Quick Answer:
When the sink next to a shower runs fine but the shower gives a weak stream, start by removing the showerhead and running water from the bare shower arm for 10–15 seconds. If the flow from the arm is strong, the showerhead is the likely cause — clean or replace it. If the flow from the arm is weak, the problem is upstream at the valve, cartridge, or local shut-off stops and usually needs more troubleshooting or a plumber.
Why This Happens
Showers have parts that can reduce flow even when the rest of the house is fine. Common causes:
- Mineral buildup or a clogged flow restrictor inside the showerhead.
- A partially closed diverter or tub spout that redirects water away from the showerhead.
- A clogged or failing cartridge/valve inside the shower control that limits flow to the shower outlet.
- Local shut-off stops (angle stops) that are partially closed or obstructed.
If you see similar symptoms elsewhere, check related topics like **Tub spout pressure weak but sinks fine** or if the problem started suddenly, see **Shower pressure suddenly dropped** for more context.
Step-by-Step What to Do
1. Confirm it’s only the shower
- Turn on the sink faucet right next to the shower and another faucet on the same floor. If they run strong, the issue is isolated to the shower branch.
- If many fixtures are weak, the cause may be main supply or pressure issues and you should broaden the check.
2. Remove and test the showerhead
- Unscrew the showerhead from the shower arm. Wrap a cloth around the arm if it’s tight and use pliers gently to avoid damage.
- Run water from the bare shower arm for 10–15 seconds. Observe flow and any sputtering or debris.
- If the flow from the arm is strong: clean the showerhead (soak in vinegar to remove mineral deposits, remove and clean the flow restrictor if possible) or replace the head.
- If the flow from the arm is weak: do not spend more time on the head — move to the valve and stops.
3. Check the diverter and tub spout (if present)
- If your tub has a diverter, make sure it isn’t stuck or partially blocking the shower line when in shower mode.
- A faulty diverter can reduce shower flow but allow the sink to run normally.
4. Inspect shut-off stops and the valve/cartridge
- Look for any local shut-off valves (angle stops or service stops) serving the shower. Ensure they are fully open.
- If the stops are open and the bare arm is still weak, the cartridge or mixing valve may be clogged or failing. To inspect the cartridge you must shut off the water to the shower (at the stop or main), remove the handle and trim, then pull the cartridge to check for debris or damage.
- Clean or replace the cartridge per the manufacturer’s guidance. If you’re not comfortable doing this, call a pro.
5. Reassemble and test
- After cleaning or replacing parts, reassemble the trim and showerhead. Turn water back on and test the shower on full hot and full cold to check flow and temperature balance.
- Monitor for leaks around the trim and connections.
What Not to Do
- Don’t assume the whole house is low if only the shower is weak—test the shower arm first; if flow is weak there, a plumber is appropriate.
- Don’t force or pry valve parts without shutting off the water first.
- Don’t use harsh chemical drain cleaners inside a valve or cartridge; they can damage seals and internal parts.
- Don’t over-tighten fittings or use excessive heat on trim pieces — you can crack finishes or plastic parts.
When to Call a Professional
- Flow from the bare shower arm is weak after removing the head.
- There are no accessible shut-off stops or the valve assembly is corroded or seized.
- You find leaks behind the wall, damaged valves, or are unsure how to shut off water safely.
- The cartridge is difficult to remove, or you’re uncomfortable reassembling the trim and ensuring proper seals.
Safety Notes
- Always shut off the water supply to the fixture before removing the cartridge or opening the valve body.
- Be cautious with hot water — run cold first when testing to avoid scalds.
- Place towels and a bucket under the work area to catch water. Keep electrical devices away from wet areas.
- If you find unexpected insulation, asbestos, or major corrosion, stop and call a professional.
Common Homeowner Questions
- Why does the sink have good pressure but the shower is weak? The showerhead or the shower valve path is likely restricted; sinks use a different branch so they can be unaffected.
- Can I fix a weak shower flow by myself? Yes, if the issue is the showerhead or a stuck diverter; if the bare arm is weak or the valve needs work, consider a plumber.
- How long should I run the bare shower arm when testing? Run it for 10–15 seconds to allow air and loose debris to clear and to get an accurate sense of steady flow.