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Roofer: Where Is Your Leak Coming From?
The Shingles
If your roof is comprised of shingles, the leak could be coming directly in through that field. A roofer will often make this his first point of inspection, as any one broken shingle could be the cause of the problem. It will depend on the materials made to create your roof whether or not you can easily get up top and take a walk around. No matter what your roof is made of, however, it is advised that you wait until the rain has dried up completely before taking a stroll.
Intersections
Unless you have a very old fashioned, v-shaped roof, you probably have some valleys in the construction. These valleys are often covered with flashing or lacing, but they are often the source of leaks. This is where a great deal of water will run off and collect if you don't have a good drainage system. If there is even the smallest hole, water will get through and cause an issue. A good roofer will have sealed this area tightly in the original construction, but you can't always count on getting careful construction, particularly if the house belonged to someone else before you.
The Chimney
If you live in a northern state, you probably have a chimney for your fireplace. This chimney may be the source of your leak. Some chimneys are created with as many as four separate forms of flashing, any of which could be the problem. A roofer who goes to inspect a house with a chimney will often make that his first point of investigation. All of the flashing must be correct to avoid leaks. Even a small crack will let water flow in and will cause a massive problem inside the house.
Finding the source of a leak on your own is fine if you feel the need to create a temporary solution, but no leak should be trusted entirely to quick fixes. Call a professional roofer and make sure your house is sealed up tightly before the next rainfall.