High Water Pressure Doesn’t Feel Like a Problem, Until It Is

Ask around and most water pressure complaints run the same direction. A weak shower, a tub that takes forever to fill, a hose that barely sprays as far as the garden. High pressure gets almost no complaints, and that’s exactly the problem with it. A shower that blasts feels like a win. Then a water heater fails years early, a supply line splits behind a wall, or a washing machine hose gives out overnight. Missouri has no statewide plumbing code, so every city and county in the region handles plumbing rules on its own. On one number, though, they agree: once household pressure passes 80 psi, a pressure-reducing valve is required.

No State Code, One Number That Keeps Showing Up

Missouri is one of only a few states with no general statewide building code. Lawmakers have tried and failed to change that more than once. Colorado still gets grouped with it: Colorado licenses plumbers and adopts a state plumbing code through its Department of Regulatory Agencies, even without a general building code of its own. Illinois used to sit in that same group, but adopted a statewide baseline building code at the start of 2025. Its own plumbing rules didn’t move: Illinois still runs a separate state plumbing code through its Department of Public Health, untouched by the new building code baseline. Missouri has none of that: no state plumbing board, no state exam, no state license. Every city and county decides for itself, right down to which code book to adopt.

Webster Groves enforces the International Plumbing Code through its own building department. The City of St. Louis and unincorporated St. Louis County both run on the Uniform Plumbing Code instead, a different book. Each has its own inspectors.

With that kind of split, you’d expect a lot of variation, but that’s not the case. St. Louis City, unincorporated St. Louis County, and Webster Groves all land on the same limit. Most other municipalities in the region run on one of these same two code books, so the number tends to carry over without anyone deciding to match it on purpose. Once pressure passes 80 psi, a pressure-reducing valve becomes required. Different books, different editions, different inspectors, same number.

A lot of homes in the area were built before that rule existed. Others have a pressure-reducing valve that failed quietly years ago and was never replaced. Pressure becomes too high much sooner than a person would notice it in the shower. Instead, it wears out a water heater, fixtures, and supply lines a little faster every year, with nothing dramatic enough to notice until something finally gives.

Three Things Worth Knowing

• Check it yourself, cheap. Thread a hose-bib gauge onto an outside spigot and you have a real number in a couple of minutes, for almost nothing. Most pressure-reducing valves ship set around 50 psi, adjustable from roughly 25 to 75. From there, a licensed plumber can go further. They can check more than one fixture, at more than one time of day, since neighborhood demand can shift the number.
• Old pipe can hide a real problem. Plenty of homes with corroded galvanized or cast iron supply lines never seem to need a pressure-reducing valve. That’s not a good sign. That old pipe corrodes from the inside, through a process called tuberculation. It narrows enough to choke off flow before pressure ever reaches a fixture. A valve that has “never seemed necessary” often means one thing: the pipe itself is throttling pressure, badly, instead of a valve doing that job on purpose. Location matters too. Utilities manage pressure across zones and booster pump stations, so two homes a few blocks apart can run at meaningfully different pressure. Repipe with copper or PEX, and pressure at that address can jump right along with it. Sometimes it jumps enough to finally need the valve nobody thought they had a use for.
• Installing a pressure-reducing valve means your water heater needs attention too. The valve closes off the path that expanding water used to take back into the street main. Once that path is closed, the water heater has nowhere to send the water it expands as it heats, unless something else is added to catch it. Both the International and Uniform plumbing codes require exactly that: a thermal expansion tank on the water heater’s cold supply line, downstream of the new valve. We’ve also covered what a water heater expansion tank does and how long it tends to last.

Worth a Look Either Way

A cheap gauge on an outside spigot is a fine start. A licensed plumber, not a salesperson, can go further, checking pressure at more than one point in the house, looking at what the supply pipe itself is doing to that number, and confirming whether a valve, an expansion tank, or neither is called for. If it has been a while since anyone looked at yours, ask about it on your next visit, or request one. You can also reach us at (314) 962-0956.

Sources

• STLPR, “Missouri doesn’t have a statewide building code. This bill seeks to change that” (stlpr.org)
• Missouri Division of Professional Registration, Listings of Boards (pr.mo.gov)
• Colorado Department of Regulatory Agencies, Code of Colorado Regulations, 3 CCR 720-1 (sos.state.co.us)
• Illinois Department of Public Health, Plumbing Program (dph.illinois.gov)
• Illinois Capital Development Board, Statewide Building Code FAQ, Public Act 103-0510 (cdb.illinois.gov)
• Webster Groves, MO Code of Ordinances, Chapter 27, The Plumbing Code (library.municode.com)
• St. Louis County, MO Code of Ordinances, Chapter 1103, The Plumbing Code (library.municode.com); City of St. Louis, MO Code of Ordinances, Chapter 25.04, Plumbing Code (library.municode.com)
• Uniform Plumbing Code, Sections 608.2 (Excessive Water Pressure) and 608.3 (Expansion Tanks) (up.codes)
• International Plumbing Code, Section 607.3, Thermal Expansion Control (codes.iccsafe.org)
• Watts, Water Pressure Reducing Valves technical literature (watts.com)
• McWane Ductile, “What is Tuberculation and Why Did it Happen in Iron Pipe?” (mcwaneductile.com)
• EPA, Distribution System Water Quality: Pressure Management fact sheet (epa.gov)