Plumbing · Lesson 6 of 13Intermediate7 min read

Pipes, Fittings and Joining

Walk into a hardware store’s plumbing aisle and you’ll see a dozen kinds of pipe and a hundred fittings. They sort out quickly once you know two things: what each pipe is for, and how its joints are made to hold water.

Supply pipe: what carries water under pressure

Copper. Rigid copper tube, joined by soldering, has been the standard in much of the country for decades; HUD’s inspection guide says copper came into wide use in the 1930s and normally lasts 50 years or more. It comes in wall thicknesses called types K, L and M, all named in HUD’s manufactured home standard, with K the thickest.
PEX (cross-linked polyethylene). Flexible plastic tubing for hot and cold water, often red and blue. It bends around corners, so it needs fewer fittings, and it’s the usual tubing for manifold systems that give each fixture its own line.
CPVC (chlorinated polyvinyl chloride). A rigid, cream-colored plastic rated for hot and cold water, joined with solvent cement.
Galvanized steel. Steel pipe coated with zinc and joined with threads, common in older homes. HUD’s inspection guide gives it a service life of 20 to 50 years; rust and mineral deposits slowly close it off from the inside, and rust-colored water, especially from hot taps, signals advanced decay.
Lead. Found in very old service lines and plumbing. HUD notes it may be a health hazard; Lesson 7 has EPA’s advice.

Any plastic used for drinking water must be made for it. HUD’s standard, for example, requires plastic water piping in manufactured homes to be listed for use with hot water and plastics not covered by its listed standards to be certified as non-toxic.

Drain pipe: what carries waste by gravity

PVC and ABS. White PVC and black ABS plastic pipe are the most familiar drain, waste and vent pipes today. HUD lists both, in Schedule 40 DWV grades. Each has its own solvent cement.
Cast iron. Heavy and quiet, used for stacks and building drains, especially in older homes. Older cast iron was joined at bell-shaped hubs; hubless (no-hub) pipe is joined with rubber sleeves clamped by stainless steel bands.
Copper DWV. A thin-walled copper tube made for drains, found in some homes.
Galvanized steel. Older drains and vents; it rusts and clogs over time.

HUD’s inspection guide says properly installed plastic pipe, supported and protected from sunlight, should last indefinitely, but some local codes restrict where plastic may be used. Your code decides which materials are allowed in your home.

Colors are typical, not a rule; plastic pipe is printed with its type and standard. Which materials are allowed is set by your local code.

Common pipe materials
Material	Used for	Joined by
Copper (types K, L, M)	Water supply	Solder; also flare, compression and press or push-fit fittings
Copper DWV	Drains and vents	Solder
PEX	Water supply, hot and cold	Crimp or clamp rings, expansion fittings, push-fit
CPVC	Water supply, hot and cold	Solvent cement
PVC, ABS (Schedule 40 DWV)	Drains, waste and vents	Solvent cement made for each plastic
Cast iron	Stacks and building drains	Hub joints; no-hub rubber couplings with steel bands
Galvanized steel	Older supply and drain lines	Threads

How joints hold water

Every joint has to stay sealed against full water pressure, or against sewage, for decades.

Soldered (“sweated”) copper. The pipe end and fitting are cleaned bright, coated with flux, slipped together and heated; melted solder is drawn into the thin gap and fills it. HUD’s standard spells out the basics: approved solder-type fittings, surfaces cleaned bright, a noncorrosive paste flux, and for drinking water, solder with no more than 0.2 percent lead.
Threaded. Tapered pipe threads wedge tighter as they’re turned. A sealing compound or thread tape fills the gaps. HUD’s standard says joint compound goes on the male threads only, threads must be fully engaged, and threads may not be wrapped with string, paper or putty.
Solvent-welded plastic. PVC, CPVC and ABS aren’t glued so much as fused. A primer and solvent cement soften the surfaces of the pipe and the fitting, and when they’re pushed together the plastics blend and harden into one piece. Each plastic needs its own cement; HUD’s standard says to follow the manufacturer’s methods.
Crimp, clamp or expansion (PEX). PEX slides over a barbed fitting and a metal ring is crimped tight around it, or the tubing is expanded with a tool, slipped over the fitting, and shrinks back to grip it. Each method needs its own matched fittings and tools.
Push-fit. Teeth inside the fitting grip the pipe and an O-ring seals it. Quick and handy for repairs, as long as the pipe is cut square, deburred and pushed in all the way.
Compression and flare. A compression nut squeezes a brass ring onto the pipe; a flare fitting clamps a flared tube end. Common on fixture supply valves. HUD calls for a proper flaring tool for flared copper joints.
No-hub couplings. A rubber sleeve over two cast iron or plastic drain pipe ends, clamped with stainless steel bands.

