Supply and Drain Lines

Here’s the rule that matters more than any other plumbing decision: a plumbing system is two completely separate stories. One story is about water arriving — clean, under pressure, ready to leap up to your shower the moment you open a tap. The other is about water leaving — dirty, heavy, falling away by gravity. Get those two systems confused and you’ll have a failure on your hands.

Supply lines and drain-waste-vent (DWV) lines run on opposite logic. Clean water arrives through relatively small pipes under pressure. Because it is pressurized, supply can run uphill, sideways, anywhere; it does not care about gravity. Drainage is the reverse. Used water and waste leave through much larger pipes, and they run entirely by gravity. Every drainage pipe must fall continuously toward the stack.

There’s a third piece most folks forget about. Every plumbing system also includes vent pipes that extend through the roof. These vents allow air to enter the drainage system, preventing vacuum conditions that could slow drainage or siphon water from fixture traps. They also allow sewer gases to safely escape outdoors rather than entering the home. Vents carry air, not water. Skip them and your drains gurgle and stink.

The cardinal sin: water supply pipes differ fundamentally from drain, waste, and vent pipes. Using PVC on a hot water supply line, for example, is a code violation and a failure waiting to happen. DWV-rated pipe is built for gravity flow, not pressure. Don’t cross the streams.

Choosing Supply Line Material

For pressurized water, three materials carry the modern load: PEX, copper, and CPVC. In Montana, freeze behavior should weigh heavily on your choice.

PEX is the default in most new work. PEX (cross-linked polyethylene) is flexible, resistant to scale and chlorine, and generally less expensive to install than copper. Because it comes in long rolls, plumbers can run continuous lengths with fewer fittings, reducing potential leak points. Fewer fittings means fewer places to leak — that matters in a wall you’re about to close up.

The freeze story is where PEX earns its keep up here. PEX is also more resistant to freeze damage than rigid piping, as it can expand slightly if water freezes inside. One catch worth knowing: PEX degrades under UV exposure, so it cannot be used outdoors or in exposed sunny areas.

Copper is the old reliable. Copper is durable, corrosion-resistant, and does not allow gases to permeate the pipe walls. Properly installed copper piping can last 50 years or more. But it’s rigid and unforgiving in a cold snap. Factors such as acidic water, high water pressure, or high mineral content can shorten its lifespan and lead to pinhole leaks. Copper is also rigid and can burst if water inside freezes. It also requires soldering for repairs — a torch and some skill.

CPVC handles hot and cold but brings a cold-climate weakness. CPVC is chemically altered PVC that tolerates higher temperatures. It handles both hot and cold water supply lines inside homes. It is more brittle than PEX and can crack during hard freezes. In Montana, that brittleness is a real consideration for any line that might see cold.

One more thing on supply: the main water line is buried underground below the frost line and brings water into the home. Given how deep frost drives across western Montana, that burial depth is not negotiable.

Choosing Drain Material and Sizing the System

On the DWV side, the common players are PVC, ABS, and cast iron — each with a trade-off.

Most modern DWV systems use PVC piping for drain, sewer, and vent lines. PVC is lightweight, corrosion-resistant, cost-effective, and relatively easy to install. Joining it takes two steps — PVC requires a two-step solvent welding process (primer and cement), while ABS uses a single-step cement.

ABS is the black pipe, and it has a Montana-relevant edge. ABS is another plastic material used in DWV applications. ABS is strong and performs well in colder temperatures. That makes it worth a look for unheated crawlspaces and below-grade runs. Whichever you use, the two materials require different types of cement and should not be directly connected without proper transition fittings. Local building codes often determine whether PVC or ABS is permitted.

Cast iron is the quiet one. For best results, use cast iron for drain lines that run through a living space. To save money, switch back to ABS or PVC under the house, but only cast iron will provide silent drains in the living space.

Sizing the lines

Undersize and you get low pressure and water hammer. Oversize and you waste money. On the supply side: the main hot and cold supply trunks are commonly 3/4 inch, while larger homes may have 1-inch mains. Most branch lines supplying individual fixtures are 1/2 inch. Bathroom sink and dishwasher supply lines are often 3/8 inch.

System	Run / Fixture	Common Size
Supply	Main trunk	3/4” (1” for larger homes)
Supply	Branch to fixture	1/2”
Supply	Sink / dishwasher feed	3/8”
Drain	Kitchen sink / lavatory	1-1/2”
Drain	Shower, tub, washing machine	2”
Drain	Any line serving a toilet	3” min (4” preferred)

A few hard rules on drains. The smallest drainpipe allowed for underground use is 2 in., and the smallest building sewer allowed is 3 in. in diameter. The toilet rule is firm — always 3” pipe, 4” preferred. And direction matters: never reduce drain pipe size in the downstream direction (toward the sewer). Only increase or maintain size as the line extends. When you’re on the fence, best practice calls for upsizing these minimums.

Slope, traps, and testing

Drains live or die on slope. Minimum 1/4” per foot slope equals a 2.1% grade — the code standard, ideal for carrying solids and maintaining self-cleaning velocity. Larger pipe can run flatter: horizontal drains usually run 1/4 in. per ft.; 3–6 in. diameters may use 1/8 in. per ft. Too flat and it chokes; too steep and the water races ahead leaving solids stranded, which also blocks.

Every fixture needs a trap. A trap is a U-bend that holds a plug of water — the water seal, typically around 50 mm deep. That little plug is the only thing standing between you and the smell of the sewer. The vent protects that seal. When a slug of water rushes down the stack, it drags air with it and creates suction — and that suction can pull the water seal straight out of a nearby trap, leaving the path open for smells.

Before the walls close, prove your work. Pressure-test the supply side and verify drain slopes — don’t rebury a problem. And always confirm your material against local code: codes specify which materials are acceptable in which applications. Before any installation, verify what is approved for your municipality.

A note on older homes: if you uncover galvanized steel supply pipe, it corrodes from the inside out, restricting water flow and eventually failing. That is one of the clearest signs to replace your plumbing.

Frequently Asked Questions

Can PVC be used for hot water supply lines? No. Never use standard PVC for hot water supply. It warps and fails under sustained heat. PVC belongs in DWV and cold-water service applications. For hot supply, use PEX, copper, or CPVC.

Which supply material holds up best against freezing? PEX has the advantage in cold climates. PEX is more resistant to freeze damage than rigid piping, as it can expand slightly if water freezes inside. By contrast, copper is rigid and can burst if water inside freezes, and CPVC is more brittle than PEX and can crack during hard freezes. None of that excuses skipping proper insulation and burial.

Can PVC and ABS drain pipe be glued directly together? No. The two materials should not be mixed without a special transition coupling. They take different cements, and local building codes often determine whether PVC or ABS is permitted in the first place.

What slope does a horizontal drain need? The standard is 1/4 inch per foot, which equals a 2.1% grade — ideal for carrying solids and maintaining self-cleaning velocity. Larger 3-to-6-inch pipe can drop to 1/8 inch per foot.

Why does a toilet need a bigger drain than a sink? Code sets the floor. A toilet always requires a 3-inch pipe minimum, with 4 inch preferred. Compare that to a kitchen sink or lavatory at 1-1/2 inch. The drain handles solids, not just water, so it needs the room.

What keeps sewer gas out of the house? The trap under each fixture. A U-bend holds a plug of water — the water seal, around 50 mm deep — and that little plug is the only thing standing between you and the smell of the sewer. The vent system keeps that seal from getting siphoned away.