Anatomy Of A Pitched Roof
Every winter, across the nation, housing-news headlines always read the same! “Roof Leaks Cause of Major Damage.” “Ceilings, Walls, Furniture and Floors Ruined.” So, how do you prevent your home from becoming “newsworthy”? Well, you might never achieve absolute perfection here, but you certainly can reduce the possibility of major damage to your home by knowing something about the anatomy of a roof. Being aware of what each component in the roof system does can help you to decide which ones to include the next time you re-roof. In fact, once you know what the possibilities are, there may be a modification or two that you will want to make to your existing roof. When it comes to your biggest single investment it’s never be too late to “make things right”.
A roof system consists of the roof covering material (roofing), the waterproof underlayment, the flashings and the ventilation system.
Most of us know that our roof covering is supposed to keep Mother Nature outside – and dry conditions inside. Also, the shape, texture and color of the roofing can act to enhance the home’s appearance. However, one component of the equation that is sometimes overlooked is whether the roofing can be walked-on without being damaged. Believe it or not, there are many roof-covering materials that readily split, crack, chip, dent or bend when walked upon. You may not have reason to be on your roof very often, but when you do go up there you should be pretty much assured that mere foot traffic won’t destroy its waterproof integrity. Test shingles on the ground before they are purchased and installed on your roof. A shingle that fails under moderate pressure is not for you.
The waterproof underlayment is the black, asphalt-impregnated felt-paper that lies immediately beneath the roof covering. With wood shingles, 30 lb. underlayment is used. With most other roof covering materials 15 lb. underlayment is used. With shingle roofs, the underlayment prevents wind-blown rain and melting snow from making their way into your home. With other types of pitched roof covering the underlayment acts as a second layer of protection against leakage. According to the experts, in snow country it is a good idea to use two layers of underlayment. For roofs pitched 4:12 and greater the paper should be laid beyond the exterior walls at least one-foot. For roofs that slope less than 4:12 the underlayment should be laid at least two feet beyond the exterior walls. In our opinion, the second layer of underlayment should extend to the end of the overhang no matter how far it is beyond the exterior wall. Also, we would like to suggest that the horizontal seams on the second layer of underlayment be staggered at least a foot away from the corresponding seams in the first layer. The building code does not require underlayment to be used when a second or third layer of composition roofing is applied over an existing roof covering. The previous roofing layer ends up acting as the underlayment.
Note: By the way, we are opposed to multiple layers of roofing. Generally, we see roof sagging as a result of the added weight. A bundle of shingles weighs as much as a small child. Can you imagine the weigh exerted on your roof by a hundred bundles?
Finally, it is important to remember that the underlayment needs to breathe. Although it is designed to keep water out of the attic it must also “breathe” allowing vapors in the attic to escape. Therefore, a vapor barrier should not be used for underlayment.
Roofing is usually flat and brittle and not conducive to folding or forming. For this reason roofing can not be easily used to create a waterproof seal in conditions, for example, where the roof meets a wall, a pipe or a chimney. These connections are made with “flashings”. Flashings are made using light gauge metal such as galvanized sheetmetal, aluminum and copper. Copper has proved to be the best because of its lasting quality. However, copper is very expensive. Galvanized sheetmetal has been most popular because it is less expensive than copper. Unfortunately, galvanized sheetmetal has a tendency to rust requiring high maintenance and resulting in a short life span. Aluminum isn’t as sturdy as galvanized sheetmetal, but for most flashing applications strength is not an issue. Better yet, aluminum doesn’t deteriorate as quickly as galvanized sheetmetal. Maybe that’s why we are seeing more flashings being made of aluminum.
When it comes to roof ventilation there can never be enough. Roof ventilation aids in the reduction of condensation thereby reducing the growth of fungi, moss and mildew on shingles and flashings. Good roof ventilation is promoted when there is proper attic ventilation. As the flow of air in the attic increases so does the movement of air surrounding shingles and flashings. In attics where poor ventilation exists it can actually become stormy – not a good condition. Isn’t it interesting how one problem in the home can tend to lead to another.