Multiple drains back up only upstairs

Quick Answer:

If several fixtures on the upper floor back up (for example, the shower and a sink) when you run water but lower-floor drains stay normal, the likely causes are a blocked upstairs branch or a venting problem. Do basic checks first: test other fixtures on the same floor, inspect P‑traps, do a controlled bucket test in the shower to watch toilet levels, and—if it’s safe—check roof vent terminals for obvious blockages. If the problem affects many upstairs fixtures or roof access is unsafe, call a pro.

Why This Happens

Two common things cause upstairs-only backups:

A partial clog in the branch drain that serves the upstairs fixtures. A partial clog can let some water pass but causes backup when several fixtures are used together.
A blocked or restricted vent stack. If the vent is plugged, drains can’t breathe and water can push back into the nearest fixture when flow increases.

Less commonly, a main sewer problem or a vent blocked lower down can produce similar symptoms, but you’ll usually see lower-floor fixtures affected too. If the toilet and sink on the same floor back up together, that points to a shared branch rather than an isolated trap.

Step-by-Step What to Do

1. Identify which fixtures show the issue

Run water in each upper-floor fixture one at a time and in combinations. Note whether toilets, sinks, and the shower behave the same way. This test helps narrow whether it’s a single fitting, a branch, or the vent system. If you observe Sink and toilet backing up together, that suggests they share a blocked branch or a common vent.

2. Compare upstairs to downstairs

Check a few downstairs fixtures while you run upstairs fixtures. If downstairs drains stay normal while several upstairs fixtures show problems, the issue is likely on the upstairs branch or its vent.

3. Remove and inspect P‑traps

For affected sinks and floor drains you can access, turn off the water, put a bucket under the trap, and remove the P‑trap to look for hair, soap buildup, or foreign objects. Clean the trap and test the drain before reassembling. Do not force plastic nuts; if they’re brittle or corroded, take care to avoid breakage.

4. Run a controlled bucket test in the shower

Pour 1–2 gallons of water slowly into the upstairs shower drain while someone watches the upstairs toilet bowl level. If the toilet water rises as you add water to the shower, that indicates the fixtures share a restricted path or a vent problem. Don’t dump large volumes at once—pour slowly and observe.

5. Check accessible vent terminals on the roof

From ground level or a safe roof spot, look at the vent terminals for bird nests, leaves, or debris. Do not climb if the roof is steep, wet, or you’re uncomfortable—this is a job to hand off. If you can reach the vent safely, remove lightweight debris by hand or with a tool; do not push objects down the pipe.

6. Try simple in-place clearing (if comfortable)

Use a hand-held drain auger (plumber’s snake) from the fixture itself, not from far below. Feed it carefully into the drain and pull back any pulled debris. Clean the trap first so the snake can reach the branch more easily.
If you don’t get results, stop and reassess rather than forcing longer tools into the system.

7. Record what you’ve tried and call for help if needed

Note which fixtures were affected, the results of the bucket test, and whether clearing traps helped. This information will speed any professional diagnosis.

What Not to Do

Avoid using a long power snake from the basement to reach an upstairs branch—this may damage fittings and joints. Reaching a branch from the wrong access point can shear connections or push debris into the main line.
Do not climb onto the roof if it is steep, wet, or icy. If you can’t safely access the vent terminals from a stable ladder or you’re unsure, call a pro rather than risking a fall.
Don’t pour large volumes of chemical drain cleaner into upstairs fixtures to clear suspected branch blockages—these chemicals can damage pipes and create hazardous conditions for anyone who must work on the drain later.
If multiple upstairs fixtures are clogging while downstairs remain clear, don’t attempt complex main-line work yourself—this is a sign to call a plumber.

When to Call a Professional

The bucket test causes toilet levels to rise or toilets back up when you pour water into other upstairs fixtures.
Several upstairs fixtures are blocked or slow while downstairs fixtures are fine—this often needs tools and camera inspection a homeowner won’t have.
Roof access is unsafe or you find a vent blockage you can’t remove from a safe position.
After trap cleaning and basic snaking you still have backups, especially when multiple fixtures are involved.

If the symptoms are intermittent rather than constant, or seem to come and go with weather or water use, consider that vents or partial blockages may be the cause; this situation can be harder to diagnose and may require professional camera inspection. See also Multiple drains back up intermittently for that scenario.

Safety Notes

Never climb a ladder or walk on a roof unless you are trained, equipped, and it’s safe. Falls cause serious injury.
Wear gloves and eye protection when opening traps or handling drain debris. Have a bucket and towels ready to control spills.
Turn off water to the fixture if you need to disassemble fittings. For toilets, shut off the fill valve at the tank if water levels rise dangerously during testing.
Avoid mixing chemicals and mechanical clearing methods; chemical residue can be hazardous to anyone working on the pipes afterward.

Common Homeowner Questions

Q: Could a roof vent blockage cause only upstairs fixtures to back up?
A: Yes. A blocked vent can prevent air from entering the drain system, causing backups or slow draining on that floor.
Q: Is it OK to run a snake from the basement to clear an upstairs branch?
A: No. Running a long powered snake from the basement can damage fittings; clear from the affected floor when possible or call a pro.
Q: If cleaning P‑traps fixes the problem temporarily, will it come back?
A: It can. Trapped debris like hair is a common recurring cause. If it returns, a deeper branch or vent issue may need professional inspection.