12. How important are roof flashings and what are the different types?


Roof flashing is one of the most critical components of your roof. The main function of your roof is to keep the elements out of your house. The most common element that your roof protects you from (besides the sun) is rain. Modern roofs do a great job and can last many years depending of the roofing material. But every roof is subject to some weak points that are more likely to leak. That’s where flashing comes in.

The most vulnerable points in any roof are joints and penetration. A joint is anywhere that two slopes meet. Valleys and dormers are examples of joints between different slopes. Penetrations are what they sound like, anything that requires a hole in your roof. Common penetrations are vents, chimneys, and skylights.

Anywhere that there is a joint or penetration in your roof, there is an opportunity for water to work its way through your roof and into your home. To prevent this, roofers install flashing in these areas. Flashing is made of materials that are not easily penetrated by water. It serves to redirect water away from the joint and down into the gutters or off the roof. Without flashing, your roof would almost certainly develop leaks over time. But if you have properly installed flashing that is kept in good shape, your chances of a leak are significantly reduced. Here is a brief description of the most common types of roof flashing:

  • Valleys: These are areas where two slopes of the roof meet. Flashing here is vital because the two slopes both direct water into the valley and the valley then directs the water down that section of the roof. The stream of water creates a high risk of leakage, both due to the large amount of water and the inevitable seam of both the shingles and the roof deck beneath. Flashing in valleys is usually made of long pieces of sheet metal that are shaped to fit into the valley. Shingles on both slopes overlap the edges of the flashing for a tight seal.
  • Chimneys: A chimney requires special flashing. It is often one of the most extensive penetrations in a roof. Generally, there are three types of flashing around a chimney: head flashing, step flashing and base flashing. On the sides of the chimney perpendicular to the slope of the roof, step flashing is used to direct water away from the chimney and onto the roof. Below the chimney, an apron is installed, a large area of sheet metal laid flat against the roof. Above the chimney, head flashing or a cricket is installed (depending on the width of the chimney). A cricket is a raised ridge that starts at a point above the chimney and widens to the full width of the chimney. It looks something like a small dormer. The cricket directs water around the chimney so that it does not pool at the upper edge of the chimney.
  • Skylights: Skylights require a tight seal with the roof. Usually, a continuous piece of sheet metal flashing is installed on each side of the skylight. Rubber gaskets may also be installed beneath the flashing as part of the skylight assembly.
  • Dormers: Dormers are an example of a joint in the roof, as well as a meeting between to slopes. However, a dormer usually does not form a valley. The seam between the roof of the dormer and the rest of the roof is protected with a length of sheet metal like the one used in a valley. The seam between the walls of the dormer and the roof are sealed with step flashing like the sides of a chimney.


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