Perc tests, soil evaluations and what happens next

What a soil evaluation actually measures

A soil evaluation is not a hole full of water with a stopwatch. A licensed Authorized Onsite Soil Evaluator (AOSE) or professional engineer digs or bores a series of pits across the proposed drain field area and reads the soil profile the way a geologist reads rock. What they are recording:

• Soil texture and structure by horizon. How much sand, silt and clay at each depth, and how the particles are aggregated. Structure matters as much as texture — a well-structured clay with strong blocky peds moves water far better than a massive, structureless clay of identical texture.
• Depth to a limiting layer. Rock, saprolite, hardpan, or a restrictive clay horizon. You need a workable thickness of usable soil beneath the trench bottom for treatment to happen.
• Depth to seasonal high water table. Read from redoximorphic features — the gray and orange mottling left behind by iron moving under saturated conditions. This is why the evaluation works in July even though the water table peaks in March. It records where the water has been.
• Estimated soil permeability. Expressed as a loading rate in gallons per square foot per day, which is what converts directly into how much trench you need.
• Slope and landscape position. Percent grade, whether the site is a ridge, sideslope or drainageway, and where water goes when it rains.

From those readings the evaluator assigns a loading rate and a system type, then designs a layout that fits the usable area with required setbacks from wells, property lines, streams, foundations and cut banks. The output is a signed evaluation and design package that VDH reviews.

Where we come in

The evaluation and the design belong to the AOSE or the engineer — that is a separate licence from installing, in Virginia and in North Carolina both. What comes out of it is a drawing: system type, dimensions, products, elevations, the lot.

That drawing is what we build from. We prepare and submit the permit paperwork, then install everything exactly to the specification — the specified products, the specified dimensions, in the specified place. No substitutions to save a few dollars, because the health department signs off against that same drawing, and a system built to spec is the one that still works in twenty years.

If the evaluation has not been done yet, say so and we will get it scheduled with an evaluator so the whole thing runs as one job instead of you chasing two trades.

Why nobody in Virginia says perc test anymore

People still search for “perc test” and we still answer to it, so here is the plain explanation. A percolation test measured how fast water dropped in a pre-soaked hole, in minutes per inch. It was cheap, it was fast, and it was unreliable — results swung wildly depending on how long the hole was pre-soaked, how the sidewalls were prepared, and how wet the season was. A hole augered in dry clay in August can smear a glazed skin on the sidewall and produce a number that has nothing to do with how the soil behaves in February.

Virginia moved to soil-profile evaluation, which reads the permanent physical evidence in the soil instead of a single moment’s water behavior. It is more accurate, it is repeatable, and it detects the thing a perc test misses entirely: a seasonal water table that will drown the field four months a year.

Practically, this matters to you in three ways. First, results do not depend on when you test — a winter evaluation and a summer evaluation should agree. Second, the evaluator produces a design, not just a pass or fail, so you leave with a specific system and a specific footprint. Third, if you are relying on an old “perc test passed” note in a listing or a decades-old file, that is not a current, permittable evaluation and should not be treated as one.

So when a realtor, a lender or a builder asks whether the lot “percs,” what they need is a soil evaluation and a VDH construction permit or a current AOSE design. That is the document with value.

The sequence from raw lot to operating system, with real timelines

Every step depends on the one before it. Skipping ahead is where projects lose months.

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Plan on four to twelve weeks from first shovel to permit in hand for a normal residential job, and longer if the design is engineered or the site needs additional work. Weather is a real factor in Piedmont clay — evaluators cannot read a profile properly out of a pit that is filling with water, and no responsible installer cuts trenches in saturated clay.

An AOSE design in Virginia is generally valid for eighteen months and is transferable to a new owner, which makes it a genuine asset if you sell the lot. We prepare and submit permit applications for our customers — we do not issue permits, and no contractor can.

How the soil report drives every downstream cost

This is the part builders and lot buyers most need in front of them. The evaluation is a small line item that determines a very large one.

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On top of the install: soil evaluation and design runs roughly $1,500 to $5,000 depending on lot size, complexity and whether an engineer is required alongside the AOSE. Permits and health department fees generally run $900 to $2,500. Those are real numbers to carry in a land budget, and they arrive before you own a single foot of pipe.

The number of bedrooms in the house sets design flow — not square footage, not the number of people currently in your family. Virginia sizes residential systems by bedroom count, so a fourth bedroom on the plan can meaningfully enlarge the field and, on a tight lot, can be the difference between a conventional system and an engineered one. If you are on marginal soil, price the three-bedroom and four-bedroom versions before you finalize the floor plan.

What a failed or limited evaluation actually means

“It didn’t perc” is almost never the end of the story, and it is almost never free to solve either. Here is the honest range of outcomes.

