Septic drain field design is five numbers in a fixed order: design flow, percolation rate, soil loading rate, absorption area and trench length. Everything else on the drawing, the pipe spec, the trench spacing, the distribution method, follows from those. This page walks the sequence with real figures so you can check a designer's numbers, or work out roughly what your lot needs before you pay anyone.
Step 1: design flow
Design flow is gallons per day, and it comes from bedroom count rather than from occupancy. A code typically assumes 150 gallons per day per bedroom with a two bedroom minimum, so a three bedroom house is designed for 450 gpd whether two people live there or six. The reason is resale: the next family might be six.
| Bedrooms | Design flow | Common tank minimum |
|---|---|---|
| 2 | 300 gpd | 750 to 1,000 gal |
| 3 | 450 gpd | 1,000 gal |
| 4 | 600 gpd | 1,250 gal |
| 5 | 750 gpd | 1,500 gal |
| 6 | 900 gpd | 2,000 gal |
Step 2: the percolation rate
A perc test measures how many minutes the water level takes to drop one inch in a presoaked hole at trench depth. Fast soil gives a low number. Most codes will not permit a conventional field faster than about 1 minute per inch, because water moves through that soil too quickly to be treated, or slower than about 60 to 120 minutes per inch, because it will not move at all.
Step 3: the soil loading rate
The loading rate converts a perc rate into gallons per square foot per day. It is a table in your county code, and the numbers below are typical of the mid-range codes. This is the step where two counties can legitimately disagree about the same soil.
| Perc rate | Loading rate | Soil |
|---|---|---|
| 1 to 5 min/in | 1.2 gal/sq ft/day | Coarse sand |
| 6 to 15 min/in | 0.8 gal/sq ft/day | Sandy loam |
| 16 to 30 min/in | 0.6 gal/sq ft/day | Loam |
| 31 to 60 min/in | 0.4 gal/sq ft/day | Silty clay loam |
| 61 to 120 min/in | 0.2 gal/sq ft/day | Clay |
Step 4: absorption area
Absorption area in square feet is design flow divided by loading rate. A three bedroom house at 450 gpd on loam at 0.6 gal/sq ft/day needs 750 square feet. The same house on clay at 0.2 needs 2,250 square feet, three times the yard, which is why the perc test result changes the price of a lot and sometimes changes whether it is buildable.
Step 5: trench length and layout
Trench length is absorption area divided by trench width. At the common three foot width, 750 square feet is 250 linear feet of trench. That is then split into runs, because a single run has practical limits: most codes cap an individual trench at 100 feet so that the far end still receives flow. So 250 feet becomes three trenches of about 83 feet, set 6 to 10 feet apart on center.
Many codes allow a reduction, often 20 to 40 percent, when you use plastic chambers instead of pipe and gravel, because the open chamber presents more sidewall to the soil. The reduction is written into the code, not chosen by the installer. See leach field chambers for what that does to the trench count.
The pipe itself
- Four inch perforated PVC or corrugated HDPE. Perforations face down, which surprises people. The water is meant to leave along the bottom, not rain from the top.
- Laid level, or at most a slight fall. A drain field trench is not a drain. Many designers specify dead level so the run loads evenly.
- Six to twelve inches of washed gravel below the pipe and two inches over it, then a permeable barrier so backfill does not wash into the voids.
- Solid pipe from the tank to the distribution box. Only the trench runs are perforated.
Distribution: box, drop boxes or pressure
Gravity distribution through a level distribution box is the default and the cheapest. On sloping ground, drop boxes fill each trench in sequence, which is more forgiving than a box that has settled out of level. Pressure distribution uses a pump and small orifices to dose the whole field evenly, and it is required for most mound and sand filter designs and for tight sites where even loading is the only thing that makes the numbers work.
What the drawing has to show
A permit drawing is not an artistic exercise. It shows the lot lines, the house, the well and any neighboring well within the setback, surface water, the tank, the field with dimensions, the reserve area, contours or slope direction, and the soil test locations with their results. If a design you are handed is missing the reserve area or the test locations, it is not finished.