Effective trench width & lateral-length conventions
A drain field is a set of gravel-filled trenches (laterals). Total trench length = absorption area ÷ effective trench width; the run is split into laterals of a limited length so effluent spreads evenly. These are labeled planning conventions, not code — confirm with a designer and your local health department. Apply them in the trench length & laterals and drain-field replacement tools.
| Convention | Typical value | Notes |
|---|---|---|
| Effective trench width | ~3 ft | Absorption width used to convert area → trench length |
| Maximum lateral length | ~100 ft | Longer runs are split into multiple laterals |
| Gravel depth (replacement) | ~2 ft | Washed stone around perforated pipe |
| Perforated pipe | 4 in | One run per lateral, laid level or to a slight fall |
Labeled planning conventions, not a code reference — confirm width, lateral length, spacing and setbacks with a designer and your local health department. Trench length = area ÷ effective width; laterals = ceil(length ÷ max lateral).