: Hi Tom,: There should be a little rock under the pipe — not more than 2 for a 4 pipe. I personally use the rigid pipe because it can be cleaned of tree roots, etc. at some future time.: : : : Here in California we build the drain with a 12 inch wide trench and rock from 3/4 to 1-1/2 inch in size. We also usually use a complete wrap of filter fabric around the pipe and rock to prevent soil intrusion into the rock. The larger the rock, the more important the filter fabric is. Ideal slope is 2%, which is 2 feet of fall per 100 feet of pipe. This is about 1/4 inch fall per foot of pipe: : : : What size should the trench be to accept 4 corrugated pipe. Is that width to be filled with 2-3 bull rock 1.5 septic rock or does it matter? What should the fall per foot be? Thanks for the answers.: : George, does the pipe lay at the bottom of the trench or is there rock below it. If so, how much rock? Is corrugated pipe yhe best for this drain sytem if is a straight run?Hey guys. Here is the dumb question of the week. Do the holes in the pipe go down or up. I cannt remember form 30 years ago. Thanks for a prompt reply, Jean
