That word "only" is a diagnostic gift. Whole-house problems — a failing pressure regulator, a partly closed main valve, a municipal issue — show up at every fixture. When the kitchen tap blasts and one shower dribbles, the cause sits somewhere in the last few feet: the showerhead, the valve behind the handle, or the short branch line feeding that bathroom. If you want to confirm the house side is healthy before you start, a cheap gauge on a hose bib does it — we cover the method in test your home water pressure. Then start at the showerhead and work backward. If the weakness turns out not to be local after all — every tap weak, and weak since yesterday — switch lists: the causes behind low water pressure in the house all of a sudden start at the street, not the showerhead.
Compare the tub spout to the showerhead. On a tub-shower combo, run the tub spout alone, then divert to the shower. Strong spout, weak showerhead means the head (or the short riser to it) is the restriction — go to step 2. Both weak means the problem is downstream of the valve or in the valve itself — skip to steps 4 and 5. Also watch the spout while the shower runs: a spout that keeps dribbling a heavy stream during showers is stealing volume, and a worn diverter spout is a cheap part to replace.
Remove the showerhead and soak it. Unscrew the head by hand or with padded pliers. First, check the little mesh inlet screen where the head threads onto the arm — it's often packed with grit and scale flakes, and rinsing it backwards under a tap can restore flow on the spot. Then submerge the head in a bowl or a tied-off bag of white vinegar for several hours or overnight to dissolve the mineral scale blocking the nozzles, and clear each nozzle with a pin or toothpick. It's the exact same grit problem as a slow faucet — the cure mirrors what we describe for cleaning a faucet aerator. Before reinstalling, run the shower for a few seconds with the head off: a strong stream from the bare arm confirms the head was the whole problem.
Test with a known-good showerhead. Swap in the head from another bathroom for one shower. If the pressure follows the head, yours is done — internally scaled up or simply a low-flow model. Some heads are rated 1.75 gallons per minute or less by design and feel weak no matter how clean they are; a head rated at the standard 2.5 gpm feels dramatically stronger. Swapping one takes minutes — our guide on how to replace a showerhead covers the thread-tape details.
Check the shutoff valves for that bathroom. Many showers have their own shutoff valves behind an access panel on the other side of the wall, or in the basement ceiling below. A valve left half-closed after a repair years ago restricts that one shower forever, and nobody connects the two events. Turn any valve you find fully counterclockwise and retest.
Suspect the cartridge in a single-handle shower. Most modern single-handle valves have a pressure-balancing spool inside the cartridge that slides to keep hot and cold in balance. Scale and age make it stick, and when it sticks it chokes flow on both temperatures. Telltale signs: the tub spout is weak too, the problem came on gradually, and temperature swings when someone flushes a toilet. Replacing the cartridge is a standard homeowner job — shut off the water, remove the handle and trim, pull the old cartridge, and seat the identical replacement. Match the brand and model; cartridges are not interchangeable.
Consider the pipes — last, and mostly in older homes. Galvanized steel pipe corrodes from the inside, and a branch line feeding one bathroom can narrow to a pinhole over decades while the copper or plastic lines elsewhere stay clean. If that one bathroom sits on old galvanized pipe and everything upstream tests fine, the pipe itself is the restriction. There's no Saturday fix for that; it's a plumber conversation about replacing the branch run.
Buying a new head before the spout test. Two minutes with the tub spout tells you whether the head is even guilty. Cranking up the pressure regulator. When one shower is weak, raising pressure for the whole house stresses every pipe and appliance and still doesn't clear a clogged cartridge. Attacking chrome with harsh acid descalers. Vinegar on a removable head is fine; strong acids left on plated finishes eat the finish. Ripping out the flow restrictor as the first move. Clean its screen first — the little plastic disc itself is often just gritty. Removing it can also put the fixture on the wrong side of local water rules in some states, and it does nothing if the real blockage is the cartridge.
One weak shower is a local problem with a short suspect list. Test the spout against the head, clean or swap the head, open the valves fully, and replace the cartridge if both outlets are weak. Only after all four should you think about the pipes themselves — and in most homes, you'll never get that far down the list.
Because they don't share the last stretch of pipe or the same fixture parts. The sink has its own aerator and supply stops; the shower has a head, a cartridge, and often its own shutoffs. A clog or a sticking cartridge in the shower line leaves the sink completely unaffected — which is exactly why the problem is local.
They rarely break, but they do clog past the point where soaking helps — internal passages scale up, especially on hard water, and rubber nozzles harden. If a vinegar soak and a clean inlet screen don't restore it, and a borrowed head works fine on the same arm, the head is done.
Run the shower with the head removed. If flow from the bare arm is weak — and the tub spout is weak too, on both hot and cold — the restriction is in the valve, which points at the cartridge. If flow from the arm is strong, the valve is innocent and the head was the problem.
It fixes it only when the restrictor itself is the bottleneck, which usually means its screen is clogged with grit — cleaning the screen gets the same result without removing anything. If the cartridge or a valve is the real restriction, a removed restrictor changes nothing. Some states and cities also regulate fixture flow rates, so check before you modify.
Call when the checks point inside the wall or under the floor: old galvanized branch lines, a cartridge seized so hard it won't pull, signs of a leak, or weak flow at several fixtures that share one branch. Those jobs stop being about cleaning and start being about opening walls and replacing pipe.