Failed in that location, fine in another. The most common result on larger tracts. The evaluator found a limitation in the area tested and there is better ground elsewhere on the parcel. Cost to resolve: another round of pits, and possibly moving the house or drive.

Conventional gravity ruled out, alternative system available. The typical Pittsylvania and Henry County outcome. The soil will accept effluent, just not at conventional depth or conventional rate. LPP or drip solves it. Cost to resolve: the gap between a conventional system and an engineered one — commonly $10,000 to $20,000 of additional cost, plus ongoing operating requirements.

Alternative system required with pretreatment. Marginal soil where the design must produce cleaner effluent before it reaches the ground. This is where sand filters and aerobic treatment units come in, and where lifetime operating cost starts to matter: annual licensed operator visits are required on alternative systems in Virginia, at minimum yearly for typical residential flows.

Genuinely unbuildable for onsite sewage. It happens. Rock at ten inches, a water table at the surface, a lot too small to hold a field and its setbacks, or a parcel that is nearly all floodplain. Some of these have engineered answers at a cost that exceeds what the lot is worth. A responsible evaluator will tell you that, and so will we.

One thing to understand about sequence: a designated drain field area and a designated reserve area both have to be protected on the lot and kept free of driveways, buildings, pools and heavy traffic. A lot that technically holds a field but has no room for a reserve area is a lot with a problem in fifteen years.

Before you buy the lot: the checklist

Run this before you sign, and make the contract contingent on the results. The cost of an evaluation is a rounding error against the cost of an unbuildable parcel.

• Ask for any existing soil work or permits. Prior evaluations, old construction permits, a repair history if there was a house on it. Health departments keep records; ask what exists for that parcel.
• Make the offer contingent on a satisfactory soil evaluation and a VDH construction permit for the specific number of bedrooms you intend to build — not just “a septic permit.” A permit for a two-bedroom does not help you build four.
• Walk the low ground after rain. Standing water, sedges and rushes, alder, and stream-cut banks all tell you where the water table is before anyone digs.
• Locate the well site and the neighbors’ wells first. Setbacks between the drain field and any well are fixed, and on a small lot they are what eats the buildable area.
• Check whether utilities, easements or a shared drive cross the good soil. An overhead power easement or a gas line running through the only workable field area is a real constraint.
• Confirm access for equipment. A narrow shared drive with a low bridge means the pump truck and the excavator may not be able to reach the site, which is its own cost.
• Ask about the reserve area explicitly. If the answer is that the whole lot is the primary field, you are buying a replacement problem.
• On steep Henry County ground, ask where the field goes relative to the house. Uphill fields require pumping. That is normal, but it should be in the budget from day one.

What the soil looks like around here, county by county

Some regional patterns worth knowing before your evaluation, so the results are not a surprise.

Pittsylvania County. Deep red Piedmont clay over saprolite is the norm. The clay is usually well-structured near the surface and gets denser and less permeable with depth, which is precisely why shallow-placed, pressure-dosed systems work here and deep conventional trenches often do not. Usable soil depth is frequently the limiting factor rather than water table.

Halifax County. Similar Piedmont soils, but the bottomland along the Dan and Banister rivers carries a high seasonal water table. A site that looks and drains beautifully in August can have free water within a foot of the surface in late winter. This is the classic case where the redoximorphic features in the profile save someone from building a system that fails every spring.

Henry County. Martinsville and Collinsville sit on genuinely hilly terrain. Slope creates two issues: usable field area is broken into smaller workable patches, and the field frequently ends up above the house. Pump-dosed and pressure-dosed systems are the norm rather than the exception, and cut banks and downslope setbacks have to be respected in the layout.

None of this substitutes for an evaluation of your specific parcel. Soils change over fifty feet. But it does mean that if someone quotes you a bargain conventional gravity system on Piedmont clay without a current evaluation in hand, be skeptical.

What we would tell you not to do

Advice that costs us work.

• Do not clear and grade the lot before the evaluation. This is the single most expensive mistake we see. Stripping topsoil and running equipment across the best soil on the parcel destroys structure and compacts it, and an evaluator has to read what is there now — not what was there before the dozer. People have turned a permittable lot into an unpermittable one in an afternoon.
• Do not rely on a neighbor’s result. “The lot next door percs” means nothing. Soil boundaries do not follow property lines.
• Do not buy on a decades-old permit or an old perc note. Regulations, the design method and the site itself have all changed. Get current work.
• Do not chase the cheapest evaluation. The evaluator’s judgment about loading rate and system type determines a five-figure construction cost and thirty years of operation. An aggressive loading rate that squeaks past review produces a system that fails in year seven.
• Do not let us install a conventional system on marginal soil because it is cheaper. If the evaluation calls for LPP or drip, that is the system. We would rather lose the job than put a gravity field into ground that will not take it.
• Do not skip the reserve area. Building the shop or the pool on it is legal-feeling right up until the field needs replacing.