Perc Test for Septic Design in Central NY

Perc test for a septic system in Central New York

A perc test (percolation test) times how fast water soaks into your soil, in minutes per inch. In New York the result, along with a deep hole test, decides whether a lot can take a conventional septic system, how long the trenches have to be, or whether the design moves to a raised system or mound.

This page covers the engineer side of the test: the exact procedure in the NYS Department of Health design handbook, the rates Appendix 75-A accepts, who witnesses the test in each Central New York county, and how the number turns into a system design. For the basics of what a perc test is, see our septic perc test and site evaluation page.

Who needs a perc test

• New house on a lot without sewer. NYS DOH Fact Sheet #1 says the design of all new residential septic systems, conventional ones included, must be done by a licensed design professional, and that design work includes running the soil percolation tests.
• Land buyers. The DOH design handbook says a field evaluation of the site and soils should be done before a property is bought or developed. See lot septic feasibility.
• Replacement of a failed field. Moving or extending an absorption field to a spot not approved before needs a licensed design professional under the same fact sheet, and that means new soil data. See septic replacement design.
• Bedroom additions. When a house adds bedrooms, the existing field gets checked against the new flow. See septic design for a bedroom addition.

How a New York perc test is run

The step by step below is the procedure in section 4.7.2 of the NYS DOH Residential Onsite Wastewater Treatment Systems Design Handbook. County health departments in Central New York point designers to this handbook.

1. Pick the test locations. The holes go inside the proposed absorption area, with the bottom of each hole at the planned bottom of the trench or bed. Onondaga County's design guidelines say the holes must be staked and widely spaced. On sloping sites the second hole is located by measuring 40 feet from the first along the contour and then 40 feet downslope.
2. Dig the hole by hand. The hole is about 12 inches square or 12 inches in diameter with vertical sides. Depth is 24 to 30 inches below final grade for conventional trenches, or at the planned trench bottom for shallower or deeper systems. The handbook says the holes shall be hand dug, and using a backhoe to dig the upper soil is not acceptable. The sides are scraped so they are not smeared.
3. Add stone. About 2 inches of washed aggregate goes in the bottom so pouring water does not scour and silt the hole.
4. Presoak. The hole is filled with water again and again and allowed to drain for at least 4 hours, starting one day before the test. The handbook skips this only for clean, coarse sand and gravel. Presoaking swells any clay in the soil so the test acts like a field that is already receiving effluent. Loose soil that falls in is cleaned out after the last presoak.
5. Time the drop. Clean water goes in to 6 inches above the bottom. The tester records the minutes it takes to fall from 6 inches to 5 inches.
6. Repeat until it is stable. The run is repeated at least 3 times until two runs in a row are nearly equal: within 1 minute for rates of 1 to 30 minutes per inch, within 2 minutes for 31 to 60. The longest one-inch time is the stabilized rate. In the handbook's own example, runs of 24, 27, 30 and 30 minutes give a design rate of 30 minutes per inch.

Appendix 75-A also says at least two percolation tests are needed at the site of each proposed system, and that a perc test is only an indicator. It has to agree with the soil found in the deep test hole. If it does not, more tests are run and the more restrictive result is used for design.

What perc rates are acceptable

Appendix 75-A defines useable soil as soil that percs between 1 and 60 minutes per inch. Soil faster than 1 minute per inch is not suitable for subsurface absorption unless the site is blended with less permeable soil. Here is how the rate lines up with system types in Table 4 of the design handbook:

So a slow 75 minute per inch soil rules out conventional trenches and raised systems, but the mound column still allows it. A very fast gravel that perks under 1 minute per inch fails every column until the soil is blended. That is why the perc number alone never picks the system. The PE reads it next to the deep hole log.

How the rate sizes the field

For conventional trenches, the perc rate sets an application rate in gallons per day per square foot (Appendix 75-A, handbook Table 6B). New homes are designed at 110 gallons per day per bedroom. A 3 bedroom house is 330 gallons per day.

The same house needs more than two and a half times the trench on a 50 minute soil as on a 4 minute soil. The handbook table also requires dosing once total trench length reaches 500 feet, and alternate dosing at 1,000 feet. On top of that, Appendix 75-A asks for an extra 50 percent of useable area to be set aside for future replacement whenever possible. A slow perc can push a small lot out of room for a conventional field. Then the design moves to a raised bed, mound or pressure distribution layout.

