Septic Bed

The tank gets the attention. It has lids, it gets pumped, and the truck that comes to empty it is hard to miss. The bed sits under a patch of lawn and asks for nothing, so most owners only think about it when something goes wrong. That is backwards. The province's SepticSmart guide, citing the US Environmental Protection Agency, puts 30 to 50 percent of the treatment in the tank and 50 to 70 percent in the soil underneath the bed.

On our builds, the septic system is part of site preparation, which I oversee, from design and installation through to the permit inspections. Here is what a septic bed is made of, the kinds the Ontario Building Code allows, what it needs around it, and what to do when it starts to struggle.

What a septic bed is

A septic bed is the part of a septic system that returns treated water to the ground. The Building Code calls it a leaching bed. You will also hear it called a drain field, a tile bed or a weeping bed. They all mean the same thing: the place where the liquid leaving the tank spreads out and soaks into soil.

The tank does the first, rough part of the job. Solids settle to the bottom, grease floats to the top, and an effluent filter at the outlet holds both back. What leaves the tank is cloudy water that still carries bacteria and dissolved waste. The bed spreads that water thinly over a large area of unsaturated soil, and the soil finishes the job.

That is why the bed is the expensive part to lose. A tank can be pumped. A clogged bed often has to be dug out.

What is inside a septic bed

The most common kind is built from absorption trenches. SepticSmart describes the layers from the bottom up:

• Unsaturated soil. Native soil, or imported sand fill where the native soil will not do.
• Septic stone. A layer of washed, crushed stone that the pipe sits in.
• Perforated pipe. PVC distribution pipe with holes that let the effluent out along its length. Some beds use half-moon plastic leaching chambers instead.
• A cover. More stone over the pipe, then a non-woven geotextile so sand cannot wash down into the stone.
• Backfill. Sand, then topsoil, then grass.

The numbers the Code sets for a trench-style septic bed

The Ontario Building Code is specific about how trenches go together. Under Article 8.7.3.2, each trench is 500 to 1,000 mm wide, 600 to 900 mm deep and no more than 30 metres long. Trenches with pipe are centred at least 1,600 mm apart, and the bottom of every trench sits at least 900 mm above the high groundwater table, rock, or tight soil.

The pipe itself (Article 8.7.3.3) is at least 3 inches for a gravity system and falls 30 to 50 mm for every 10 metres, so effluent runs the full length instead of pooling at the start. It sits in at least 150 mm of stone below and 50 mm above, and the stone is covered with untreated building paper or permeable geotextile before the backfill goes on. However small the house, the Code asks for at least 40 metres of distribution pipe. The full length is set by the house’s daily flow and how fast the soil drains, which we cover in septic system design.

Why the soil under a septic bed matters most

SepticSmart describes the soil under the stone as a biological, chemical and physical filter. Bacteria living in the soil feed on what is left in the effluent. Fine particles of soil hold back what the bacteria do not catch. The water moves down slowly enough for both to work.

That only happens in soil with air in it. Saturated soil, below the water table, has no room for the effluent to spread and none of the oxygen that the treatment depends on. That is the reason for the 900 mm rule: there has to be a deep enough layer of dry, open soil between the bottom of the bed and the water table or the rock.

The Code also sets limits on the soil itself. Under Article 8.7.2.1, a bed cannot go in soil with a percolation time faster than 1 minute or slower than 50 minutes. Soil that drains too fast passes the effluent through before it is treated. Soil that drains too slowly cannot take it at all. The same article keeps a bed off ground steeper than 1 in 4 and out of areas where flooding could damage it.

The five kinds of septic bed in Ontario

The Building Code allows five ways to spread effluent into soil. Which one fits depends on the soil, the space and the level of treatment ahead of it.

Absorption trenches

The conventional bed described above. SepticSmart notes it is usually the least expensive option where the native soil meets the design requirements, and it usually runs by gravity, with no pump. The trade-off is space: on a small lot it may not fit.

Filter bed

Instead of separate trenches, a filter bed is one continuous layer of stone over a bed of specified filter sand. Under Article 8.7.5.3, that sand is at least 750 mm deep and must stay unsaturated for its full depth. Each filter bed is between 10 and 50 square metres, and for a house sending up to 3,000 litres a day, no more than 75 litres can land on each square metre. The footprint is smaller, which helps on a tight lot. SepticSmart adds two cautions: the filter sand is costly because it is hard to find and usually has to be processed, and a filter bed does not handle heavy flows as well as trenches do.

Shallow buried trench

Small pipes inside open-bottomed plastic chambers, close to the surface. A pump doses the effluent under pressure at set intervals so it spreads evenly along the whole trench. These take only effluent that has already passed through an advanced treatment unit. SepticSmart points out that the trenches can follow an irregular line, around trees for example.

Type A and Type B dispersal beds

Both follow an advanced (Level IV) treatment unit. A Type A bed is a layer of stone over a specified sand, and SepticSmart notes it sits lower above the existing grade than a raised leaching or filter bed. A Type B bed is a pressurized stone bed that was added to the Code in 2017. It is laid out long and narrow, with its long side following the slope’s contours.

The advanced treatment unit is what makes these smaller beds possible, and it comes with a mandatory maintenance agreement. The trade-offs are covered in our post on the cost to put in a septic system.

