Kitchen Sink Draining Slowly

What's happening?

WaterSafe lists grease as the most common cause of a blocked kitchen sink: cooking oil and fat harden as they cool and line the pipe. Soap scum, which forms when soap reacts with the minerals in the water, and food scraps such as coffee grounds, pasta and rice add to it. The result is a pipe that has narrowed, so it drains slowly long before it blocks. A trap that is clean but still slow suggests the blockage is beyond it in the waste pipe or, if more than one fitting is slow, in the drain. WaterSafe says that if several fixtures are slow to drain or gurgle, the blockage is likely to be outside.

What usually causes it?

• Grease and fat set hard in the trap and waste pipe
• Soap scum and food scraps building up with the grease
• A blockage further along the waste pipe, or in the outside drain, when the trap is clean

Is it dangerous?

No. A slow sink is a nuisance, and standing water can smell and breed bacteria. WaterSafe warns that boiling water can damage PVC pipes, so use very hot water rather than boiling.

Can I fix it myself?

Scoop out standing water, pull out any visible debris with gloves on, and pour very hot but not boiling water down to loosen grease. WaterSafe says a plunger can help force a blockage out. Put a bowl under the trap and unscrew it to clean it. If the trap is clear and the sink is still slow, do not keep trying: the blockage is further along. Stop pouring fat down the sink and use a strainer.

When should you call a drainage engineer?

Call a drainage engineer when the trap is clear and the sink is still slow, when more than one fitting is slow or gurgling, or when the slow drainage returns within days.

What will the drainage engineer actually do?

• Check whether it is the sink waste or the main drain by testing the other fittings and the outside chamber
• Clear the waste pipe with a drain machine or jetting, and clean the trap
• Camera the line if the blockage returns, to check for a sag, a collapse or roots
• Advise on keeping grease out of the system

Who fixes this

• Blocked toilet and sink
• Blocked drain clearing

What it usually costs

Drain Unblocking Cost

Priced on survey

Questions people ask

Sources & References

1. WaterSafe: why is my sink not draining
2. WaterSafe: how to unblock a sink with standing water
3. WaterSafe: how to tell if your outside drain is blocked