A drain clog sits just downstream of the last fixture that still drains and just upstream of the first fixture that backs up. Tell the Clog Locator which fixtures are slow or overflowing and whether the problem comes and goes, and it narrows the blockage to a fixture trap, branch line, stack, building drain or lateral, then suggests the auger class, cable size and when to hire it out.
- One slow fixture almost always means a clog in that fixture's trap or trap arm, not in the main.
- When several fixtures misbehave at once, the clog sits where their drain lines join, and the lowest opening shows it first.
- A basement floor drain that gurgles or backs up when an upstairs toilet flushes points to the building drain or lateral.
- Intermittent symptoms usually mean a partial obstruction such as roots, grease or a sag, which an auger opens but rarely cures.
- The locator narrows the search; a camera inspection is what confirms the cause.
Clog Locator
01 /How does the Clog Locator work?
The Clog Locator works by elimination: a blockage stops water from every fixture upstream of it and has no effect on any fixture downstream of it. So the pattern of which drains are slow, which are backing up and which are fine tells you where along the drain system the obstruction must be. You tick the fixtures that are misbehaving, say whether the problem is constant or comes and goes, and the tool maps that pattern onto the typical layout of a Canadian house drain.
It returns four things: the most likely clog location (fixture trap, branch, stack, building drain or lateral), the auger class that reaches it (closet auger, hand drum, drum machine or sectional machine), the cable diameter that suits the pipe at that point, and a stop line that tells you when the job belongs to a drain professional instead of a rental counter.
02 /How do drain lines branch in a house?
A house drain is a tree that runs backwards: small branches at each fixture join larger branches, which join a vertical stack, which drops to a horizontal building drain under or along the basement floor, which leaves the house as the building sewer (the lateral) and runs to the municipal main or a private septic system. Pipe size grows as you move downstream, because each junction has to carry more fixtures.
| Part of the system | What drains into it | Typical nominal size |
|---|---|---|
| Fixture trap and trap arm | One fixture: bathroom sink, kitchen sink, tub, shower | 1-1/4 in (bath sink) to 1-1/2 or 2 in |
| Toilet trapway and closet bend | One toilet | About 2 in trapway, 3 in bend |
| Branch drain | A group of fixtures, usually one bathroom or kitchen | 1-1/2 to 3 in |
| Stack | Every branch on the floors above | 3 in in most houses, 4 in in some |
| Building drain | The stack plus basement fixtures and floor drain | 3 to 4 in (75 to 100 mm) |
| Building sewer (lateral) | The whole house, out to the main | 4 in (100 mm) typical, sometimes 5 or 6 in |
Two consequences matter for augering. First, the further downstream the clog, the bigger the pipe and the heavier the cable you need. A 1/4-inch hand-drum cable that is perfect in a bathroom sink will simply coil up inside a 4-inch building drain without touching the blockage. Second, the further downstream the clog, the more fixtures it affects, which is exactly what the locator reads.
03 /Why does the lowest fixture show the clog first?
The lowest fixture shows a main-line clog first because water backed up behind a blockage rises until it finds the lowest open drain above the blockage, and spills out there. In most Canadian houses that is the basement floor drain, a basement shower, or a basement toilet. Upstairs fixtures keep draining normally until the water level in the building drain has climbed all the way up to the basement fixture openings.
This is why a floor drain that gurgles, bubbles or pushes up grey water when the washing machine empties or an upstairs toilet flushes is a much more serious signal than a slow upstairs sink. The upstairs sink tells you about one trap. The floor drain is telling you about the pipe that every fixture in the house relies on.
- Upstairs fixture slow, basement fine: the clog is above the basement, in a trap, branch or the stack.
- Basement fixtures back up when upstairs water runs: the clog is in the building drain or lateral, downstream of where the stack drops in.
- Only basement fixtures drain slowly, upstairs fine: the clog may be in a basement branch, or the building drain may be partly blocked and the upstairs flow is still getting past.
04 /What does each result mean?
| Result | Symptom pattern | Auger class and cable | Hand it off when |
|---|---|---|---|
| Fixture trap or trap arm | One sink, tub or shower slow; everything else normal | Hand drum auger, 1/4-in cable, 15 to 25 ft | Cable will not pass the trap arm, or the fixture has a chemical cleaner sitting in it |
| Toilet trapway or closet bend | One toilet slow or overflowing; other fixtures normal | Closet auger, 3 or 6 ft, with a rubber or vinyl bowl guard | Object is hard and stuck (toy, brush head); the toilet needs to come off |
| Branch drain | Two or more fixtures in one room or on one wall slow together | Drum machine or heavy hand drum, 5/16 to 3/8-in cable, 25 to 50 ft | No access point short of pulling a fixture; the branch is cast iron with heavy scale |
| Stack | Several fixtures on upper floors slow; basement fine | Drum machine, 3/8 to 1/2-in cable, often from a roof vent or stack cleanout | Access is from the roof, or the stack is cast iron and cracked |
| Building drain or lateral | Floor drain or basement toilet backs up when upstairs water runs | Drum machine with 1/2-in cable or sectional machine with 5/8 or 7/8-in cable, 75 to 100+ ft | Any backup into the basement, repeat clogs, roots suspected, no usable cleanout |
Each result is a starting point, not a verdict. If the locator says fixture trap but the hand auger passes the trap and hits a hard stop 8 metres out, you have learned the clog is further down than the symptoms suggested, and you should step up to the next result rather than forcing a small cable.
