Forcing a large object down your bathroom sink is likely to clog it, so most people avoid doing this. Most sink clogs, however, aren't caused by a single large obstruction but by an accumulation of smaller ones. Common culprits are hair, toothpaste and soap, which, when combined into a mass over time, can completely stop water from flowing. Clogs may also be the result of insufficient drain slope and inadequate venting.
Hair is one of the most pervasive causes of drain clogs. One or two strands may catch on the sink strainer or in one of the bends of the P-shaped trap under the sink and collect other small particles flowing past it, in time forming a gooey mass that blocks the drain. Hair isn't easy to clear out with a plunger or snake because it is usually caught in the piping. You can dissolve it with chemicals, but often the best way to clear it is to disassemble the drain and remove it manually.
Soap and Toothpaste
Although they are water-soluble, soap and toothpaste can still collect inside the drain to form a clog. Either substance will probably dissolve over time and the drain will eventually clear, but if the drain is stopped completely, you may have to plunge it or use a snake to move them along. When a drain is partially clogged by soap or toothpaste, water flows more slowly, making it easier for other contaminants to settle out and add to the mess. The clog grows quickly if you're in the habit of putting water-insoluble substances like nail polish down your drain.
Pipe Slope and Size
If the bathroom drain pipes are sized incorrectly or installed with insufficient slope, they may inhibit the water flow. The effect may not be noticeable at first, but eventually enough sediment will settle out and collect in the bottom of the P-trap to block it. Counter-intuitively, this may happen if the pipes are too large. When the pipes are sized correctly, water flowing through them creates a vacuum that clears sediment out of the P-trap. When they are too large, the water doesn't seal them completely. The water also doesn't create suction if the pipes are sloped too gradually to get the water moving fast enough.
Plumbing codes require drain lines to be vented to allow air in to fill the vacuum created by flowing water. If the vents are blocked or nonexistent, the vacuum may be strong enough to impede the flow and allow sediment to collect in the P-traps or elsewhere. An incorrectly placed vent can also cause clogs. If it is too near the exit from the P-trap, it cancels the suction force created by the water as it flows out and promotes the collection of sediment in the bottom of the trap.
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