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What is Roof Flashing?

By Adam Hill
Updated May 16, 2024
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The roof of a residential home serves one major purpose, and that is to be a barrier against the weather outside. The most difficult weather element to control is water, and roof flashing acts as the last line of defense against water leakage. It is simply a piece of sheet metal -- usually aluminum or galvanized steel -- which is placed over joints in roof and wall construction to prevent water seeping into the house and causing damage. The major cause of residential roof leaks is a fault in the flashing.

Roof flashing represents a very simple and intuitive engineering technique that has been known for hundreds of years. The whole idea depends almost entirely on gravity. If the flashing has been installed correctly, gravity will work with the flashing material to shed the water onto the regular roofing materials, where it is then directed harmlessly to the ground. Roof flashing is commonly used around chimneys, fan vents, plumbing vent stacks, and other items that project out of residential roofs.

For roof flashing to be effective, it must be durable, low-maintenance, weather resistant, and also accommodate movement. Movement must be accommodated because of the constant expansion and contraction between roofing materials and things that are not roofing materials. This expansion and contraction can happen on a daily and/or seasonal basis. For example, sunlight can create very high surface temperatures during the day, causing the roof materials to expand, but at night the temperature drops and the materials contract. Seasonal changes can affect the framing lumber, and the flashing materials must be able to withstand all these changes and remain leak-proof.

The stresses that roof flashing experiences over time can make it more prone to failure when extreme weather occurs, such as heavy snow, high winds, or excessive heat. During installation, care must be taken to preserve the flashing materials. Professional roofers typically cut and shape their own flashing materials from sheet metal, but they are also available pre-formed. Flashing materials which can rust, such as steel, should always be properly painted. They must first be washed with soapy water or paint thinner, and then must have a special primer applied. The paint must then be applied in the correct number or coats.

Flashings are the most technical and most difficult aspect of roofing in most cases. Traditional roof flashing methods and materials have stood the test of time, but they require experience and are time-consuming. Since flashings are the weak link in the chain when it comes to roofing, the skills of a professional are an important asset with these types of installations and repairs.

HomeQuestionsAnswered is dedicated to providing accurate and trustworthy information. We carefully select reputable sources and employ a rigorous fact-checking process to maintain the highest standards. To learn more about our commitment to accuracy, read our editorial process.
Discussion Comments
By emartin74 — On Feb 17, 2011

@Kalley – I agree with @JillT, but if you really want to try repairing or replacing the roof flashing yourself, you can buy roof flashing online although I wouldn’t recommend it. Your best bet would be to go to a hardware store and ask one of the sales people which type of roof flashing is best, and find out if they have any tips on how to install the roof flashing so your roof won’t leak anymore.

By JillT — On Feb 16, 2011

@Kalley - I wouldn’t recommend doing that unless you’re a professional roofer – you’re likely to make problems worse by either using low-quality roof flashing material or installing the roof flashing wrong, which will only make your problems worse. In addition, it’s dangerous for anyone other than a professional roofer to be on your roof. Save yourself the time, money and possible medical expenses and call a professional.

By Kalley — On Feb 14, 2011

How can I repair or place the roof flashing myself?

By anon97964 — On Jul 21, 2010

Excellent, straightforward, informative advice.

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