PercTallyTank gallons, trench length and what the perc test really decides.

What a 90 minute per inch perc rate means for a 3-bedroom house

90 minutes per inch, 3 bedrooms
conventional system not permitted
the soil will not accept the water — an engineered system is required
Soil this rate indicatesa conventional trench field is not permitted in most jurisdictionsclay — too slow
Hydraulic loading rateoutside the table at both endsnone permitted
Absorption area required—
Design flow3 bedrooms × 150 gal/day — the most common figure in state code450gal/day
Percolation rate90min/inch

Notes

  • Both ends of the scale fail, and that is the part nobody expects. Everyone assumes faster is better. Below about 1 minute per inch the water runs through before the soil has treated it, and a conventional field is refused for exactly the same reason clay is refused — the difference is that clay fails visibly and sand fails invisibly, into the groundwater somebody drinks.
  • 90 min/inch is outside what a conventional trench field can do. The answer is usually a mound system, a sand filter, or an aerobic treatment unit — all of which cost several times a conventional field and most of which need annual servicing. Get the design from an engineer, not from a table.
  • A perc test is a snapshot of one day. Run it in August after a dry month and the soil takes water it will not take in March. Many counties now require the test in a defined wet season, or require a soil profile pit read by a licensed evaluator instead, precisely because the fast-dry-season result kept producing failed systems two winters later.
  • The number that matters more than perc rate is the water table. A perfect 20 min/inch loam with seasonal groundwater 18 inches down still fails: the code requires vertical separation, typically two to four feet of unsaturated soil below the trench. That is found in a soil pit, not in a perc hole, and it is the single most common reason a site with an acceptable perc rate is still refused.
  • Design flow here is 150 gal/day per bedroom, which is the most widely used figure. A few states use 120, and some count fixtures instead of bedrooms. Bedrooms are used because they predict occupancy better than square footage does — and because a den with a closet becomes a bedroom the day after the inspector leaves.

90 minutes per inch, in plain terms

A percolation test drops water into a hole and times how long the level falls one inch. 90 minutes per inch puts this site in clay — too slow, and the loading rate that goes with it is none permitted gallons per ft² per day.

For 3 bedrooms the design flow is 450 gallons a day, so the absorption area works out at — ft².

Why bedrooms and not square footage

Codes count bedrooms because bedrooms predict how many people sleep in the house, and people are what produce the flow. A 3,000 ft² house with two bedrooms generates less wastewater than a 1,400 ft² house with four. It also closes an obvious loophole — otherwise every plan would show a "study" that becomes a bedroom the week after the inspection.

The three numbers that decide whether you can build

The perc rate is only the first. The other two are the depth to seasonal groundwater and the depth to bedrock or any restrictive layer. Codes require vertical separation — commonly two to four feet of unsaturated soil beneath the trench — and a site with a perfectly acceptable 90 min/inch rate still fails if the water table sits eighteen inches down in March.

Those two are found in a soil pit read by a licensed evaluator, not in a perc hole. Many counties have moved to requiring the pit precisely because perc alone kept approving sites that failed two winters later.

Nearby sizes

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