Septic Fields for Landscapers, Fence and Deck Builders
Why this matters
On a septic property, the nicest flat stretch of lawn is often the one place nothing should go: the drain field. It looks like grass. Under it, a set of shallow trenches is treating every gallon the house sends down the drain, and it only works if the soil above stays loose, open to air and free of weight. A skid steer driven across it once, a row of fence posts through its trenches, a patio poured over its end or a line of trees planted along it can shorten its life by years or end it. A field replacement is one of the most expensive repairs a homeowner faces, and it usually needs a permit, a soil evaluation and a new area of the yard, which may already have your deck on it.
Safety first
- Never enter a septic tank or lean into an open one. The air inside can hold hydrogen sulfide and methane and be short of oxygen; people die in tanks, often a second person going in after the first. If a lid is open or broken, keep everyone back, cover the opening with something that will carry a person's weight if you can do it without leaning over it, and call the septic company.
- A cracked or sagging lid, or ground that drops when you step near the tank: stop, keep people and equipment off, and mark it off. Old tanks with failing lids are a fall-in hazard for children and pets.
- Soggy ground with a sewage smell, or gray or black water on the surface: keep people, children and pets off it, do not dig in it, wash hands before eating, and tell the homeowner to call the septic company. Surfacing sewage carries pathogens. A smell fading after a few minutes is not a sign it is safe; hydrogen sulfide dulls the sense of smell.
How a typical system is laid out
PLAN VIEW, CONVENTIONAL SEPTIC SYSTEM
+---------+
| HOUSE |--cleanout--building sewer, sloped
+---------+ |
[ SEPTIC TANK ] lids or risers
inlet outlet at grade
|
solid pipe
|
[D-BOX] distribution box,
| | | splits flow
============|====|====|===== TRENCH 1
============|====|===== TRENCH 2
=============|===== TRENCH 3
o inspection ports along trenches
---- shallow, gravel or chambers ----
. . . . . . . . . . . . . . . . . . . .
. RESERVE AREA: kept open for a .
. future replacement field .
. . . . . . . . . . . . . . . . . . . .
The tank. A buried box, usually concrete, plastic or fiberglass, that holds solids back and lets clarified wastewater, called effluent, flow out. Its lids may be at grade, under a few inches of soil, or brought up to the surface with plastic risers (round green or black lids).
The distribution box (D-box). A small concrete or plastic box past the tank that splits effluent between the trenches. Its lid is usually buried. A D-box tipped by a vehicle or a frost heave sends all the flow down one trench and overloads it.
The trenches (laterals). Perforated pipe in gravel, or plastic chambers, in trenches commonly a few feet wide and only a couple of feet deep. Effluent soaks out into the soil below, where soil microbes finish treating it. That treatment needs oxygen, which reaches the soil from the surface.
The reserve area. Many states and counties require a second area, sized and tested at permit time, to be kept open for a replacement field. It is often recorded on the permit. Building on it can leave the property with no legal place for a new field.
Why nothing goes on the field
- Vehicles and equipment. A loaded truck, a skid steer or even repeated passes of a mower trailer compact the soil and can crush pipe and chambers. Compacted soil stops absorbing, and the field fails from the surface down. Route equipment around the field, and carry materials across it by hand if you must.
- Posts and footings. An auger through a trench breaks the pipe or chamber and lets gravel and soil into it. Concrete in or near a trench blocks flow. Footings also bring point loads the trench was not designed for.
- Paving, patios, decks and sheds. Covering the field cuts it off from air and from evaporation, and blocks access for the inspection and repair it will someday need. Many local health codes prohibit structures and impervious surfaces over the field and reserve area.
- Trees and large shrubs. Roots go to water and nutrients, which is exactly what is in the trenches. Willows, poplars and other water-seeking trees near a field clog it. Grass is the best cover. Many guides give a distance tied to the tree's mature height; your septic designer or health department can give the local number.
- Irrigation and downspouts. Sprinkler heads aimed at the field, or a downspout draining onto it, add water the soil has to handle on top of the house's.
See related: Septic System Basics Reference.
How to find the field before you plan anything
- Ask for the permit drawing. The county or local health department usually has the septic permit and an as-built drawing showing the tank, the field and often the reserve area. This is the best single source and is often a free records request.
- Find the tank. Look for risers or lids, a cleanout pipe between the house and the tank, and the direction the building sewer leaves the foundation.
