Guide
Drainfield types, sizing, setbacks and the habits that decide whether a field lasts eight years or thirty.
Tanks are cheap and durable. Drainfields are neither. Replacing one means new excavation, often a new permit, sometimes a soil evaluation that says the original design is no longer allowed. Everything in septic maintenance is really about protecting this component.
Perforated pipe in gravel-filled trenches, fed by gravity. Cheapest, simplest, longest-lived. Needs decent soil and enough depth to groundwater.
Arched plastic chambers replace the gravel. More expensive per foot, but no gravel hauling, a larger open infiltration surface, and easier installation on awkward sites.
A pump doses effluent through small orifices so the whole field is loaded evenly rather than the first few feet taking everything. Extends field life on marginal sites, at the cost of a pump that will eventually need replacing.
Where soil is too shallow, too tight, or the water table too high, the field is built above grade in imported sand. Effective and expensive — typically two to three times a conventional field.
Air is blown through the effluent so aerobic bacteria treat it before it reaches the soil. The resulting effluent is clean enough that the absorption area can be roughly halved, which is why these appear on small or difficult lots. They need power, an annual service contract, and they fail quietly when neglected.
Absorption area equals daily design flow divided by the soil application rate. A three-bedroom house at 450 gallons per day needs 750 square feet on sandy loam and 4,500 on clay. Same house. The soil is the variable that matters.
| From | Typical minimum |
|---|---|
| Private well | 100 ft |
| Stream, pond, wetland | 50 – 100 ft |
| Property line | 10 ft |
| Building foundation | 10 – 20 ft |
| Driveway or parking | Not permitted over |
Your county sets the real numbers. Check before you buy a lot, not after.