Who witnesses the test, county by county

Appendix 75-A lets a local health department accept or require other soil tests when its own staff run or observe them. Many Central New York counties also run their own witness and review rules. Here is what each county publishes:

• Onondaga County. The Bureau of Public Health Engineering reviews plans (Office of Environmental Health, John H. Mulroy Civic Center, 421 Montgomery St., Syracuse). The county's design guidelines put the site evaluation and soil testing on the design professional who submits the plan, and say county technicians may run confirming tests in suspect areas. In the Skaneateles Lake watershed, 10 NYCRR 131.1 requires that all site and soil appraisals be witnessed by a City of Syracuse watershed inspector.
• Madison County. Madison County Public Health (138 N. Court St., Building 5, Wampsville) lists observing percolation and deep hole tests as one of its services. The county reviews alternative system designs. Town code officers issue permits for ordinary systems.
• Cayuga County. For new construction, the design professional must run a perc test and deep test hole before preparing plans. A Health Department inspector then visits the site to confirm wetlands, watercourses, wells and perc holes (8 Dill St., Auburn).
• Oswego County. The Environmental Health Division (70 Bunner St., Oswego) requires new residential systems to be designed by a licensed professional engineer or architect, with four sets of plans submitted for review. Its published pages do not state a separate witness requirement for perc tests.
• Oneida County. The Health Department (185 Genesee St., Utica) lists an individual sewage site survey fee of $150. Its sewage program page describes approving alternative treatment systems for individual homes and giving technical help to towns and engineers.
• Cortland County. The county says the engineer conducts the percolation and deep hole tests, designs the system and inspects the install.

Seasons, frozen ground and wet springs

The NYS DOH handbook allows percolation tests any time except when the ground is frozen or when rain adds water to the test hole. Winter work in Central New York often waits on frost.

Groundwater is a separate issue from the perc. The handbook says spring rain and snowmelt normally push groundwater to its high level between March 15 and June 30, and that drought or wet years shift it. The Skaneateles watershed rules say soil analyses should be done in that March 15 to June 30 window. If they are done at another time, soil mottling is used to set the high groundwater level, and more observation during a wet period may be required. Onondaga County's design guidelines say some alternative system sites need to be evaluated during the period of seasonal wetness.

Problems a perc test turns up, and what the design does

Too slow (over 60 min/in). Conventional trenches and raised systems are out on that soil, per Table 4. A mound can still be designed on in-place soil up to 120 minutes per inch if there is at least 1 foot of it and the slope is 12 percent or less. See failed perc test and clay soil.

Too fast (under 1 min/in). Effluent would move through without treatment. Appendix 75-A says the site has to be blended with less permeable soil first.

Perc and soil do not agree. A fast perc in a hole that shows dense clay in the pit wall means more tests, and the slower, more restrictive one sets the design.

Good perc, high water. A passing rate does not help if the deep hole shows mottling 18 inches down. Conventional trenches need 2 feet between trench bottom and high groundwater. See deep hole soil test and high water table.

Timeline

The perc itself is a two day event: presoak on day one, timed runs on day two. Scheduling around a county or watershed witness, frozen ground and rain adds to that. After the tests, the PE drafts the plan and it goes to the county or town for review. Cayuga County says plan approval may take up to 4 to 6 weeks. The full sequence is on the septic engineering process page.

What Backwell does on a perc test job

Backwell digs the deep test hole, sets up the perc holes where the design area will go, handles the presoak and timing logistics, and coordinates the witness where the county or watershed requires one. The results go straight into the stamped design and the health department submittal, and the same company can install the system the plan calls for.

Designs are prepared and stamped by a New York licensed Professional Engineer (PE). Backwell coordinates the design, the health department submittal, and the install.

Sources

• Backwell, Perc Test and Site Evaluation (typical Central New York testing fees)
• NYS DOH, 10 NYCRR Appendix 75-A, Wastewater Treatment Standards: Residential Onsite Systems
• NYS DOH, Residential Onsite Wastewater Treatment Systems Design Handbook (includes Appendix 75-A and Fact Sheet #1)
• Onondaga County Health Department, Design Guidelines for Individual Sewage Systems
• Onondaga County Health Department, Septic Systems (Bureau of Public Health Engineering)
• 10 NYCRR 131.1, Skaneateles Lake watershed rules and regulations
• Madison County Public Health, Water and Septic: Individual Systems
• Cayuga County Health Department, Instructions for New Construction
• Oswego County Health Department, Septic Plan Review
• Oswego County Health Department, Water Supply Protection
• Oneida County Health Department, Plan Review Fee Schedule (rev. 3/2022)
• Oneida County Health Department, Public Water Supply and Sewage Program
• Cortland County Health Department, Sewage Systems
• Onondaga County Health Department, Fee Schedule
• HomeAdvisor, Perc Test Cost (2026)
• Angi, How Much Does a Septic System Cost (2026)