Quick Answer:
If only the shower is weak while sinks and toilets work normally, it’s usually a local problem: a clogged or failing showerhead, a blocked cartridge or pressure‑balance spool, or a partially closed shower stop. Start by removing the showerhead and testing the flow straight from the shower arm to separate fixture issues from supply issues.
Why This Happens
When the rest of the house has normal water flow, the supply from the street and the main plumbing are likely fine. That narrows the cause to the shower fixture itself or the immediate valves feeding it. Common causes:
- Mineral buildup or debris in the showerhead or inline flow restrictor.
- Debris or corrosion in the cartridge or pressure‑balance spool that limits flow.
- Partially closed shutoff “stops” behind the access panel or under the tub/shower that reduce flow to that fixture only.
- A single hot or cold side issue at the shower cartridge — for related scenarios, see Low pressure on hot side only at one fixture.
Step-by-Step What to Do
1. Quick sink vs shower check
Turn on a nearby sink (both hot and cold) and observe the flow. Then remove the showerhead and turn the shower on to test flow directly from the shower arm. This proves whether the problem is the fixture or the supply line.
2. Remove the showerhead and inspect
- Use an adjustable wrench or pliers with a cloth to avoid scratching the finish. Unscrew the head counterclockwise.
- With the head off, turn the water on briefly and watch the flow from the arm. Strong flow here means the supply and valving are okay; the head or its internal screen is the likely culprit.
3. Clean or replace the showerhead
- Soak the head in a vinegar solution or clean out mineral deposits and debris. Pay attention to rubber nozzles and the inlet screen.
- If cleaning doesn’t restore flow, replace the head—many low‑flow heads can clog with minerals and are inexpensive to swap out.
4. Verify shower stops are open
- Locate the shutoff stops (usually behind an access panel or in the basement). They look like small valves on the hot and cold feeds to the shower.
- Turn them fully open (usually counterclockwise) and recheck flow. Partially closed stops are a common reason a single fixture is weak.
5. Inspect the cartridge or pressure‑balance spool
- Turn off water to the shower at the house shutoff or at the shower stops.
- Remove the trim and handle, then pull the cartridge or spool according to the manufacturer’s instructions.
- Look for debris, mineral deposits, or damaged seals. Flush the valve body briefly with water (catch it in a bucket) to remove grit before reinstalling the cartridge.
- If the cartridge or spool is visibly damaged or won’t clean up, replacing it is usually the fix.
What Not to Do
- Don’t assume ‘low pressure’ means the city is down—prove it with a sink test vs showerhead‑off test.
- Don’t force a stuck cartridge or valve without shutting off and relieving pressure first; that can cause damage or a leak.
- Don’t overtighten plumbing fittings when reinstalling; use thread tape where needed but avoid excessive torque that can crack fittings.
When to Call a Professional
- If flow from the shower arm is weak even with the head removed and the shower stops are fully open—this suggests a hidden restriction or supply piping issue.
- If removing or replacing the cartridge uncovers damaged valves, corroded pipes, or persistent leaks.
- If you’re uncomfortable shutting off the main water or working behind walls, call a licensed plumber to avoid water damage.
Safety Notes
- Always shut off the water to the fixture before removing the cartridge or internal parts. Open the shower to relieve any residual pressure.
- Be careful with hot water—run both hot and cold briefly to avoid scalding when testing.
- If you open an access panel, keep tools and small parts organized so nothing falls into the wall or drain area.
Common Homeowner Questions
- Why is only my shower weak? Most often the showerhead, cartridge, or a partially closed stop is restricting flow.
- Can I fix a cartridge myself? Yes if you’re comfortable turning off water and following the manufacturer’s steps; otherwise call a plumber.
- Will a new showerhead fix it? If the flow is fine from the shower arm, replacing a clogged head usually solves the problem; if not, the valve or piping needs attention. For a related check of nearby sink behavior, see Kitchen sink pressure suddenly dropped.
For more related articles, see the Low Pressure at Single Fixture hub.