If your roof is leaking you can typically guess what kind of roof leak to look for based on the age of the roof. If you've just had a storm and your roof is leaking more than likely you're going to look for obvious storm damage to your roof.
If your roof is brand new and leaking after the 1st couple rains, it's more than likely due to a bad install and something with flashing, pipe flashing, chimney flashing, etc etc done really bad.
If your roof is about 7 to 15 years old and just started leaking, your roof has not suffered any significant storms, there's a good chance what you're looking for is probably going to be nails in or close to the key ways, that are rusting out causing a hole.
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