A customer of mine has a patio in his back yard. His drainage is very poor. Last year the lawn cutter could not cut the grass in the back area because of this. I was telling him that 4" perforated corrgated piping needed to be installed to take away the surface water. Say you are looking at the back of the house. The area in question is 45' wide and 21' long. The 21 feet slopes towards you (wooded area) with a drop of 6-8 inches. Its on a very level lot. In order to insure all surface water to be drained off, how far do I put the pipes on center. Would it be 3', 5', 7' or more? Thanks guys/gals. Chris.