A slow drain almost always means there’s a partial blockage somewhere in the pipe, not a broken fixture. The water still moves, just not fast enough. Here’s what’s actually going on, by fixture type, and how to tell whether a quick surface check will fix it or you need a licensed plumber.
What Builds Up Where
The blockage material depends almost entirely on which drain is slow.
Bathroom sink and tub drains fill up with hair, mostly. Hair tangles around the stopper or the pivot rod on a pop-up drain assembly, then catches soap scum and toothpaste residue. After a few months you get a dense, greasy mat sitting just below the drain cover. You can often see it with a flashlight. It smells bad when you pull it out.
Shower drains are the worst for hair. There’s no stopper mechanism to collect it neatly, so it spreads down the drain arm. In a household with multiple people, a shower drain can pack surprisingly fast.
Kitchen sinks are a grease problem. Cooking fat goes down liquid and coats the inside of the pipe, then cools and solidifies. Dish soap alone doesn’t reliably dissolve solidified grease in drain pipes the way people think. It needs hot water to work at all. Over time the coating thickens and the effective pipe diameter shrinks. Food particles (coffee grounds, pasta starch, rice) stick to the grease layer and accelerate it. A slow kitchen drain that smells like old food is almost always grease buildup.
Bathroom sink, specifically slow after years of use: soap scum. Most bar soaps contain tallow or other fats that leave a residue on the pipe walls, similar to kitchen grease but usually building up more uniformly.
How Far Down Is the Blockage?
This matters for understanding what you’re dealing with.
If only one fixture is slow, the blockage is local, usually within a few feet of the drain opening.
If two fixtures on the same wall are slow, or if a slow drain is accompanied by gurgling from another drain (toilet bubbling when you run the sink), the blockage is further down in a shared branch line.
If multiple fixtures throughout the house are slow, or if sewage is backing up into low-lying drains like a floor drain or tub, that’s a main line issue. This needs a licensed plumber.
How a Licensed Plumber Diagnoses It
The first step is usually a visual inspection of the drain opening and accessible trap. If there’s obvious material in the first few inches, that gets removed manually.
For persistent or deeper clogs, a drain cable (snake) tells a lot by feel: soft material (grease, hair) gives differently than a hard object or pipe scale. If the cable goes in easily but water still drains slow, the problem may be partial and further down, or it may be a venting issue. A blocked vent stack creates the same slow-drain symptom by preventing air from entering behind the water.
Camera inspection gets used when the cause isn’t clear, when clogs recur quickly after clearing, or when there’s suspicion of root intrusion, pipe offset, or corrosion scale. That’s more common on older cast iron or clay sewer lines. The scope lets the plumber see exactly what’s there before deciding on a clearing method.
Quick Checks Before You Call
A few things are safe to check yourself.
Pull visible hair out. For bathroom sink and shower drains, remove the drain cover or stopper and pull out whatever’s there. A cheap plastic barbed drain tool from any hardware store handles this. It’s unpleasant but usually effective for surface clogs.
Plunge a flat sink or tub. A cup plunger works on flat surfaces. Cover the overflow hole first (the small hole near the top of the sink or tub) with a wet rag so you’re building pressure in the drain, not venting it sideways. Steady, firm strokes. This works for soft, near-surface clogs.
Hot water for a slow kitchen sink. If there’s no complete blockage, slowly pouring very hot tap water can soften grease buildup enough to flush it along. Useful as a maintenance habit, though it won’t fix an established buildup.
Skip the chemical drain cleaners. They’re hard on pipe fittings and joints, dangerous to skin and eyes, and if the drain is fully blocked they just sit in the trap as a hazard for whoever opens the pipe next. Repeated use can damage PVC and accelerate corrosion in older metal pipes.
When to Call a Licensed Plumber
If the basic checks above don’t clear things up, it’s time to call a pro. Specifically:
- The drain is completely blocked (standing water that won’t move)
- Multiple fixtures are slow at the same time
- You hear gurgling from other drains when you run water
- The clog came back within a few weeks of clearing it yourself
- There’s any sign of sewage smell from a floor drain or outdoor cleanout
- The house has older galvanized or cast iron drain pipes (internal corrosion scale looks like a clog but won’t clear with a snake, and forcing one through severely corroded pipe can cause more damage)
In California, verify your plumber’s license at cslb.ca.gov before any work starts. A licensed contractor will have a current, searchable record on file. The diagnostic conversation costs nothing; the guesswork does.