What a perc test actually measures
A percolation (perc) test measures how fast water moves through your soil, expressed as minutes per inch. Combined with a deep hole test (which checks water table depth and soil layering), the result tells us what kind of septic system your lot can support, where it can go, and whether you need an engineered design.
Why you need one
- New construction. No septic permit without it. Required by every county health department in NY.
- Buying land. Most rural lot purchase contracts include a perc contingency. A failed perc kills the deal or drops the price.
- Failed system replacement. Existing field design data is often missing or outdated. New perc establishes what the soil can handle today.
- Subdivision. Each lot needs its own perc on the proposed septic location.
How we run it
- Site walk. We identify the best candidate area: away from wells, downhill of structures, away from drainage easements.
- Deep hole. We dig a 7 to 9 ft test hole with a mini excavator. The county health inspector witnesses it. We log soil horizons, mottling depth (indicates seasonal high water table), and depth to bedrock or refusal.
- Perc holes. Three to six 12-inch diameter holes, 24 to 36 inches deep, depending on county.
- Pre-soak. Holes filled with water and allowed to saturate the surrounding soil for at least 4 hours (most counties require 24-hour pre-soak in dry months).
- Timed drop. Water added, drop measured over a fixed interval. We record minutes-per-inch at each hole.
- Inspector sign-off. County health department signs the field log on site.
What the result means
| Perc rate (min/inch) | What you can install |
|---|---|
| 1 to 5 | Soil is too fast (sand/gravel). Needs sand filter or modified design. |
| 5 to 30 | Ideal. Conventional gravity or chamber system. |
| 30 to 60 | Marginal. Pressure distribution or extended field. |
| 60+ or no perc | Engineered mound, ATU, or alternative system. |
What it costs
| Item | Typical fee |
|---|---|
| Site evaluation (no excavation) | $200 to $350 |
| Perc test + deep hole | $400 to $800 |
| County health department witness fee | $75 to $250 |
| Engineered design (if required) | $1,500 to $3,500 |
When the perc test is bundled with a Backwell septic install, the test cost is rolled into the install price. Standalone perc tests for land purchases or subdivisions we charge separately.
What can go wrong on a perc
- Wet season false fail. Tests in March or November with saturated soil can fail an otherwise good site. Best months: June through early October.
- Water table within 24 inches of surface. Disqualifies conventional systems.
- Bedrock at 18 inches. Forces mound or engineered system.
- Clay layer above gravel. Looks fine on the surface, blocks downward flow.
If the first location fails, we will test a second candidate area at no additional excavation charge. Most lots have at least one workable spot.
Counties we test in regularly
Onondaga, Oswego, Madison, Oneida, Cayuga, Cortland, Wayne, Jefferson, Lewis, Herkimer.