Quick Answer:
If the shower feels weak only when the tub is being used or when the diverter is pulled for shower mode, the diverter or tub spout is likely letting some water leak back out the spout. Check whether water continues to run from the tub spout during shower mode (diverter leakage); if it does, repair or replace the diverter or spout and retest the shower pressure.
Why This Happens
Most bath/shower setups send water up to the showerhead through a diverter inside the tub spout or in the valve body. If the diverter does not seal fully, some flow escapes out the tub spout instead of going up to the showerhead. That reduced flow to the showerhead feels like low pressure. Other causes can include a clogged showerhead, a partially closed shutoff, or restrictions at the mixing valve, but the pattern you described (only weak when tub/diverter used) points to diverter leakage.
Step-by-Step What to Do
1. Confirm the symptom
- Turn on the shower and set any other taps in the bathroom off.
- Watch the tub spout while the shower is running. If you see water running from the spout, the diverter is leaking.
2. Compare shower head flow
- With the diverter engaged, note how weak the shower flow is. Turn the shower off and then on again, engaging the diverter fully, to make sure the condition is repeatable.
- If possible, remove the showerhead and look for mineral buildup that might also reduce flow. Clean or soak the showerhead if it’s clogged, then test again.
3. Test with the spout blocked (simple diagnostic)
- Briefly place a finger or a small cloth over the spout outlet while someone runs the shower. If the shower strengthens noticeably when the spout is blocked, that confirms flow is being lost through the spout/diverter.
- Only do this momentarily to test; avoid full pressure blocking for long periods.
4. Repair or replace the diverter/spout
- If the tub spout diverter is the type that pulls up, it often can be replaced without touching the main valve. Shut off water to the bathroom if you will remove the spout, and follow the spout manufacturer’s instructions or shop for a matching replacement.
- For diverters built into the valve body, the valve cartridge or diverter assembly may need replacement. After replacement, retest shower pressure.
- Check whether water continues to run from the tub spout during shower mode (diverter leakage); if so, repair/replace the diverter or spout and retest shower pressure.
5. Retest and confirm resolution
- Once repaired or replaced, run the shower and verify the spout is dry while shower mode is on and the showerhead flow has returned to normal.
- If pressure is still low, compare with other fixtures or consult the related topics below for additional troubleshooting.
What Not to Do
- Don’t ignore diverter leakage (water at spout during shower)—it steals pressure and often gets worse over time.
- Don’t assume only the showerhead is at fault without checking the spout and diverter first.
- Don’t force parts you don’t understand. If the replacement involves the main valve or plumbing inside the wall, a plumber is appropriate.
When to Call a Professional
- If the diverter is inside the valve body and replacing it requires removing the trim or cartridge you are not comfortable with.
- If the spout or diverter replacement does not stop the leak or restore shower flow.
- If multiple fixtures in the house have low pressure or you suspect a larger supply problem.
- For complex valve work, call a licensed plumber to avoid damage and ensure the correct parts are used.
Related reading: Low pressure after installing new valve, Pressure weak only when multiple showers used
Safety Notes
- Shut off water to the bathroom before removing spouts or valve cartridges to avoid unexpected water damage.
- If you are unsure which shutoff controls the fixture, shut off the main water supply before working on the valve.
- Use basic eye and hand protection when working with tools, and follow manufacturer instructions for replacement parts.
Common Homeowner Questions
- Why does water still drip from the spout after I pull up the diverter?
Because the diverter seal or internal parts are worn and not seating fully; replacement usually fixes it. - Will a clogged showerhead cause this exact symptom?
Clogs reduce flow but won’t cause water to run from the spout while in shower mode — that points to the diverter. - Can I replace the diverter myself?
If it’s a simple pull-up spout diverter and you’re comfortable with basic plumbing, yes; for in-wall valve diverters, a plumber is recommended.