Answer: -2% or 1/4" per foot No less - No More This is standard for laterals from the house to the main in the street. Larger pipes can lay slightly flatter -When the toilet flushes, it dumps a surge of water into the sewer drain pipe. The human waste and paper are carried along by the surge. If the degree of fall in the pipe is too steep, then the water gets ahead of the solid wastes and leaves it sitting in the pipe. The next flush allows more waste to build up and before long, you have to snake the sewer line. The opposite of "not enough" fall, the wastes settle out and begin to build up causing a clogged sewer line. This is why good plumbers take such time consuming pain to get down on hands and knees with a four foot level and make sure their new sewer line has the correct amount of fall. The pipe lays on solid tamped ground and is covered on both sides and at least six inches of soft earth over the top. All this is tamped in as you fill in the ditch. Absolutely NO rocks or pebbles near the pipe so you do not have a puncture in the future if a truck drives over the sewer line some day. * 1st 2.00 #7040 * Being digital is independent option from confining standards