I recently purchased a used modular home that I moved to a new location and is now sitting on a foundation with a crawl space. I want to install the drain lines underneath the house and and hook them up the septic tank. The house is 28 X 60 and has one 4" drain at one end of the house and another 4" drain at the other end. My question is what kind of slope should In use for the pipe? I have heard 1/4" every 1' and 1/4" every 10". Also any other suggestions would be greatly appreciated.