Supply Plumbing and PEX Manifolds for the Trades That Share the Walls
Why this matters
Supply plumbing is the only system in the wall that is under pressure all the time. A screw through a cable is a fire risk you may never see; a screw through a supply line is a leak, sometimes today and sometimes months from now when the screw corrodes or shifts. Drywallers, electricians, insulators, cabinet installers and anyone cutting an opening in a finished wall share cavities with these lines. Knowing how the water gets into the house, how a manifold system is laid out, and what to do in the first minute after you hit a line saves a ceiling, a floor, and sometimes the whole job.
If you hit a line and water is moving
- Water near electrical parts comes first. If water is spraying or pooling near a receptacle, a box, a fixture or the panel, keep everyone out of the wet area and do not touch anything electrical. Only switch off a breaker if you can reach the panel from dry floor; otherwise wait for the electrician.
- Stop the water at the closest valve you can reach without walking through water or past live electrical parts. On a manifold system that may be the port valve for that one line. Otherwise it is the house main.
- If a screw or nail is in the pipe, leave it in. It is plugging most of the hole. Pulling it turns a drip into a stream.
- Call the plumber, then tell the customer. Catch what you can with towels and a bucket, and get water off the floor and away from finished surfaces.
How water gets into the house
From the street (or from a well's pressure tank), the water service enters the house at one point and almost everything else branches from there.
- Curb stop or meter valve. The utility's valve at the street or in a meter pit. It belongs to the water utility and usually needs a special key. You do not operate it.
- House main shutoff. The homeowner's valve where the service enters: on a basement wall, in a crawlspace near the foundation, in a garage, in a utility closet, or on a slab house often near the water heater. Ball valves turn a quarter turn; older gate valves take several turns and sometimes break when forced.
- Pressure reducing valve (PRV). A bell-shaped valve just after the main in many homes. The 2021 IRC (P2903.3.1, as adopted) caps static house pressure at 80 psi and calls for a PRV where the street supply is higher; many are set somewhere around 50 to 60 psi. Most PRVs also stop water flowing back to the street, which turns the house into a closed system, which is why there may be an expansion tank on the water heater; see related: Expansion Tanks and Pressure Valves for Non-Plumbers.
- Distribution. Either a branch layout (a main trunk with tees feeding each fixture) or a manifold layout (described below), and often a mix.
SERVICE ENTRY TO FIXTURES (manifold layout)
street -> [MAIN SHUTOFF] -> [PRV] -> trunk
|
+----------------+-------+
| |
v v
[WATER HEATER] COLD MANIFOLD
| port 1 kit sink -->
v port 2 dish -->
HOT MANIFOLD port 3 hall lav -->
port 1 kit sink --> port 4 toilet -->
port 2 dish --> ... one line per
... one line per fixture, each with
fixture its own port valve
and a label
The manifold layout
A home-run manifold is a header pipe, usually near the water heater, with a row of outlets, one per fixture. Each fixture gets its own small line, typically 3/8 or 1/2 inch, running uninterrupted from the manifold to that fixture: no tees inside walls. Most manifolds have a small valve on each outlet and a label showing which fixture it serves.
For other trades that means two things:
- More lines in fewer places. Instead of one trunk, there can be a bundle of a dozen or more lines rising through the floor or top plate in one bay, then spreading out. The bundle is usually in a predictable path from the manifold toward the bathrooms and kitchen.
- You can often isolate one line. If you hit the master bath lavatory hot line, the port valve labeled for it stops that line alone, and the rest of the house keeps water.
Labels drift. A remodel can leave a port serving a different fixture than its label says. Confirm by closing the port and opening the fixture.
What PEX does not tolerate
PEX (cross-linked polyethylene tubing, usually red, blue or white) is now the most common supply pipe in new houses and remodels. It is tough in some ways and fragile in others.
Sunlight. Ultraviolet light breaks PEX down. Manufacturers set a maximum exposure, measured from days to a few months depending on product, before it must be covered. PEX left uncovered in a skylit attic, run along an exterior wall, or stored in a truck bed on site has used up that budget. If you see exposed PEX in a sunny place, report it.
Heat. PEX has temperature limits. Keep heat guns, torches, recessed light housings and hot flue pipe away from it. Many PEX makers' instructions keep PEX off the first 18 inches of pipe at a gas water heater and call for metal pipe there; the specific rule is in the manufacturer's install guide.