- Find the inspection ports. Short white plastic pipes with caps standing in the lawn, often at the ends of trenches. A line of them is a line of trench.
- Read the grass. In dry weather the field shows as stripes of greener, lusher grass over each trench. In snow, the stripes melt first. A narrow sunken line can mark an old trench.
- Ask a septic company to locate it when the records are missing or the yard does not match them. They can trace the lines with a transmitter run from the tank. A steel probe pushed hard into the lawn by a crew that does not know what it is looking for can puncture a chamber or a pipe.
See related: Private Wells and Septic for Non-Specialists.
Worked example: a pet fence across a septic field
A fence crew is asked for a 4 foot pet fence that runs straight across the back yard, posts 8 feet on center. The county as-built drawing shows the field directly in its path: three trenches, each 3 feet wide, spaced 6 feet center to center, running parallel to the house. The reserve area lies to the east, off the fence line. The fence line crosses the trenches at a right angle.
How much of the crossing is trench. With trench centers at 0, 6 and 12 feet and each trench 3 feet wide, the field runs from 1.5 feet before the first center to 1.5 feet past the last: 12 + 3 = 15 feet of crossing. Trenches occupy 3 x 3 = 9 of those 15 feet, or 60 percent. A post dropped anywhere in the crossing at random lands in a trench more often than not, and an 8 foot spacing that starts from the house side lands where it lands.
Where a post could go. Between trench 1 (center at 0, far edge at 1.5 feet) and trench 2 (near edge at 4.5 feet) is a 3 foot strip of undisturbed soil; the same gap lies between trenches 2 and 3, from 7.5 to 10.5 feet. Their centers are at 3 and 9 feet.
The first question is whether to cross at all. The crew calls the county health department, which for this property says a fence across the field is acceptable if no post lands in a trench and no equipment drives on the field. Some jurisdictions say no to any posts in the field; that answer is local, and it is the first call, not the last.
The layout. The crew sets posts outside the field at 2.5 feet before the first trench edge (at -4 feet) and 2.5 feet past the last (at 16 feet), and one post in the center of each gap (at 3 and 9 feet). The spans are 3 - (-4) = 7 feet, 9 - 3 = 6 feet and 16 - 9 = 7 feet, all within the 8 foot maximum the rails allow. A gap post sits 1.5 feet, 18 inches, from each trench edge; a 10 inch post hole reaches 5 inches from center, leaving 18 - 5 = 13 inches of undisturbed soil on each side.
Confirming the trenches are where the paper says. Drawings drift. The crew does not probe. The septic company traces the trenches with a transmitter and flags their edges along the fence line; all three trenches sit 8 inches west of the drawing. The crew shifts the two gap posts 8 inches west with them, which keeps each centered in its gap. The end posts stay put and still sit outside the field: the first span shortens by 8 inches to 6 feet 4 inches, the middle stays 6 feet, and the last lengthens by 8 inches to 7 feet 8 inches, still under 8.
How the posts go in. Hand auger, no truck or skid steer on the field, posts set in gravel and soil tamped by hand rather than concrete inside the field, and spoil carried off. Outside the field the crew uses its normal method.
The tally. Fifteen feet of crossing, two posts inside the field, both in the gaps between trenches with 13 inches of undisturbed soil on each side, no machine on the field, nothing on the reserve area. The customer gets the fence and the field keeps working.
What changes the answer
Mound, drip and pressure-dosed systems. A mound is a raised bed of sand built above grade; a drip system runs small tubing a few inches down across a wide area. Both are more fragile than a trench field. A drip field in particular has tubing everywhere, not in a few lines. Treat the whole area as off limits and ask the service provider.
An alarm or control panel. Systems with a pump or an aerobic treatment unit have a control panel and an alarm on a post or the house wall, and power run to the tank. That buried power line is private and unmarked by 811, like the effluent lines.
A deck near the tank. A deck that covers the tank lids blocks pumping, which many health departments require to stay possible. Leave an access hatch the size the septic company asks for, or move the deck. See related: The Deck Footing That Landed on the Septic Line.
References
- Your county or local health department's septic permit and as-built records for the property.
- State onsite wastewater rules, which set setbacks, reserve area requirements and what may be built over a field.
- See related: Private Wells and Septic for Non-Specialists
- See related: Septic System Basics Reference
- See related: How to Find the Private Lines a Utility Locate Will Not Mark