Every method has its own fittings, tools and listing. Use the method the pipe and fitting makers specify and your local code allows.

Lead-free rules

Lead used to be common in solder, brass and even pipe. The Safe Drinking Water Act changed that. EPA explains that the 1986 amendments banned the use of any pipe, fitting, fixture, solder or flux that wasn’t “lead free” in drinking water systems after June 1986. Back then, “lead free” meant solder and flux with no more than 0.2 percent lead and pipes with no more than 8 percent. The Reduction of Lead in Drinking Water Act of 2011 tightened the definition to a weighted average of 0.25 percent lead on the surfaces that touch water.

Some products are exempt because they don’t carry drinking water, including toilets, bidets, urinals, fill valves, flushometer valves, tub fillers, shower valves and service saddles, and plumbing used only for non-drinking water. That’s why you’ll see “lead free” or a certification mark on faucets and fittings meant for drinking water.

Mixing metals

When two different metals touch in water, one can corrode the other, an effect called galvanic corrosion. HUD’s inspection guide says to separate dissimilar metal pipes with a dielectric coupling, and calls mixed galvanized steel and copper or brass piping a sign of potential trouble. No separation is needed between metal and plastic.

Tools of the trade

A homeowner’s kit covers most first jobs: adjustable wrenches, tongue-and-groove pliers, a basin wrench for faucet nuts under a sink, a tubing cutter, a deburring tool, thread tape, a flashlight, a bucket and rags, a cup plunger and a flange plunger, and a hand drain snake. Plumbers add pipe wrenches, PEX crimp or expansion tools, press tools, a torch and soldering kit, drain machines and cameras, test gauges and pipe threaders. Lesson 12 has a full starter kit.

StopTorches, saws and hidden pipes

Soldering means an open flame near wood framing and insulation, and the Bureau of Labor Statistics lists burns from hot pipes and soldering equipment among plumbers’ common injuries. Cutting into walls risks hitting wires, gas lines and pipes. Learn these skills from a pro, and check whether your work needs a permit or a licensed plumber.

Check yourself

Tap an answer to check it, or open “Show the answer.”

Which pipe is a flexible plastic tubing used for hot and cold water supply?
Show the answer
PEX. PEX bends around corners and is joined with crimp, clamp, expansion or push-fit fittings.
Where does pipe joint compound go on a threaded joint, according to HUD’s standard?
Show the answer
On the male threads only. Male threads only, with the threads fully engaged.
How does solvent welding join PVC?
Show the answer
Softens the surfaces so they fuse into one piece. Primer and cement soften the plastic so the pipe and fitting fuse.
Under the Reduction of Lead in Drinking Water Act of 2011, “lead free” plumbing may have no more than what weighted average of lead on the surfaces that touch water?
Show the answer
0.25 percent. 0.25 percent, under the Reduction of Lead in Drinking Water Act of 2011.
Why put a dielectric coupling between galvanized steel and copper?
Show the answer
To stop galvanic corrosion between different metals. Dissimilar metals in contact in water can corrode; the coupling separates them.

Go deeper

U.S. EPA: Use of Lead Free Pipes, Fittings, Fixtures, Solder, and Flux for Drinking Water EPA on the Safe Drinking Water Act’s lead-free rules.
24 CFR 3280.605, Manufactured Home Standards: Joints and Connections (via Cornell LII) An official code text on joints and connections (manufactured homes).
U.S. Department of Energy, Building Science Education: Manifold Plumbing DOE on manifold plumbing.
Lesson 7: Water Quality and Safety Next: what’s in the water itself.

Sources

I finished this lessonSaved on this device only.

A free course from Teddy Rook Book. It explains how plumbing works in general and follows U.S. sources; your local plumbing code, the authority having jurisdiction and manufacturers’ instructions are the final word.