French Drain Sizing: Corrugated vs. Smooth pipe capacity and the “wedge” stone error

A homeowner forum thread digs into a drainage detail that trips up a lot of yard drainage projects: French drains, the gravel-and-pipe trenches used to route water away from a house. The question centered on how much water these systems can actually carry and how much stone they need.

On pipe capacity, the poster pointed out that standard plumbing tables assume smooth-wall pipe. Flexible corrugated pipe, which is bumpier inside, moves water more slowly. At a typical slope, a 4-inch smooth pipe can carry about 113 gallons per minute, while the same size corrugated pipe carries only about 57 gallons per minute — roughly half. If a system was sized using tables meant for smooth pipe but installed with corrugated pipe, it could end up undersized for heavy rain or roof runoff.

On stone, the trench has to slope slightly for water to drain, even on flat ground, so it gets deeper as it goes. In one example, a 50-foot trench started 18 inches deep but ended nearly two feet deep. Calculating stone needs based only on the starting depth, rather than the average depth, can shortchange the order by around 17 percent.

A contractor who replied on the thread said he avoids corrugated pipe altogether, citing installation and maintenance issues, and suggested measuring trench depth at the midpoint rather than doing precise wedge math, since stone is inexpensive and it is easier to simply order a bit extra. For homeowners planning a French drain around their own house, the takeaway is that pipe type and slope both affect whether the drain works as expected.