Holes Up or Holes Down?
Perforated PVC pipes used for underground drainage have offset holes drilled along the centerline of the pipe. It seems that some philosophical disagreements exist related to the placement of the holes in gravel filled trenches (i.e. French Drains). A review of installation instructions from several State DOT manuals reveals that the holes should be down (i.e. placed facing the bottom of the trench). The reason is that this allows the water to enter the pipe and drain from the soil at the lowest level. If the holes are located along the top of the pipe, then the water must rise to the level of the top of the pipe before it can be drained. In most cases, the purpose of these buried pipes is to collect water in the ground before it enters a building basement or seeps up through parking lot pavement, so it makes sense to collect the water at the lowest level available. Water will follow the path of least resistance, so once it enters the pipe, gravity will make it travel until it reaches the discharge point at the low end of the pipe.