Perforated Pipe - Holes Up or Down?

Q: When I use perforated pipe in a drainage system, should the holes go up or down?

A: Steven Baczek, a residential architect from Reading, Mass., who specializes in designing durable, low-energy homes, responds: I hear arguments for both methods, but it really depends on how the pipe functions in your drainage system. Before going any further, the pipe we are discussing here is heavy-duty schedule-40 PVC pipe with either two or three courses of holes with the outer courses usually 120° to each other. I don’t recommend using corrugated flex-pipe in drainage systems.

When designing a water-management regimen for a home, I try to drain to daylight (“A Primer on Water Management,” Jun/17). In this system, drainage pipe around the perimeter of the house links with the downspouts as well as with a perimeter drain inside the basement, all of which drain by gravity to a pipe that exits the ground at a safe distance from the building. Here, the perforated pipe has two functions: collection and conveyance, with the latter being the primary function. I place the pipe with the perforations facing up and count on the streaming water from the downspouts to help keep silt and debris from accumulating in the pipe.

I also place the perimeter piping in what I call a ground gutter, a trench filled with crushed stone and wrapped on all sides with filter fabric—a pipe within a pipe. Water draining from the walls or dripping from the eaves diffuses through the filter fabric and the crushed stone, with most of the liquid being distributed by the ground gutter. The ground gutter would need to saturate to the level of the perforations before any significant water would enter the pipe, and the likelihood of that happening is usually pretty slim. In this scenario, the pipe would have to be completely occluded with silt and mud to become ineffective.

With the perforations facing down, the primary function is collection and distribution. Even when placed in a ground gutter as described above, the pipe fills with groundwater more quickly. When more water enters one area than another, it flows to another area of the pipe and drains away. This would seem to work best in a French drain system where excess water drains to a sump pit to be pumped out. In the systems I install, it’s much more difficult for the debris to be washed away with the perforations facing down. Either way, though, when silt and debris fill the pipe to the level of the perforations—essentially half the diameter of the pipe—it can no longer take on water and no longer is effective for drainage.

So there are good arguments for both methods. Having the holes facing up is just the most effective plan for the systems that I’ve designed. Regardless of your preference for perforation placement, I always recommend installing clean-outs in strategic locations for clearing the pipe should it become blocked or sluggish.