Ontario · Calculations · 2026
T-time: the one number that sizes your whole septic bed
Every quote you get, every square metre of bed, every truckload of stone traces back to a single number: how fast water moves through your soil. Here is what T-time is, how it is measured, and exactly what it does to your design and your bill.
The definition: T-time (percolation time) is the minutes it takes water to drop 1 cm in your soil. Sand might be T=4. Loam T=15–25. Clay T=40+. The Code feeds it straight into the bed formula: pipe length L = daily flow Q × T ÷ 200.
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Costs, permits, contractor vetting, and the owner-builder path — updated for the 2026 Building Code.
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1 Something’s wrong2 What it costs3 The permit4 Who designs it5 The paperwork6 Approval
The part nobody tells you
Designers charge $800–$2,000 just for the paperwork in these quotes.
Ontario law lets you do that part yourself, on your own property. OntarioSepticDesigner.ca turns your answers into the same 26-page package the township clerk sees every day — calculations, drawings, forms, all of it.
Why one number matters this much
A leaching bed works by letting effluent soak away through soil. Slow soil needs more area to absorb the same daily flow — so the Code scales the bed directly with T-time. Same house, different soil:
| Soil | T-time | Pipe needed (3-bdrm, Q=1,600 L/day) | What that means |
|---|---|---|---|
| Coarse sand | T = 6 | 48 m | Small, cheap bed |
| Fine sandy loam | T = 18 | 144 m | The typical Ontario bed |
| Silty loam | T = 30 | 240 m | Big bed — lot size starts to matter |
| Clay | T = 50+ | Conventional bed no longer permitted | Filter bed or raised bed on imported sand |
Triple the T-time, triple the pipe, the stone, the excavation and most of the cost. This is the single biggest reason quotes vary between neighbours.
How T-time is actually determined
1Identify the soil — from your test holes, the layer the bed will sit in. Many designs use the Code’s soil-classification tables to assign a conservative T-time from the soil description
2Or test it directly — a percolation test: dig a small hole in the bed layer, soak it, refill, and time the drop with a ruler and a watch
3Or send a sample — a soils lab grades the sample and reports the percolation rate; some authorities want this on borderline ground
The mistake that fails reviews
Testing the wrong layer. The T-time must come from the soil AT BED DEPTH — not the topsoil, not the fill somebody spread years ago. An examiner who sees “T=8, sand” on the worksheet and “silty clay at 60 cm” in the test-hole log will bounce the package in one read. The two documents must describe the same ground.
Where the number lands in your paperwork
Your T-time appears on the design worksheet, drives L = Q × T ÷ 200, sets the run count and spacing, and has to agree with the soil description in your test-hole log and the layers on your cross-section. Three documents, one number, zero tolerance for contradiction — that consistency is most of what a reviewer checks. The full package list shows where everything goes.
The $18,950 line that never appears on a quote
A real worked example: 3-bedroom bungalow, conventional bed. Tank supplied and set, pipe, stone, fabric, one day of machine and labour, permit allowance — $13,049 with HST. The same job quoted at $32,000. The gap is design fees, markup, and labour you may not need to buy.
Two minutes on the free checker shows the numbers your own lot generates — daily flow, tank size, risk level — before anyone quotes you.
What the software does with it
Enter your soil description or measured perc rate once. The builder assigns the Code T-time, runs the bed math, sizes the runs, prices the material — and writes the same number on every worksheet, log and drawing so nothing can disagree. The free check will also warn you when your T-time pushes the design out of conventional-bed territory before you spend anything.
T-time questions, answered straight
What is a good T-time for a septic bed?
Lower is better. Under 10 is excellent; 15–25 is normal Ontario ground; over 50 means a conventional bed is off the table and the design moves to a filter bed or raised bed.
Can I do my own percolation test?
On your own property, yes — dig to bed depth, pre-soak the hole, then time the drop per centimetre. Log the method and readings; some authorities will ask to witness it or want a lab report on difficult soil.
Does T-time change with the seasons?
The soil’s rate does not change much, but testing in frozen or saturated ground gives false slow readings. Test in reasonable conditions and pre-soak properly.
What T-time does clay have?
Typically 50 and up — past the limit for a conventional in-ground bed. Clay lots build filter beds or raised beds on imported sand, which is a cost question, not a feasibility death sentence.
One number in, the whole design out
The builder turns your T-time into the sized bed, the material order and every worksheet the township wants — consistent to the last digit. $99.99.