roof deck. A front wall (or a headwall) is behind a sloping roof deck. When you add flashing to these joints, water is blocked from getting between the shingles and redirected down towards the gutters.

There is also valley flashing, which is meant to be installed under the edges of shingles where the roof ends. This means that when it rains, the shingles help funnel the water towards the exposed valley flashing. In turn, the flashing sends the water downward, towards the gutters.


Now you should know that the purpose of flashing is to act as both a dam and a miniature gutter for your roof. When it rains, flashing acts to shuttle the way down the roof towards the eavestroughs while preventing moisture from seeping beneath the shingles. Don’t overlook the importance of roof flashing when repairing or replacing shingles, and make sure you check them out when doing routine maintenance. Roof flashing is essential!