I have a 4" PVC drain from a downspout that runs parallel to the garage for about 25' before joining another drain. This run is about 12" deep and with a very slight grade. While the driveway is being repaved, I am replacing the thinwall PVC with schedule 40 (the pipe was clogged because the inlet was slightly below grade and had filled with mud and gravel over the years).
Is the 12" (approx) depth adequate? What should the pitch be (there is room to lower the downstream end as much as 12" if needed)?