Quick Answer:
If the tub spout is weak while sink faucets are fine, the problem is usually local to the tub spout or the diverter path. Start by confirming the low flow is only at the spout, then remove the spout if it’s the removable type to check for debris or a loose washer. Also test shower mode to see whether the water takes a different route or if the issue affects both outlets.
Why This Happens
There are a few common causes when only the tub spout has low flow:
- Mineral buildup or trapped debris at the spout opening or inside the spout body.
- A loose or damaged washer or screen inside the spout that limits flow.
- A partially stuck diverter valve (the part that switches water to the shower) or blockage in the tub-side plumbing path.
- Valve cartridges or internal stops that are not fully open or are partially clogged.
Problems like this are often limited to the tub branch, which is why sinks can still have normal pressure. If you see sudden or broader drops, consider reading about Shower pressure suddenly dropped or Shower low pressure after plumbing repair for related scenarios.
Step-by-Step What to Do
1. Confirm the scope
Run water from:
- The tub spout alone.
- The shower mode (if using a diverter).
- Nearby sink and other fixtures on the same supply lines.
This shows whether the weak flow is isolated to the spout, the diverter path, or a larger supply issue.
2. Remove the tub spout (if it’s the removable type)
Most slip-on or threaded spouts can be removed:
- Turn off the water to the tub if you can, or be ready to catch water.
- Look for a set screw under the spout—loosen it and pull the spout straight off, or unscrew the spout if threaded.
Inspect the inside for mineral deposits, debris, or a loose washer. Clean the openings with a cloth and mild vinegar if there is scale. Replace a crumbled washer or O-ring if you find one.
3. Inspect the diverter and compare shower mode
With the spout off, operate the tub valve to switch to shower mode:
- If shower flow is normal but the spout path is weak, the blockage is likely in the spout or its immediate connection.
- If both are weak, the issue is upstream—diverter, cartridge, or a supply valve.
If you see a piece of debris come out when the spout is removed, clean the pipe stub and reinstall the spout.
4. Reinstall and test
After cleaning or replacing small parts, slide or screw the spout back on and tighten the set screw (do not overtighten). Restore water and test tub fill and shower mode again.
5. Try simple valve checks
If removing the spout didn’t help and shower mode is also weak, try:
- Fully opening the supply stops for the tub if accessible.
- Operating the faucet handle through its full range to ensure internal cartridges or stops are fully open.
If those don’t restore flow, the issue is likely deeper in the valve or plumbing branch and may need a professional.
What Not to Do
- Don’t try to “boost” tub fill by removing parts from the spout or diverter incorrectly—this can damage seals or let debris into the system.
- Don’t force a stuck spout or diverter with excessive clamping or hammering; that can break fittings or crack tiles.
- If the spout is corroded, seized, or the diverter is stuck, don’t attempt aggressive on-site repairs—hiring a plumber is the safer choice.
When to Call a Professional
Call a plumber if:
- You can’t remove the spout without risking damage.
- Cleaning the spout doesn’t restore normal flow and the shower mode is also weak.
- You find corrosion, a broken diverter, or evidence of leaking inside the wall or under the tub.
- The job requires replacing a cartridge, diverter valve, or cutting into plumbing where soldering or complex connections are needed.
A pro can diagnose blockages inside the valve body and replace worn parts safely.
Safety Notes
- Shut off water at the fixture shutoffs or main before doing work where possible.
- Wear gloves and eye protection when scraping mineral deposits or when using cleaning agents like vinegar.
- Don’t use excessive force when removing metal parts—use the correct tools and work gradually to avoid breaking fittings.
Common Homeowner Questions
- Why is only the tub spout weak while the shower is fine? The spout or its connection may be blocked or have a damaged washer; the shower uses a different outlet path so it can remain unaffected.
- Can I just replace the spout myself? If the spout is removable and you’re comfortable with basic tools, you can replace it—stop if corrosion or hard-to-reach parts appear.
- How long should I wait before calling a plumber? If a simple spout clean doesn’t fix it, or the diverter/shower is also weak, call a plumber rather than risk worsening the issue.