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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.