How to Read a Perc Test Result in Ontario (And What to Do If You Fail)

Your Perc Test Results Just Came Back. Here Is What They Actually Mean.

A percolation test result is a number. Most Ontario landowners stare at it without knowing what it tells them, what it means for their septic system, or what their options are if the number is bad. This guide explains all three.

The percolation test — almost always called a perc test, and officially called a T-time test in Ontario — is the single piece of information that determines what kind of septic system your property can support and how large it needs to be. Get a good number and you have options. Get a bad number and your options narrow. But a bad number is not the end of the road — and understanding exactly what the number means is the first step to knowing what to do next.

This guide is written specifically for Ontario. Perc test standards, units, and procedures vary by province and country. The rules here are governed by Part 8 of the Ontario Building Code, and what follows is specific to those requirements.

What a Perc Test Actually Measures

A percolation test measures how quickly water drains into the soil under saturated conditions. The official Ontario definition, from the Building Code, is the average time in minutes it takes for the water level to drop one centimetre in a percolation test hole. This is expressed as minutes per centimetre, abbreviated as min/cm and referred to as T-time.

That last unit matters. In the United States, perc tests are commonly expressed in minutes per inch. Ontario uses minutes per centimetre. If you are researching perc tests online and reading American sources — which most of them are — the numbers will not match Ontario’s system. A T-time of 10 min/cm in Ontario is not the same as 10 minutes per inch in a US context. Do not cross-reference the two without converting.

The lower the T-time number, the faster water drains — and generally, the better the soil conditions for a conventional septic system. A T-time of 2 min/cm means water drops one centimetre every 2 minutes. Sandy, well-draining soil. A T-time of 50 min/cm means water drops one centimetre every 50 minutes. Dense, slow-draining soil. Both are measured in the same units; the implications for your septic system are completely different.

How the Test Is Conducted in Ontario

Understanding the procedure helps you evaluate whether a test was done properly — and whether the results are reliable.

A qualified designer or engineer excavates test pits on your property — typically with a backhoe, though hand-digging is permitted. The Ontario Building Code requires that test pits be deep enough to reach the soil horizon where the leaching bed will actually sit, typically 1.5 to 1.8 metres. This matters because the percolation rate of the surface soil is irrelevant — what counts is the rate of the receiving soil at leaching bed depth.

Within the proposed leaching bed area, a minimum of three separate percolation tests must be conducted, suitably spaced to capture variability across the site. This is a requirement under Section 8.2.1.2 of the Ontario Building Code — not a recommendation. One test hole is not sufficient.

The test procedure, per Ontario’s guidelines and the Canadian Standards Association (CSA B65-12), involves:

• Digging the test hole to the appropriate depth and scarifying the sides to remove compacted soil from the excavation process
• Pre-soaking the hole — keeping it filled with water for a period to fully saturate the surrounding soil, simulating actual in-use conditions. This step is critical. Testing unsaturated soil produces artificially fast results that do not represent real-world performance.
• Filling the hole to a consistent depth and measuring how long it takes for the water level to drop exactly one centimetre
• Repeating the measurement until three consecutive readings are the same — confirming the rate has stabilized
• Repeating across all three test locations

When multiple test results are obtained, the highest T-time (the slowest result) is used for design purposes. Not the average. Not the best. The worst. This is a Building Code requirement. The system must be designed for the worst soil condition encountered, because the worst section of the bed is where failure begins.

How Ontario Determines T-Time: Two Methods

Ontario’s Building Code recognizes two legitimate methods for establishing T-time, and this is one of the most commonly misunderstood aspects of the perc test process.

Method 1: The Percolation Test

The field test described above. Water is poured into a prepared hole and timed. This is what most people picture when they think of a perc test and it is the most commonly used method on straightforward sites.

Method 2: Soil Classification (The Lab Method)

The soil from the test pit is collected and sent to an accredited laboratory, where it is classified using the Unified Soil Classification System (USCS), as described in MMAH Supplementary Standard SB-6 (“Percolation Time and Soil Descriptions”). The classification puts the soil into a category — well-graded gravels, silty sands, inorganic clays, and so on — and each category has an established range of corresponding T-times under Ontario’s standard.

