Quick Answer:
If a musty or stale smell appears only at upstairs sinks, tubs, or toilets, it’s usually trapped air, a dry P-trap, or an unused branch of pipe rather than the main vertical stack going bad. Start by comparing how the cold taps upstairs and downstairs behave, check traps and vents, and clear or run water through any rarely used fixtures before calling a plumber.
Why This Happens
Homes have runs of pipe and many small branches. If a sink or shower isn’t used often, its trap can dry slightly or drain slowly, letting sewer air or stagnant water odors escape. Upstairs branches can be less used than main floor fixtures and can also trap warm, moist air that smells musty.
Temporary events can make smells show up in odd places — for example, pressure changes after a city water interruption can stir odors from low-flow branches (see Musty odor after city outage for more on that). Repairs or changes to plumbing can also leave residues or disturb trapped air; people often see a similar smell after work is done (see Moldy smell after plumbing repair).
Step-by-Step What to Do
1. Note where and when you smell it
- Which fixtures smell: sink, shower, toilet, laundry tub?
- Is it constant or only when water runs?
- Does it happen after heavy use, after sitting unused, or at certain times of day?
2. Compare upstairs and downstairs cold taps
- Turn on a cold tap at an upstairs sink and an equivalent tap downstairs. Notice water clarity, smell at the faucet, and whether the upstairs water smells stronger.
- If upstairs cold water smells but downstairs does not, that points to a local branch, trap issue, or unused line rather than the main supply or stack.
3. Run water and refill traps
- Run each upstairs cold tap for 30–60 seconds, then run tubs and showers long enough to refill the P-trap (few minutes if rarely used).
- Flush toilets on that floor a few times. This refills traps and often clears trapped odors.
4. Clean drain strainers and pop-ups
- Remove and clean hair or debris in sink strainers and shower drains. Organic buildup can cause musty smells.
- Use a cup of baking soda followed by vinegar and hot water only if drains are slow; otherwise, flushing with hot water is safer.
5. Check venting and roof vents visually
- From the yard or with binoculars, look at roof vents for blockages like leaves, bird nests, or animal activity. Blocked vents can slow drain flow and cause odors to migrate into branches.
- If safe, have someone inside run a fixture while you observe vent or roof behavior from outside—don’t climb on the roof yourself if you’re not comfortable.
6. Inspect for low-use lines or recent changes
- Identify fixtures rarely used (guest bathrooms, floor drains). Occasional use lets traps evaporate and smells appear.
- Note any recent plumbing work upstairs; disturbed traps or lines can cause temporary odors.
What Not to Do
- Do not open walls before isolating branches. Cutting into walls or ceilings can make the problem worse and create mold or structural damage if you haven’t confirmed the source.
- Do not pour large amounts of harsh chemicals down the drains; they can damage pipes and harm septic systems.
- Do not assume the vertical stack is “stagnating” and needs replacement without basic checks — most issues are local to a branch or trap.
When to Call a Professional
- Persistent smell after you’ve refilled traps, cleaned drains, and compared upstairs/downstairs taps.
- Visible sewer gas symptoms (sulfury rotten-egg smell), repeated drain backups, or multiple fixtures affected.
- Suspected vent blockage high on the roof, or if safe inspection isn’t possible from the ground.
Safety Notes
- Work in well-ventilated areas. Open windows to clear odors while you check fixtures.
- Wear gloves when cleaning strainers or removing debris from drains. Avoid skin contact with standing water in traps.
- Do not use open flames or test with a candle to locate sewer gas. That is dangerous. Use scent and careful observation only.
Common Homeowner Questions
- Why does the smell only happen upstairs? Short answer: quieter, less-used branches and traps upstairs are more likely to dry or trap stale air than busy downstairs fixtures.
- Will running water fix it? Often yes—running cold taps, showers, and flushing toilets refills traps and clears trapped air that carries the odor.
- Could this be dangerous? Most musty smells are nuisance odors. If you smell a strong rotten-egg odor or feel sick, leave the area and call a professional immediately.
For more related articles, see the Musty, Earthy, or Moldy Water Smell hub.