Nicks, scores and kinks. A cut from a utility knife, a gouge from a drill bit, a scrape from a stud saw, or a sharp kink weakens the wall of the tube. Manufacturers generally require a damaged section to be cut out and replaced, not taped. A nick that does not leak today can still fail later.
Chemicals. Some solvents, adhesives, paints and spray foams are not compatible with PEX. If you are spraying or gluing near exposed PEX, check the product label for compatibility or keep it off the pipe.
Where lines run, and what protects them
Supply lines run up from below through bottom plates, or down from above through top plates, to fixtures at predictable heights: toilet supplies low on the wall behind the toilet, lavatory and kitchen sink supplies around 18 to 22 inches off the floor, tub and shower lines to the valve body and up to the showerhead.
Where pipe passes through framing close to the face, the 2021 IRC (P2603.2.1, as adopted) wants steel shield plates over the spot. A plate on a stud in a bathroom or kitchen wall is the same signal as on an electrical run: stop, something is behind it.
Worked example: a bathroom remodel above a manifold
A remodel crew is gutting and rebuilding a second-floor master bathroom. The house has a manifold system in the basement mechanical room: a cold manifold with 10 ports and a hot manifold with 8, all labeled, 18 ports in all.
The electrician needs a new circuit for a heated floor. He plans to drill a 3/4 inch hole up through the bottom plate of the bathroom's interior wall from the first floor ceiling below. Before drilling, he reads the manifold labels: five ports are labeled master bath (tub cold, tub hot, lav cold, lav hot, toilet cold). Five lines, then, go up somewhere toward that bathroom. He opens a ceiling access panel below the bathroom and sees the five-line bundle rising into the stud bay directly under the vanity wall. He chooses the next bay over, 16 inches away, where no lines are visible, and drills there. The tally on the electrician's side: five lines expected, five found, zero touched.
The drywaller cuts the vanity box opening. While cutting the new drywall for the vanity supply stub-outs with a utility knife, he scores the outside of the lav hot line, a shallow cut along about 1/2 inch of the tube. It is not leaking.
What he does:
- Stops cutting and marks the spot with tape on the drywall edge.
- Does not tape over the cut or try to heat or smooth it.
- Tells the lead, who tells the plumber still on the job that week.
- The plumber cuts out the scored section and replaces it with a coupling, as the manufacturer requires, and pressure-checks the line.
What changes if it had been a screw. If a drywall screw had gone into the lav hot line, the drywaller leaves the screw in, the lead goes to the basement and closes hot port 6, labeled master lav hot, then opens the lav faucet to confirm the line is dead (no flow after a few seconds as the line drains). If the label were wrong and water still flowed, he would close the hot side at the water heater or the main and tell the plumber the label is wrong.
The tally for the job. Five master bath lines; one scored and replaced by the plumber; zero leaks; five master bath labels matched to a five-line bundle; one bay chosen to avoid the bundle. The plumber's visit took less time than the drying-out would have.
The version where nobody checked. The scored line goes behind tile. It holds for six months, then splits at the score, and the first sign is a stain on the kitchen ceiling below. By then nobody remembers who cut what.
What changes the answer
Copper or CPVC instead of PEX. Copper is harder to puncture but dents and cracks, and a nail can still go through it. CPVC (cream or tan rigid plastic) is brittle, especially when old, and can crack when bumped or flexed during demolition. Old galvanized steel corrodes from inside and can start leaking when disturbed.
Branch layout instead of manifold. No port valves. A hit line usually means closing the main, and the tees in the walls mean more fittings in more places.
Well homes. The pressure comes from a well pump and pressure tank, not a utility, and the main shutoff is usually near the tank. See related: Private Wells and Septic for Non-Specialists.
Radiant floors. The PEX in a floor may be heating loops, not supply. See related: Radiant Floor Heating for Flooring Installers and Carpenters.
How to verify you got this right
- You know where the main shutoff is before you cut or drill, and whether there is a manifold with port valves.
- You checked where lines run in the wall or bay before you drilled or cut.
- Any line you nicked, scored or punctured is reported to a plumber the same day, and any screw in a pipe stayed in until the plumber arrived.
- Exposed PEX in sunlight or near heat is reported.
References
- International Residential Code, 2021 edition, Sections P2603.2.1 and P2903.3.1, as adopted by your jurisdiction.
- The PEX manufacturer's installation guide for UV exposure, heat clearance, chemical compatibility and damaged-section repair.
- See related: Expansion Tanks and Pressure Valves for Non-Plumbers
- See related: How to Scan a Wall Before You Cut or Drill