Between the septic tank and the drainfield of most gravity systems sits a small buried box with one pipe in and several pipes out: the distribution box, or d-box. Its only job is to split the flow evenly between the drainfield's trenches. It is one of the simplest components in the system, and one of the most consequential when it goes wrong.
Effluent leaves the tank through a single outlet pipe, but a drainfield is several trenches working in parallel. The d-box receives the single stream and distributes it so each trench takes its share. Even distribution is what lets the whole field share the load; a field loaded evenly ages evenly.
Because tilt is the common failure, service work on a d-box is often just re-leveling it, or fitting adjustable outlet levelers that trim each pipe's flow. A d-box check belongs in a full septic inspection: lid off, flow observed, levels compared. It is quick when someone knows where the box is.
The d-box appears on the as-built drawing with tie measurements, usually a short run downstream of the tank outlet. Pull the drawing before anyone starts probing: our as-built lookup guide covers how, and the parcel map shows what we hold for your property. Where no drawing exists, our generator's free estimated design shows the likely position between tank and field. It is estimated, NOT a county-stamped document.
A tilted d-box mimics drainfield failure. The as-built tells a contractor exactly where to look first; find yours through the records index or the parcel map.
Find your county's septic records Open the parcel map and design tool
No. Gravity systems with multiple trenches usually have one; pressure-distribution, drip, and single-trench layouts split flow by other means. The as-built shows whether yours has one.
One soggy or odorous stripe over a single trench while the rest of the field looks dry is the classic sign that one outlet is taking most of the flow.
Often, yes. Re-leveling or replacing the box and fitting outlet levelers restores even distribution, provided the overloaded trench has not been permanently damaged.
Shallower than the tank in most layouts, on the line between tank outlet and drainfield, but the real answer for your system is the tie measurements on the as-built drawing.
Guide last reviewed 2026-09-08.