Your sink is backing up into the other sink **because of a clog or blockage** within your drain pipes. This is the primary reason water from one side of a double sink system can't drain properly and instead rises or overflows into the other side.
## Understanding Sink Clogs
A clog or blockage occurs when materials accumulate within the drain line, obstructing the flow of water. According to the reference provided, common culprits for these blockages include:
* **Food accumulation:** Scraps of food, particularly those not properly disposed of.
* **Fat:** Grease and oils poured down the drain cool and solidify, clinging to pipe walls.
* **Hair:** Especially common in bathroom sinks, but can also find its way into kitchen drains.
* **Other objects:** Small items accidentally dropped down the drain.
When these materials build up, they create a barrier that water cannot pass through easily.
## How a Clog Causes Backup in Double Sinks
Double sinks are typically connected to a single drain pipe further down the line. Imagine an inverted 'Y' shape, where the two sink drains join together before heading towards the main plumbing system.
Sink 1 Drain --> \
-- Shared Drain Pipe --> Main Plumbing
Sink 2 Drain --> /
If a blockage occurs in this shared drain pipe (somewhere after where the two sink drains meet), water attempting to drain from either sink will encounter the obstruction. Since the water has nowhere to go downstream, it will rise back up the pipe. In a double sink setup, this often means water from one side will back up and appear in the other sink basin because they share the same restricted pathway.
Essentially, the clog acts like a dam in the shared section of the pipe. Water fills up behind the dam, and because the two sink drains connect above the dam, the water level rises simultaneously in both drain lines, causing the backup into the sink basins.
Addressing the Issue
To resolve the backup, the clog or blockage needs to be located and cleared from the shared drain pipe. Common methods involve using plungers, drain snakes, or sometimes professional plumbing services for more stubborn blockages.
In summary, the reason your double sink is backing up into the other basin is a restriction in the shared drain pipe, most often caused by an accumulation of food, fat, hair, or other debris.
