Roofing – Roof Flashing
Residential Roof Flashings
The sources of a wide majority of residential roof leaks are roof flashings. For the most part, the correct installation of flashing material is what determines whether or not your roof will leak. The reason for this is quite simple. The vast majority of roofing products are quite easy to install. The manufacturers have engineered these products to be practically leak proof when they are installed on a simple roof.
However, there are very few simple roofs. Examples of simple roofs are dog houses and small outdoor sheds. Rarely will you find a plumbing vent stack, chimney, fan vent, pot vent, valley, side wall projecting above the roof, etc. on a dog house. You will, on the contrary, almost always find one of these items projecting through a residential house roof.
The engineering concerning flashings has been known for hundreds and hundreds of years. It is, in fact, very simple engineering. The system depends almost entirely on gravity. Gravity works with the flashing material and sheds the water onto the regular roofing materials. If you use a flashing material that depends on some other method to shed water you can quite possibly develop a leak. Caulks, roofing cement, pitch and similar materials depend on their stickiness or adhesive qualities to seal out water. These qualities can easily break down under constant exposure to the elements. If you want a leak-proof roof, stay away from these type of compounds.
For the most part, flashings are the transition materials and media between the primary roofing materials and the things that don't get covered with roofing materials. For example, when a chimney projects through the center of a roof, the roofing products must terminate against the chimney. However, this junction must be engineered to allow for expansion and contraction and be leak-proof. This sounds like a difficult task, but it is not.
This same condition is true for any other thing that projects through your roof or where two sloping roofs form a valley, saddle or a cricket.
Many people expect roofs to perform for a minimum of 15 to 20 years. Some roofs can be expected to last 50 to 80 years. This means that the materials which are used for flashings must have the same, or greater, life span as the roofing material itself. These materials must also be able to be shaped easily and have the capability to be easily soldered, brazed or welded. This second point is crucial. Copper, tin coated steel, lead and galvanized steel are examples of traditional flashing materials.
The reason the flashing materials must have the capability of being soldered or brazed is quite simple. The soldering or brazing process of metal is similar to welding. These processes take two separate pieces of metal and make it one. There are many, many times in flashing work where one has to bend around corners or splice two pieces of metal together. These instances create seams which must be soldered or brazed. As I mentioned earlier, the flashing material must have a projected life span equal to or greater than that of the roofing material. Soldering or brazing metals together passes this test.
Flashing systems must also pass another test. They must be able to handle the constant expansion and contraction between roofing products and those things which are not roofing products. This battle of expansion and contraction can take place on a daily or seasonal basis. The sun can create extremely hot surface temperatures during daylight hours, causing roofing materials to expand. At night, the temperature drops and the materials contract. Seasonal changes can cause the framing lumber to shrink and swell depending upon humidity and wood moisture content. This shrinking and swelling causes the roof sheathing and rafters to move. The flashing materials must be able to withstand this movement and continue to be leak-proof. Well-engineered and installed two-part metal flashing systems can handle this movement with no problem. On new construction, the framing lumber can shrink dramatically in the first year after construction. If one-part flashing systems are used, leaks can easily develop from the gaps that develop from the shrinking lumber.
Two part flashing systems consist of a base or step flashing which is usually in contact or laced into the primary roofing material, and these are then covered by a counter flashing. The counter flashing is a separate piece of metal or building material which laps over the base or step flashing. It should not be attached (mechanically or soldered) to the base or step flashing. The two materials must be able to move independently of one another. The counter flashing can be made of a wide variety of materials. It can be wood siding, stucco, vinyl or aluminum siding, or other material which forms the outside surface of a wall. In chimneys and along brick walls, the counter flashing is almost always the same metal which is used for the base or step flashing.
Two part metal flashing systems are not harmed in any way by the sun's ultra-violet (UV) rays. UV rays can easily defeat caulking, plastic roof cement or most other flashing substitutes. UV radiation can remove the elasticity of these products and make them brittle. When they become brittle, they can no longer handle the daily expansion and contraction. Also, very few of these products have a life span equal to or greater than roofing products. They are temporary remedies at best. Only consider using them for emergency type repairs.
Flashing materials which can rust (tin coated steel or galvanized steel) should always be painted.They often are not painted correctly by roofers. These metals must have special primers applied first and then the proper number of finish coats of paint. Also, it is extremely important to wash these metals with soap and water or paint thinner before painting!!! These metals have a very light coating of oil which is applied during the milling process. If the oil is not removed, the paint job will fail. You will be battling peeling paint long into the future.
Avoid using aluminum flashing material. Aluminum cannot be soldered. Also, if it is used in flashing chimneys or brick walls, it will rapidly corrode. The chemical makeup of the mortar attacks and eats the aluminum! Do not allow your roofer to use aluminum!!!
Flashings are, for the most part, the most technical aspect of roofing. Their installation requires an experienced individual in almost all cases. Soldering vertical surfaces requires skill and knowledge. Professional roofers possess these talents. Professionals realize the importance of flashings. They know that flashings are the weak link in the chain. You should realize this also.
The art of installing flashings cannot be described in this short bulletin. In fact, many books do not fully cover the subject. You have to research several books to get the full picture. I have developed a partial list of some publicationswhich do a good job of illustrating and explaining roof flashings. Many of these are available at libraries or larger bookstores. I highly recommend that you consult several publications to get a full understanding of the complexity of roof flashings. The more you realize how complicated flashings are, the better your chances of hiring the best professional. Good luck!