Roof Flashing: What is it and why is it important?

ROOF FLASHING! Matt is always harping on flashing while walking around the exterior. What’s the big deal you ask?

Roof flashing is one of the most critical parts of your roof. Flashing secures joints in your roof and places that tend to receive a lot of water, like valleys between slopes. With adequate flashing, water is directed away from vulnerable areas of the roof and into gutters. Without flashing a roof would almost certainly develop leaks in sensitive areas. The importance of roof flashing cannot be overstated. It is vital to the health of your roof, so make sure that your roof flashing is installed properly and is kept in good condition. Twin Lights Home Inspection is always on the lookout for proper flashing protection, especially here in the Boston area of New England with our swings in weather patterns.

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The main function of your roof is to keep the elements out of your home.

The most common element that your roof protects you from (besides the sun) is rain. Humans have been building shelters to protect them from the rain for thousands of years. Modern roofs do a great job and can last anywhere from 15 to 100 years or more depending on the roof. But every roof is subject to some weak points that are prone to leaking. 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. If your house is any shape other than a perfect rectangle, chances are that you have some valleys in your roof where slopes meet. Dormers are also a common example of a joint between different slopes. Penetrations are what they sound like, anything that requires a hole in your roof. Common penetrations are vents, chimneys, and skylights. Less common penetrations may include specialty installations that have connections both above and below your roof. This may include some solar panel installations or plumbing for swamp coolers or solar water heaters.

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, modern builders 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.

Materials Used for Roof Flashing

You may be surprised to learn that flashing was once quite difficult to install. Before the introduction of sheet metals, flashing was time-consuming and labor intensive to produce and install. In some New England homes, birch bark was used as a flashing material.

In many instances, builders used other methods to try to direct water away from penetrations, most commonly chimneys. One often-used method was to apply mortar flaunching, basically a large buildup of mortar around a chimney that was sloped to direct water away from the joint with the roof. Other methods included angling shingles away from joints or placing penetrations such as chimneys at the ridge of the roof where the least amount of water would flow against them. With the introduction of sheet metal, modern flashing came into general use. One of the first materials used for flashing was lead.

Surprisingly, despite the environmental and health hazards associated with lead, lead is still used for some flashing. That is due to its extreme durability. Lead flashing has been known to last up to 500 years in some constructions. For this reason, it is commonly used today in small areas that are hard to access and would, therefore, be difficult to maintain or replace.

One of the most durable metals for flashing beside lead is copper. It is also a relatively expensive metal, but for high-end builds or historic homes, copper is the way to go. Other common materials include a wide variety of sheet metals, often various alloys of steel, zinc, and aluminum. Less common but still in wide use are rubber and plastic flashing. While they are cheap to install, they tend not to last as long as metal. Both rubber and plastic degrade over time with exposure to the sun.

What Areas of your Roof Need Flashing?

Flashing is used anywhere that there is joint or penetration in the roof. Here are some common areas that require flashing:

Valleys

These are areas where two slopes of the roof meet. Flashing here is essential because the two slopes both direct water into the valley and the valley then directs the water down that section of the roof. Because of this, during even a relatively light rain a gushing stream of water can gather at the valley. 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 substantial penetrations in a roof. Generally, there are three types of flashing around a chimney. On the sides of the chimney perpendicular to the slope of the roof, step flashing is used to directs waters away from the chimney and onto the roof. These are smaller pieces of sheet metal shaped to fit snuggly against the roof and the side of the chimney. Multiple pieces are installed in overlapping “steps” along the side of the chimney. There may also be flashing attached to the chimney that overlaps the top of the step flashing to further direct water away from the joint with the roof. Below the chimney, an apron is installed, a large area of sheet metal laid flat against the roof. Above the chimney, a cricket is installed. 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 similar to that used in a valley. The seam between the walls of the dormer and the roof are sealed with step flashing similar to the sides of a chimney.

Inspection and Repair of Flashing

Depending on the type of roofing and flashing materials you have, they may not age at the same rate. For long-lasting roofing materials like tile and slate, flashing will often need to be replaced well before the roof does. For less long-lasting roofs, like the very common asphalt shingle, flashing may last the life of the roof, but should still be monitored and maintained with as much care as the shingles.

We generally suggest inspecting asphalt shingle roofs about every three years. For tile or slate roofs, once every five to seven years is sufficient. When your roof it inspected, we’ll check your flashing, too. Don’t wait for a leak to fix your flashing. And as always, Twin Lights Home Inspection is here to help provide you with the info you need to successfully maintain your home’s long term health.