CSLB C-36 Plumbing ContractorService, Repair, and MaintenanceHard
A plumber is inspecting a residential sewer cleanout and observes that wastewater containing solids is backing up into the cleanout when the toilets are flushed. However, the kitchen sink and laundry drains are still flowing slowly but not backing up at the same rate. What does this pattern of backup most likely indicate?
- AA complete blockage in the main sewer line upstream from the cleanout.
- BA localized blockage in the toilet's P-trap.
- CA vent stack blockage affecting only the bathroom fixtures.
- DA partial blockage in the main sewer line downstream from the toilet and laundry connections.
Show answer & explanationAnswer & explanation
Correct answer: D. A partial blockage in the main sewer line downstream from the toilet and laundry connections.
If wastewater backs up at the cleanout primarily when toilets are flushed (which discharge larger volumes of solids and water) but other drains are only slowing, it suggests a partial blockage in the main sewer line *downstream* from where the toilet and laundry lines connect. The larger volume from the toilet overwhelms the partially obstructed pipe, causing a backup, while smaller, slower flows from other fixtures can still pass, albeit slowly.
Why the other options are wrong
- A. A complete blockage in the main sewer line upstream from the cleanout would mean no wastewater could pass that point, leading to backups in *all* fixtures above that point, not just preferential backups from toilets.
- B. A localized blockage in a toilet's P-trap would only affect that specific toilet, not cause a backup at the main cleanout.
- C. A vent stack blockage would typically cause gurgling and slow drainage due to siphoning, not a backup of solids and wastewater at the main cleanout, which indicates a physical obstruction in the drain line itself.
Main Sewer Line Partial Blockage
An obstruction in the main drainpipe that allows some wastewater to pass but causes backups with larger discharges.
- Symptoms worsen with high-volume fixtures (toilets, tubs).
- Lower-volume fixtures (sinks) may drain slowly but not back up.
- Often due to tree roots, grease, or foreign objects.
- Requires snaking or hydro-jetting of the main line.
Memory trick: Big flush, big rush, meets a partial hush.