Before we purchased our house we could see that there had been leaks coming through the ceiling in a few of the rooms. No one had lived in the house for a couple of years, and it was a foreclosure, so we didn't know whether the places in the roof had been repaired or whether they were still letting water enter the house.
After we bought the house and we got a big rain storm we saw that there were places where the roof still leaked, but we couldn't find the exact spots where the water was entering. We decided to call a roofer to take a look and find the leaks.
He was only on the roof for about 20 minutes and then he came down and said we were all set. As it turned out, most of the flashing around the chimneys had been loose. It was strange that when I would go into the attic and look for the leaks the water did not appear to be entering the attic around the chimneys, but that is where the water was entering initially.
Once the roofer repaired the flashing, most of the leaks stopped. According to him, loose flashing is usually the cause of leaks on the houses he repairs.