When lab results give a range of T-times, the designer uses the highest value in that range for design purposes. The same worst-case rule applies.

In practice, many sites use a combination of both methods. If the soil is visually obvious — pure sand or clear clay — the designer may classify it in the field without lab analysis. If the soil analysis suggests a T-time of 50 min/cm or greater (indicating clay), the Ontario Building Code explicitly states that you are not required to prove the specific T-time with further testing. Clay is clay. The implication for system design is clear regardless of the exact number.

What Your T-Time Number Means for Your System

This is the table that turns a number into a decision. T-times under Part 8 of the Ontario Building Code determine both the system class that is permitted on your lot and how the leaching bed area is calculated.

T-Time (min/cm)	Soil Type (Approximate)	What It Means for Your System	Typical Outcome
Under 1 min/cm	Coarse gravel, highly permeable	Too fast — insufficient treatment time before effluent reaches groundwater	System may not be permitted in native soil. Engineered solution required.
1 – 12 min/cm	Sandy soils, well-draining loam	Excellent drainage. Conventional gravity-fed Class 2 system typically possible.	Best case. Smallest leaching bed footprint. Lowest cost.
12 – 50 min/cm	Loam, silty soils, mixed conditions	Acceptable drainage. Conventional system still possible but bed area increases significantly as T-time rises. Filter bed may be required at higher end.	Workable. Bed area and cost increase with T-time. Pressure distribution may be required.
Over 50 min/cm	Clay-heavy soils	Native soil cannot support a conventional in-ground leaching bed. Raised bed, mounded system, or Class 4 advanced treatment unit required.	System possible but significantly more complex and expensive.
No workable result	Solid bedrock at shallow depth, saturated peat	No percolation possible in native soil. Engineered solution or imported fill required.	Most complex and expensive scenario. Not all lots can be developed.

What “Failing” a Perc Test Actually Means in Ontario

The phrase “failing a perc test” is misleading and commonly misused. In Ontario, a perc test does not produce a simple pass or fail result. It produces a T-time number. What that number means for buildability depends on the specific lot, the proposed system, and the available space.

What actually limits development is not the T-time itself but whether a compliant system can be designed and installed within the constraints of your specific property — setbacks, available area, water table depth, and proximity to water bodies. A T-time of 60 min/cm does not mean your lot is unbuildable. It means a conventional gravity system is not an option and you need to design something more complex.

The situations that genuinely prevent development are:

• T-time too fast (under 1 min/cm) — effluent passes through the soil too quickly for adequate treatment before reaching groundwater
• Bedrock at the surface with no usable soil depth and no room for an imported fill system
• Lot too small to accommodate the required leaching bed area at any system class, including advanced treatment
• Setbacks to wells, water bodies, or property lines that eliminate all possible system locations regardless of soil quality

These situations are genuinely limiting. But a high T-time number alone is not. It is the combination of T-time, lot constraints, and available system options that determines whether a lot can support a sewage system.

What Happens When T-Time Exceeds 50 min/cm

This is where most Ontario landowners need the most clarity, because it is the most common difficult result in the regions we serve.

When T-time exceeds 50 min/cm, the native soil cannot support a conventional in-ground leaching bed. The soil drains too slowly to safely accept the effluent volume from a residential system without saturating and surfacing. The Ontario Building Code requires a system that either bypasses the slow native soil or pre-treats the effluent to a higher standard before it reaches the native soil.

The main options are:

Class 3 Raised Bed or Mounded System

Instead of installing the leaching bed in the native soil, certified imported fill — sand or other compliant material with a known good T-time — is brought in and mounded above the existing ground surface. The leaching bed is installed within the imported fill. The effluent percolates through the engineered fill rather than the slow native soil.

