For most of the 20th century, residential plumbing meant cutting rigid copper pipe with a tubing cutter and sweating every joint with a torch. Today, the majority of new construction and remodels use some form of flexible plumbing. Flexible plumbing pipe comes in three main categories: PEX tubing for main water distribution, braided stainless steel supply lines for appliance and fixture connections, and corrugated stainless steel flex for gas lines. Each has specific applications, ratings, and installation rules. This guide covers what to use where and what to avoid.
- PEX-A vs PEX-B vs PEX-C
- Where PEX Replaces Copper
- Braided Stainless Steel Supply Lines
- Where Flexible Supply Lines Belong
- Corrugated Stainless Steel Gas Flex
- CSST for Whole-House Gas Distribution
- Fittings and Connections
- Common Failure Modes
- Cost Comparison for a Typical Bathroom Rough-In
- Final Recommendations
PEX-A vs PEX-B vs PEX-C
PEX (cross-linked polyethylene) has replaced copper as the default water distribution pipe in most new homes. The three flavors are chemically similar but manufactured differently:
- PEX-A (Uponor/Wirsbo): Most flexible, best freeze tolerance (expands without bursting). Uses expansion fittings. Highest cost, typically $0.70-$1.20 per foot for 1/2-inch.
- PEX-B (SharkBite, Apollo, Viega): Stiffer, uses crimp or press fittings. Cheaper at $0.40-$0.70 per foot. Most common in DIY projects.
- PEX-C: Least common, less flexible, primarily used commercially.
For a DIY whole-house repipe, PEX-B with SharkBite or PEX-crimp rings is the easiest option. For an install where every bend matters and you want maximum freeze resistance, pay the premium for PEX-A with ProPEX expansion fittings. Both carry a lifetime warranty when installed per manufacturer instructions.
Where PEX Replaces Copper
PEX can run from the meter to every fixture in the house. Typical uses:
- Hot and cold water distribution mains
- Branch lines to sinks, toilets, showers, and laundry
- Manifold (home-run) plumbing where each fixture has a dedicated line
- Radiant floor heating loops
- Hydronic baseboard feeds
PEX should not be used outdoors in UV-exposed locations or within 18 inches of a gas water heater’s vent draft. Local codes vary; check your area’s adopted plumbing code (UPC, IPC, or state-specific).
Braided Stainless Steel Supply Lines
The short connections between a shutoff valve and a fixture or appliance are almost always flexible supply lines today. They come in three inlet/outlet configurations (3/8 inch compression, 1/2 inch FIP, and 7/8 inch ballcock for toilets) in lengths from 9 to 72 inches.
Two types to choose from:
- Braided stainless steel over PEX core: $5-$12 per line. Most common. Good 5 to 10 year life.
- All-stainless (Fluidmaster Click Seal, Eastman Flextron PRO): $10-$20 per line. Higher burst pressure, 10 to 20 year life.
Do not use the old-style white plastic PVC-braided supply lines that came with some toilets pre-2000. They have failed in well-documented class-action lawsuits. Replace any you find with stainless braided lines the next time you pull the toilet.
Where Flexible Supply Lines Belong
Standard applications:
- Toilet tank to angle stop
- Bathroom and kitchen faucets to shutoffs
- Dishwasher to hot water supply
- Washing machine hot and cold hookups
- Ice maker line to refrigerator (1/4-inch variety)
- Water heater inlet and outlet (larger 3/4-inch supply lines or dedicated water heater connectors)
Choose the shortest length that reaches without kinking. Excess slack is fine; a line that has to bend sharply fails at the bend point within a few years.
Corrugated Stainless Steel Gas Flex
Gas appliances connect to the supply system with a corrugated stainless steel flex line (CSSF). These look similar to water supply lines but use yellow brass or aluminum end fittings with a distinct beveled seat and require yellow PTFE tape or an approved thread sealant.
Applications:
- Gas range or cooktop to shutoff valve
- Gas dryer to shutoff valve
- Gas water heater to supply (6-inch or 12-inch heavy-duty flex)
- Gas fireplace inserts
Gas flex lines are not interchangeable with water supply lines even if they look similar. Check the yellow labeling on the packaging. Install per manufacturer instructions and always leak-test with soapy water after installation. Bubbles mean a leak.
CSST for Whole-House Gas Distribution
Corrugated stainless steel tubing (CSST) is the flexible gas equivalent of PEX. Brands like TracPipe, Gastite, and HOME-FLEX are yellow or black jacketed tubing that runs through walls and floors to connect appliances. CSST installation requires a certified installer in most jurisdictions; it is not a DIY material.
CSST must be properly bonded to the electrical grounding system per NFPA 54 because lightning strikes have been linked to CSST failures when it was left unbonded. If you see yellow flex gas lines in your basement without a bonding clamp, call a licensed plumber or electrician to inspect.
Fittings and Connections
Each flexible pipe type has its own fitting system:
- PEX-A: ProPEX expansion tool, one-piece fittings.
- PEX-B crimp: Copper crimp rings with a jaw-style crimp tool ($80-$200).
- PEX-B cinch clamp: Stainless cinch clamps with a cinch tool ($40-$80). Easier for tight spaces.
- SharkBite push-to-connect: No tools beyond a deburring tool. Premium per fitting ($5-$12) but fastest.
- Supply line compression nuts: Hand tight plus a quarter turn with an adjustable wrench.
- Gas flex: Thread-on fittings with yellow PTFE tape or pipe dope rated for gas.
SharkBite fittings carry a 25-year warranty when installed correctly and work on copper, PEX, and CPVC. They are the rescue fitting of choice for in-wall repairs where access to solder or crimp is limited.
Common Failure Modes
Flexible lines fail differently than rigid pipe. Watch for:
- Kinking: Permanent flow restriction or pinhole at the bend. Replace the line.
- UV degradation: PEX left in sunlight becomes brittle. Do not run PEX outdoors above ground.
- Rodent damage: Mice chew PEX. Use metal shielding in spaces with rodent activity.
- Braid corrosion: Stainless braid eventually rusts in humid bathrooms or garages. Visual inspection once a year.
- Fitting failure at compression: Overtightening splits the nut. Hand-tight plus a quarter turn is the rule.
Cost Comparison for a Typical Bathroom Rough-In
For a bathroom with sink, toilet, and shower/tub, materials run roughly:
- All copper with sweat fittings: $180-$280 in materials, plus $25 for solder and flux. Requires torch experience.
- PEX-B with crimp fittings: $80-$140 in materials. Requires crimp tool ($120 one-time purchase).
- PEX-A with expansion fittings: $130-$220 in materials. Requires expansion tool ($200-$400 one-time).
- All SharkBite push-to-connect: $140-$250 in materials. No specialty tools needed.
For a one-time project, all SharkBite is often the cheapest total cost after factoring in tool purchase. For ongoing plumbing work, invest in a crimp or cinch tool and buy PEX in bulk.
Final Recommendations
Use PEX-B with cinch clamps for whole-house water repipes to save money on fittings. Use PEX-A for cold climate installs or radiant heat where expansion matters. Always use stainless braided supply lines at fixtures, never plastic. For gas connections, leave CSST to a pro but you can install a simple appliance flex yourself with yellow tape and a leak test. Each type of flexible plumbing pipe earns its place when matched to the right job.
