I am wondering what the general rule is on how much a kitchen sink drain pipe should drop every 10 ft? I am relocating my kitchen and when I looked at the new 30 ft. kitchen sink/ garage disposal drain pipe under the house the total drop was only 1/2". It appears to almost sit level across the pipe span. I am afraid my kitchen sink will not drain properly and will back up if this pipe doesn't have more of a drop over the 30'.