What a 45 minute per inch perc rate means for a 3-bedroom house
| Soil this rate indicatesworkable, but the field grows and so does the price | silt loam |
|---|---|
| Hydraulic loading ratethe standard table used by most state codes | 0.45gal/ft²/day |
| Absorption area required | 1,000ft² |
| Design flow3 bedrooms × 150 gal/day — the most common figure in state code | 450gal/day |
| Percolation rate | 45min/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.
- 1,000 ft² is absorption area, not the size of your yard. The trenches need spacing between them, an intact reserve area of the same size in most counties, and setbacks from the well, the property line, the foundation and any surface water. Plan for roughly two to three times this figure of usable, undisturbed ground.
- 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.
45 minutes per inch, in plain terms
A percolation test drops water into a hole and times how long the level falls one inch. 45 minutes per inch puts this site in silt loam, and the loading rate that goes with it is 0.45 gallons per ft² per day.
For 3 bedrooms the design flow is 450 gallons a day, so the absorption area works out at 1,000 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 45 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
- What a 40 minute per inch perc rate means for a 3-bedroom house
- What a 50 minute per inch perc rate means for a 3-bedroom house
- What a 35 minute per inch perc rate means for a 3-bedroom house
- What a 55 minute per inch perc rate means for a 3-bedroom house
- What a 45 minute per inch perc rate means for a 2-bedroom house
- What a 45 minute per inch perc rate means for a 4-bedroom house
- What a 45 minute per inch perc rate means for a 5-bedroom house
Read more
- Why a Perc Test Fails — And Which Failures Can Be Re-TestedMost people assume a failed perc test means clay. Often it means the wrong month, the wrong hole, or a water table nobody looked for.
- Septic Drain Field Cost by State — 1,000 ft², All 51This trade has the tightest state-to-state spread of any on the site — which is itself the useful finding when two local quotes differ by half.
- Adding a Bedroom to a House on Septic — What the County ChecksThe septic system does not care how many bedrooms the plan says. The county does, and it re-checks the tank and the field before it stamps anything.
- Septic Alarm Going Off — What to Check, in OrderThe alarm is not the emergency. It is the warning that buys you about a day, and what you do with that day decides the cost.