Why some septic beds sit above the ground

Where there is not 900 mm of good, dry soil above the water table or the rock, the bed is built up instead of dug down. SepticSmart puts it plainly: a high water table, bedrock or clay soil means a raised bed. The pipe and stone sit in imported sand fill, and the whole bed rises above the yard as a long, low mound with sloped sides.

A raised bed needs more than the mound itself. Under Article 8.7.4.2, the soil it sits on has to be unsaturated for at least 250 mm, and that good soil has to extend at least 15 metres beyond the outer pipes in the direction the water will travel. That apron, often called the mantle, is where the effluent goes once it leaves the mound. If someone builds on it, digs into it or paves it, the bed loses part of its capacity even though the mound looks fine.

A raised bed also needs more room around it. SepticSmart gives the rule: take the height of the bed above the existing grade, double it, and add that to every separation distance. A bed 1.5 metres high adds 3 metres, so a drilled well that would need to be 15 metres away now needs 18.

How much space a septic bed needs around it

Table 8.2.1.6.B of the Building Code sets the minimum clearances from the distribution pipe of a septic bed:

• 5 metres from any structure, including the house
• 15 metres from a well with a watertight casing at least 6 metres deep
• 30 metres from any other well, which includes most dug wells
• 15 metres from a lake, pond, reservoir, river or stream
• 3 metres from a property line

Those distances shape a whole lot plan, not just the bed. The house, the well, the driveway, the garage and the bed all have to fit with these gaps between them, and with the extra margin a raised bed adds. On some lots, the bed decides where the house goes. That is one reason we treat the well and septic as part of site preparation and not something added at the end.

What can go on top of a septic bed

Grass. That is the short answer, and it is close to the whole answer.

The Code asks that a bed be protected from compaction and from any load that could crush a pipe or smear the soil (Article 8.7.2.1). Smeared soil seals over and stops taking water. SepticSmart’s Don’t list says the same thing in plain terms:

• Do not drive or park on it, including trailers, boats and heavy equipment.
• Do not pave it. The Code bars covering a bed with anything that will not let water and air through.
• Do not plant trees or shrubs close to it. SepticSmart’s layout diagram shows 6 metres to a tree.
• Do not send gutters, sump pumps, hot tub water or surface runoff onto it.

Grass is the right cover because it holds the soil, uses some of the water and leaves the surface open to air. The Code also asks that the surface be shaped to shed rainwater, so a bed should never sit in a low spot where water collects.

Sheds, decks, pools, gardens with raised boxes, and later additions to the house all run into the 5-metre clearance. If you are planning something new on a property with a septic system, find the bed first. Our post on septic tank maintenance covers locating the system and keeping a record of it.

Signs a septic bed is in trouble

SepticSmart lists the symptoms of a malfunctioning system. Several of them show up at the bed before they show up in the house:

• grass over the system that is unusually green, or spongy underfoot
• water pooling on the surface
• a sewage smell in the yard
• drains slowing down and toilets backing up
• bacteria or nitrate showing up in well water tests

Effluent on the surface is more than unpleasant. Article 8.9.1.2 of the Code says a system must never discharge sewage or effluent onto the ground. SepticSmart also notes that effluent coming up through the tank lids points to either a blocked effluent filter or a saturated bed.

A bed usually fails in one of three ways: too much water, solids leaving the tank, or soil that has been compacted or sealed. Too much water comes from leaks, from running laundry all in one day, or from a sump pump tied in. Solids come from a tank that has gone too long without being pumped, or from a missing or clogged filter. Compaction comes from traffic and from building over it. SepticSmart adds that a system failing soon after it is built may have been poorly assessed, designed or built in the first place.

Can a septic bed be repaired, or does it need replacing?

Start with the cheapest explanation. SepticSmart’s advice is to have the system inspected first, because the tank may just need cleaning, or the filter may be blocked. Both are routine fixes that take the pressure off the bed.

If the problem is in the bed itself, the guide is honest about the range. Repairs can be anything from cleaning out a few lines to replacing the whole distribution system and digging out the contaminated, clogged soil. It recommends a second opinion, and lists the people qualified to give one: installers, professional engineers, certified engineering technologists and registered sewage system designers.

Two things are worth knowing before you start:

• Repairing a bed needs a permit. SepticSmart says to get the permit from your local regulatory agency before work begins. Depending on the municipality, that may be the township, the health unit or the conservation authority.
• A replacement bed may not go where the old one was. The site is evaluated again, and the bed is designed to the current Code. If the clearances or the soil have changed, the new bed may need to be a different type, or raised. On a tight lot, an advanced treatment unit with a smaller bed is one of the options SepticSmart lists for replacing a failed system.

SepticSmart puts the life of a septic system at roughly 15 to 40 years. Much of where yours lands in that range comes down to the bed, and our post on how long a septic system lasts goes through what pushes a system toward either end.

Where to start with a septic bed

If you own a property with a septic bed, find it, mark it, and keep traffic, roofs and runoff off it. Grass only. Have the tank opened and the filter cleaned on schedule, because most of what kills a bed starts in the tank.

If you are building, the bed is decided early, before the house is placed on the lot. On our projects, septic design and installation are part of site preparation, and the septic permits and inspections are handled alongside the rest of the build’s permits and approvals. If you are weighing a lot and want to know where a bed could go on it, tell us about the property. We reply within two business days.

Frequently Asked Questions