For the technique behind each result, see how to snake a sink, how to snake a bathtub or shower drain, how to auger a floor drain, how to auger a toilet and clearing a main line from the cleanout. For the sizing logic, see the auger cable size chart; once you know the line, the Auger Cable Sizer turns it into a cable, a starting head and a machine class.
05 /Why does 'intermittent' change the answer?
An intermittent clog, one that drains fine for days and then backs up, is almost never a single wad of hair or paper. It is usually a partial obstruction that catches debris until it closes and then lets go: tree roots at a joint, a ring of grease, heavy scale inside cast iron, an offset joint, or a sag (belly) where water and solids sit. The locator treats intermittent symptoms on a main-line pattern as a strong signal to add a camera inspection.
An auger can punch a hole through all of those, and the drain will run again, but the cause is still there. That is why the tool's intermittent answer steers you toward when augering is not enough and why the clog came back. If roots are the suspect, the sister site SewerRoots.ca goes deep on root intrusion; if you want to see inside the pipe, SewerInspector.ca explains camera inspections frame by frame.
- Intermittent and worse after heavy rain or snowmelt: could be groundwater entering a cracked lateral, or a surcharged municipal main. Augering will not help the second one; call the municipality (in Calgary, 311).
- Intermittent and worse after laundry day: lint and soap scum in the laundry branch or floor drain line, or a building drain that is nearly closed.
- Intermittent and worse after holiday cooking: grease in the kitchen branch. An auger opens it; jetting cleans it.
06 /What are the Clog Locator's limits?
The locator assumes a conventional gravity drain layout. It is less reliable in these situations, and you should weigh its answer accordingly:
- Houses with a sewage ejector or sump-style pump for basement fixtures. A failed pump looks like a clog but is a mechanical problem.
- Duplexes, row houses and condos where your building drain is shared, or where a neighbour's blockage can show up in your unit.
- Houses with a mainline backwater valve. A stuck or debris-fouled flap produces a building-drain pattern even when the pipe is clear. Check the valve first; see how to auger a floor drain for the cautions.
- Vent problems. A blocked plumbing vent causes slow drains and gurgling with no clog in the drain at all. Augering the drain does nothing; see snaking from the roof vent.
- Frozen lines. In a Canadian cold snap, a lateral that is shallow or passes under an unheated space can freeze. Do not auger ice with a steel cable; thawing is a different job.
- Septic systems. A full tank or failed field produces main-line symptoms that no auger can fix.
07 /How should you use the result?
- Stop the water
If anything is backing up, stop using fixtures before you start. You will diagnose faster with a stable water level.
- Run the locator
Tick only current symptoms and choose intermittent only if the problem has cleared and returned at least once.
- Check the access point
Find the cleanout or fixture opening that sits just upstream of the predicted clog. The right entry point matters as much as the right cable.
- Match the cable to the pipe
Use the size the result gives, and check it against the auger cable size chart. Undersized cable kinks; oversized cable will not turn the bends.
- Respect the stop line
If the result says hand it off, or if the cable stops twice at the same distance, stop. Read drain auger safety before you run any powered machine.
If the answer points to the building drain or lateral, or the clog keeps returning, the practical next step is a quote that includes augering plus a camera look. You can request quotes from drain professionals and compare what each includes; the drain augering cost guide shows typical Canadian planning ranges.
FAQQuestions people ask
Can a slow sink mean my main sewer line is clogged?
Rarely on its own. If only one sink is slow and every other fixture drains normally, the clog is almost always in that sink's trap or trap arm. A main line clog affects the lowest fixtures in the house first, usually the basement floor drain or basement toilet.
Why does my floor drain gurgle when I flush the toilet upstairs?
Air and water are being pushed back through the lowest opening because the building drain or lateral is partly blocked downstream of where the toilet's stack joins it. Treat it as an early main line warning and stop heavy water use until it is checked.
Is the Clog Locator accurate for condos?
Only for fixtures inside your unit. In a condo the branch and stack are usually shared, so a pattern that looks like a stack clog may be a building problem your condo corporation's plumber should handle.
What if the locator says fixture trap but the auger does not clear it?
Note how far the cable went before it stopped. If it passed the trap and stopped further along, the clog is in the branch or beyond, so step up to the next result and a heavier cable rather than forcing the small one.
Can a blocked vent cause the same symptoms as a clog?
Yes. A blocked vent causes slow drainage, gurgling and siphoned traps with a clear drain. If augering finds nothing and the fixture still drains slowly, have the vent checked.
Does intermittent mean I need a camera inspection?
On a main line pattern, usually yes. Intermittent main line clogs are typically caused by roots, grease, scale, offsets or a sag, and a camera is the only way to tell which before you spend money on the wrong fix.
NETWORKGo deeper across the HAUSE Sewer network
Signs of roots in your sewer line
Signs of roots in your sewer line: gurgling, slow floor drains, seasonal backups and what a camera shows. Ten symptoms ranked, plus how to confirm roots fast.
SewerCleaners.caIs my main sewer line clogged? Symptoms, causes and a simple test
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