This works well and is very common across Ontario. The trade-off is cost — importing, placing, and compacting 100 to 300 cubic metres of certified fill material adds $4,000 to $24,000 to the project before a single pipe is laid. See our raised bed vs conventional system guide for the full cost breakdown.

Class 4 Advanced Treatment Unit (ATU)

An advanced treatment unit — Ecoflo biofilter, Waterloo Biofilter, Bionest, Advanced Enviro-Septic, or similar — pre-treats the effluent to a much higher quality before it enters the soil. Because the effluent has already been significantly treated, the receiving soil does not need to do as much work, and the system can function on soils that would otherwise be unsuitable.

ATUs are more expensive to install and require a mandatory annual maintenance contract under Ontario regulations. But they enable development on lots where a raised bed is not practical due to size or topography, and they produce effluent that meets a higher environmental standard. For a comparison of the main ATU brands approved in Ontario, see our Ontario ATU comparison guide.

High Water Table Considerations

T-time and water table depth are separate issues, but they often appear together on challenging Ontario lots. The Building Code requires a minimum vertical separation distance between the bottom of the leaching bed and the seasonal high water table. If the water table is high — common in Georgian Bay lowlands, parts of Simcoe County, and Prince Edward County — this vertical separation requirement may force a raised system regardless of T-time. See our full guide on high water table septic solutions in Ontario.

What Happens When T-Time Is Too Fast (Under 1 min/cm)

This is the less common but equally important problem. Soil that drains too quickly — coarse gravel, fractured bedrock — does not provide adequate treatment time for septic effluent before it reaches groundwater. The biological and physical treatment processes that happen in the soil require contact time, and if water moves through in seconds rather than minutes, those processes do not occur.

On lots with very fast-draining soils near drinking water sources or water bodies, a standard leaching bed in native soil may not be permitted. Options include filter beds with specific media specifications, advanced treatment units that treat the effluent before it enters the fast-draining soil, or — in some cases — setbacks and lot conditions that make any in-ground system impossible.

This situation is less common in Ontario than high T-times, but it occurs in areas with fractured limestone bedrock, coarse outwash gravel, and some coastal Georgian Bay properties. If your designer raises this concern, take it seriously — it is not a problem a larger leaching bed solves.

Can You Challenge or Retest a Perc Test Result?

Yes, in some circumstances. If you believe the test was conducted improperly — wrong season, insufficient pre-soaking, inadequate number of test holes, or test locations that do not represent the proposed leaching bed area — you can commission a new assessment from a different qualified designer.

Legitimate reasons to retest:

• The original test was conducted during a dry summer period rather than near the seasonal high water table
• Only one or two test locations were used rather than the required minimum of three
• The test holes were not pre-soaked adequately before testing
• The test pit locations were not in the area of the proposed leaching bed
• Site conditions have changed — for example, drainage improvements that may have affected the water table

What is not a legitimate basis for retesting: simply not liking the result. The health unit has seen every strategy for getting a better number, and a retest that produces a dramatically different result on the same site will invite scrutiny of the methodology.

How T-Time Affects Leaching Bed Sizing

For those who want to understand the math: under Part 8 of the Ontario Building Code, leaching bed area is calculated from two variables — the daily design flow (which is determined by bedroom count and fixture units) and the T-time of the receiving soil.

The daily design flow for a standard three-bedroom Ontario home is approximately 1,100 litres per day. The required leaching bed area is then calculated using a formula that incorporates the T-time, producing a square metre figure for the minimum bed area. As T-time increases, the required bed area for the same flow increases — sometimes dramatically.

This is why your designer cannot give you a firm system size or cost estimate until the T-time is known. The number that comes back from the perc test is not just a soil quality indicator — it is an input into every downstream calculation in the system design. Changing the T-time changes the bed area, which changes the required lot space, which may change the system class, which changes the cost. Everything downstream of the perc test flows from that one number.

A perc test result is information. A high T-time is expensive information — it tells you your lot requires a more complex and costly system than you may have hoped. But it is not a death sentence for your project in most cases. Understanding what the number means, what system it points toward, and what your real options are is how you move from confusion to a plan.