Most drain clogs come from one of six things: grease, hair and soap scum, foreign objects, mineral scale, tree roots, or a structural defect in the pipe itself. Which one you have is usually obvious from a single observation — how many fixtures are affected.
One slow fixture means the problem is in that fixture’s branch line. Several slow at once means it is in the main line that serves all of them. That one distinction narrows six causes down to two or three before anyone opens a toolbox, and it is the most useful thing a homeowner can know.
Start here: what is actually slow?
| What you are seeing | Where the problem is | Likely cause |
|---|---|---|
| One sink or tub draining slowly | That fixture’s branch line or trap | Hair, soap scum, grease, or an object |
| Kitchen sink only, recurring | Kitchen branch line | Grease, almost always |
| Several fixtures slow at once | Main line | Roots, scale, wipes, or a structural defect |
| Toilet gurgles when the shower runs | Main line, partially blocked | Roots or accumulated buildup |
| Sewage backing up into a tub or floor drain | Main line, blocked | Stop using water and call now |
| Same fixture clogs every few weeks | That branch, structurally | A belly, offset joint, or scale — not a habit problem |
1. Grease, oil and fat
The leading cause of kitchen drain failure, and the one most homeowners believe they have avoided.
Cooking fats go down warm and liquid. Ten feet into a cold pipe under a Washington house they are neither. They congeal onto the pipe wall, and every subsequent wash adds a layer. The line does not clog suddenly — it narrows over months until one ordinary dinner’s worth of water will not fit through.
Running hot water afterward does not solve this. It moves the grease further down the line before it cools, which relocates the problem to a spot that is harder to reach. See how to keep grease out of your lines.
How it is cleared: cabling bores a channel through grease and leaves the coating on the wall, which is why snaked kitchen lines re-clog quickly. Hydro jetting scours the wall back to pipe.
2. Hair and soap scum
The bathroom equivalent, and a genuine chemical process rather than just accumulation.
Traditional bar soaps contain fatty acids. Where water is hard, those acids react with dissolved calcium and magnesium to form an insoluble residue — soap scum — that bonds to pipe walls and to itself. Hair catches in it, and the two build a mat that water passes through until it does not.
This is why a shower drain that has been fine for years starts slowing and then slows faster: the deposit grows on its own surface.
How it is cleared: often a simple mechanical clearing at the trap. If it recurs across multiple bathrooms, the issue is water hardness rather than habits, and a whole-house filtration or softening system addresses the cause.
3. Things that should not have gone down
Foreign objects cause the abrupt clogs — fine yesterday, blocked today.
- “Flushable” wipes. They are not. They do not break down like toilet paper, and they are among the most common causes of both household and municipal blockages. The label is marketing.
- Feminine hygiene products, cotton swabs, dental floss. Floss is disproportionately bad — it tangles around anything else in the line and turns loose debris into a net.
- Paper towels and tissues. Engineered to hold together when wet, which is the opposite of what a drain needs.
- Food solids and coffee grounds. Grounds do not dissolve. They settle in low spots and compact.
- Children’s items. A predictable share of our toilet calls.
Clogged toilet repair covers the fixture side of this.
4. Mineral scale
Where water is hard, dissolved minerals deposit on the inside of pipe over years, narrowing the bore. In aging cast iron this combines with internal corrosion, and the two together can reduce a 4-inch line meaningfully.
Scale rarely causes a sudden blockage on its own. What it does is remove your margin — the line still works, but it has no capacity to absorb a bad day, so ordinary grease or a handful of debris backs it up.
Severe scaling may point past cleaning toward replacement — why cast iron laterals get replaced.
5. Tree roots
The dominant main-line cause in this region, and a structural problem wearing a clog costume.
Roots grow toward moisture. A hairline crack or a slightly separated joint releases enough vapor into the soil that roots find it, enter, and thicken into a mass that catches everything moving past. Puget Sound is close to a worst case: mature tree cover, a long wet season, and a large stock of homes on clay and cast iron laterals.
The important part: clearing roots does not fix why they got in. Jetting or cutting removes the mass and buys an interval. The crack stays. If a line has been root-cleared repeatedly, the honest conversation is about repair, not a fourth cleaning. See how jetting handles root intrusion.
6. Structural defects in the pipe
The cause nobody suspects, and the reason some drains clog on a schedule no matter what goes down them.
- A belly — a section settled below grade where water and solids pool instead of flowing through
- Offset joints — ground movement has shifted two sections out of line, leaving a lip that catches debris
- Cracks and fractures — rough edges that snag, plus an entry point for roots and soil
- Partial collapse — usually terminal for that section
If a fixture clogs repeatedly despite good habits and repeated clearing, stop paying to clear it and find out why. A camera inspection settles it in under an hour, and every item on this list is visible on video.
When to book a cleaning even though nothing is wrong
These are occasions rather than causes — the original version of this article mixed the two together.
- Buying a house. A standard home inspection does not include the sewer lateral, and that line becomes your problem at closing. Detail in drain and sewer work on a newly purchased home.
- Before hosting. Holiday cooking and a full house is when a marginal kitchen line finally gives out.
- After any backup. Clearing restored flow. It did not tell you what caused it.
- On an older home you just inherited or moved into. Establish a baseline while nothing is wrong.
- Annually, if you have mature trees near the lateral. [CONFIRM — if you offer a maintenance plan, this is where to link it. Your /maintenance-plan/ page currently has nothing pointing at it.]
Frequently asked questions
How do I know if it is one drain or the main line?
Run water in a fixture on the other side of the house. If that one is fine, the problem is local to the first fixture. If both are slow, or a toilet gurgles when a shower runs, you are looking at the main line.
Do chemical drain cleaners work?
Occasionally on a minor, close-to-the-trap clog. They are hard on older pipe, frequently never reach a main-line blockage, and create a hazard for whoever opens the line next. Tell your technician if you have used one.
How often should drains be professionally cleaned?
Annually is reasonable for most homes. More often with heavy kitchen use, older plumbing, or mature trees near the sewer lateral. Less often for a newer home with no symptoms.
Why does the same drain keep clogging?
Because the clearing addressed the symptom. Recurring clogs in one fixture usually mean a belly, an offset, roots, or scale — something structural that catches debris every time. That is a camera question, not a cabling question.
Is snaking or jetting right for my line?
A cable is often the right, cheaper answer for a single solid obstruction. Jetting is for a coating on the pipe wall — grease, scale, root hair. A good technician carries both. More on snaking and on the methods generally.
Can I prevent clogs entirely?
You can prevent most of the habit-driven ones — strainers in every drain, no grease down the sink, nothing but toilet paper and waste flushed. You cannot prevent roots or a settling pipe. Prevention in more detail.
My drains smell but nothing is slow. What is that?
Usually a dry trap in a fixture nobody uses, or buildup in a line that is still draining. Occasionally a venting problem or a crack releasing sewer gas. If running water in the unused fixture does not fix it within a day, it is worth looking at.
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