Perforated pipe: holes up or down
Written 18 Aug 2026. Ask five drainage contractors and you get two camps and one shrug. Holes down: water fills the pipe from the bottom, so the trench drains fully and the pipe never holds a stagnant inch. Holes up: falling water enters directly and silt settles below the holes, keeping them open longer. Both are right about their own failure; neither admits the other's.
The two claims, side by side
| Holes down | Holes up | |
|---|---|---|
| Claim | Pipe empties completely; no standing water | Water enters from above; silt settles below the holes |
| Strongest for | Low, flat, wet trenches | Downspout-fed, silt-heavy flows |
| Weakness | Holes sit in the silt line | Bottom inch of pipe stays wet |
| What it must have | Deep rock below the pipe, real outlet | Clean rock filter above, real outlet |
Where the manuals land
Manufacturer drawings vary by product line; stormwater manuals rarely rule on the question at all, because by the time a design reaches them, the pipe's orientation is the least of the trench. The desk's position: both camps survive or die on the same two things — the rock envelope that filters the water before it reaches the holes, and an outlet that actually leaves. A holes-up pipe in dirty rock clogs exactly as surely as a holes-down pipe in a sagging trench holds water. Pick a camp; then build the trench that makes either camp work.
The disagreement sentence
A contractor who insists the hole orientation is the difference between a working and a failed drain has probably never watched silt arrive. The orientation is a detail; the envelope and the outlet are the drain.
Where to go next
- French drain — the four failure modes, orientation included.
- Sock marketing — the other pipe argument vendors love.
- Measure grade — because sag beats orientation every time.
Sources
- Minnesota Stormwater Manual, underdrain guidance — stormwater.pca.state.mn.us
- USDA NRCS, subsurface drains — nrcs.usda.gov
- ICC International Plumbing Code — iccsafe.org