Quick Answer:
If the showerhead has full pressure but the tub spout is weak, the supply to the fixture is usually fine. This points to a problem with the tub spout itself or the tub/shower diverter (debris, partial engagement, or an internal restriction) rather than a house‑wide pressure issue.
Why This Happens
Most tubs share the same water line for tub and shower. A strong showerhead but weak tub spout means the valve and hot/cold supply are delivering pressure, but the path through the spout or diverter is obstructed or not fully opening. Mineral buildup, trapped debris, or a partially engaged diverter are common causes. If you recently worked on fixtures, see Pressure loss after replacing faucet for related troubleshooting steps.
Step-by-Step What to Do
1. Test tub spout flow with the shower off
- Turn the shower off completely so the diverter is not directing water to the showerhead.
- Turn on the tub spout to full hot and cold separately and note the flow strength.
- If the tub spout flow is still weak while the showerhead flow is strong, the issue is localized to the spout/diverter area.
2. Confirm the diverter isn’t partially engaged
- Operate the diverter several times—pull up or push down the spout lever (or use the valve diverter, if present) to ensure it moves freely.
- Sometimes the diverter sits slightly open and sends most flow to the shower; make sure it seats fully for tub flow.
3. Inspect the tub spout for internal restriction
- Look for visible debris or mineral scale at the spout outlet and around the diverter area.
- If the spout has an internal diverter, small particles can lodge inside and reduce flow to the spout.
4. Remove the spout and flush the nipple
- Shut off the water to the tub at the shutoff valves or the main if necessary.
- Remove the spout—either by unscrewing or by loosening the set screw, depending on style.
- With the spout off, briefly open the water to flush water and any debris from the pipe nipple into a bucket. Be ready for sudden flow.
- Inspect the removed spout. Clean out any scale or debris and reassemble.
5. Reinstall and test
- Reattach the spout securely, restore water, and test with the shower off. Confirm tub flow returns to normal.
- If cleaning helped but the problem returns, the internal diverter or spout may be worn and should be replaced.
What Not to Do
- Don’t assume it’s a “shower problem” and replace or adjust the showerhead without first testing the tub flow—test tub flow first.
- Don’t force fittings or use excessive leverage; you can damage plumbing finishes or threads.
- Don’t rely on aggressive chemical cleaners inside the spout or valve without checking manufacturer guidance—those can damage certain finishes or seals.
When to Call a Professional
- If flushing and cleaning the spout/diverter doesn’t restore flow, the diverter inside the valve or the valve cartridge may be failing.
- Call a plumber if you find corroded or damaged pipe nipples, stripped threads, or if the valve assembly leaks when the spout is removed.
- If multiple fixtures drop pressure at the same time, see a pro—this can indicate a wider supply issue. For patterns where one fixture’s pressure improves when others run, consult the guide on Fixture pressure improves when others run.
Safety Notes
- Always shut off water before removing fixtures to avoid spraying and potential water damage.
- Use proper tools and protect finishes with cloths when using pliers or wrenches.
- Wear gloves and eye protection when flushing debris to avoid contact with mineral particles or rust.
Common Homeowner Questions
- Will replacing the tub spout fix low tub flow permanently?
Often yes if the spout or its diverter is the only problem; persistent issues may require valve repair. - Can I clean the spout without shutting off the main water?
It’s safest to shut off local shutoffs; if none exist, be prepared to shut off the main or contain the flush flow. - How do I know if the valve cartridge needs replacement?
If cleaning the spout and flushing the nipple don’t help and the diverter doesn’t move freely, the cartridge or diverter assembly may need replacement by a pro.
For more related articles, see the Low Pressure at Single Fixture hub.