CSLB C-36 Plumbing ContractorService, Repair, and MaintenanceMedium
A client reports that their basement floor drain is backing up with sudsy water whenever the washing machine discharges, but the toilets and other drains in the house function normally. What is the most likely cause?
- AA broken or collapsed sewer line under the foundation.
- BA blockage in the main sewer line downstream of all fixtures.
- CA partially clogged branch line specific to the laundry drain.
- DA clogged plumbing vent for the basement fixtures.
Show answer & explanationAnswer & explanation
Correct answer: C. A partially clogged branch line specific to the laundry drain.
If only the basement floor drain backs up when the washing machine discharges, but other fixtures are fine, it indicates a localized blockage. The most likely culprit is a partial clog in the branch line serving the laundry area, which is too small to handle the washing machine's high-volume discharge. The sudsy water confirms it's laundry waste.
Why the other options are wrong
- A. A broken or collapsed sewer line would likely cause more severe and widespread issues, potentially affecting all drains or causing persistent slow draining, not just specific to laundry discharge.
- B. A blockage in the main sewer line would cause ALL drains (toilets, showers, etc.) to back up, not just the basement floor drain with washing machine discharge.
- D. A clogged plumbing vent primarily causes slow drains or gurgling, not typically a backup of sudsy water from a specific appliance into a floor drain, especially if other fixtures drain normally.
Laundry Drain Backup
A plumbing issue where a basement floor drain or laundry standpipe backs up, usually with sudsy water, when the washing machine discharges.
- Triggered by high volume discharge (washing machine)
- Often sudsy water
- Other drains may function normally
- Indicates localized blockage in laundry branch line
Memory trick: Washer's sudsy flood? Branch line's the snag, understood!