Plumbing Picks is reader-supported. As an Amazon Associate, we earn from qualifying purchases at no extra cost to you.
Quick answer from a licensed plumber: Black iron — schedule 40 steel pipe — is still the default for exposed gas piping in a mechanical room, a garage, or anywhere the pipe can get hit. CSST (corrugated stainless steel tubing) wins where you are fishing a line through a finished structure and do not want to thread and assemble a dozen joints. The part people get wrong is not the material, it is the bonding, and it is not one rule for all CSST — the manufacturer’s own guide splits it by product. And a run labeled “1/2 inch” does not carry the same gas in both materials. Both facts are below, quoted from the pages they came off.
CSST vs black iron at a glance
| Black iron (schedule 40 steel) | CSST | |
|---|---|---|
| What it is | Rigid threaded steel pipe | Flexible corrugated stainless tubing with mechanical fittings |
| Joints | Threaded, one at every fitting and direction change | Often a single continuous run, fittings only at the ends |
| Install speed | Slow — measure, cut, thread, assemble | Fast, especially through finished framing |
| Impact resistance | High | Lower — the manufacturer says so in writing |
| Bonding | Bonded as ordinary rigid metal piping | Depends on the product — see below |
| Capacity at the same nominal size | Higher | Lower — roughly half at 1/2 inch |
| Best at | Exposed runs, mechanical rooms, garages, anywhere it can be struck | Retrofits, long fished runs, seismic areas |
The bonding rule is two rules, and that is the whole ballgame
Almost every article on this topic says “CSST has to be bonded.” That is close enough to be dangerous, because the requirement depends on which CSST is in your hand. Gastite’s own Design & Installation Guide (January 2020 edition) puts the two cases side by side in section 4.10, and I am quoting it rather than summarizing it:
“Unlike Gastite, there are no additional bonding requirements for FlashShield+ imposed by the manufacturer’s installation instructions. FlashShield+ is to be bonded in accordance with the National Electrical Code NFPA 70 Article 250.104 and Canadian Electrical Code, CSA-C22.1, in the same manner as the minimum requirements for rigid metal piping.”
And for the classic yellow product:
“Direct bonding of Gastite yellow CSST is required for all gas-piping systems incorporating Gastite CSST whether or not the connected gas equipment is electrically powered. This requirement is for single-family and multi-family buildings.”
Read that second one twice. Whether or not the connected gas equipment is electrically powered. A gas log set with no cord on it still triggers the requirement. That is the sentence I have seen missed on real jobs, usually with the reasoning “there is nothing electrical on it.”
The same section spells out what “direct bonded” means, and the specifics matter because an inspector will check them:
- “A single bond clamp attachment to rigid pipe or rigid component at any point within the gas piping system”
- “Bond clamp attachment downstream of individual gas meter or 2nd stage regulator for propane systems, and in accessible location”
- “Metallic contact is required (remove paint or plating on steel pipe)”
- “Bonding clamp listed to UL 467”
- “Bonding conductor is #6 AWG copper (minimum) or equivalent, and not exceeding 75 feet in length”
- “Direct bonding to be performed by a person qualified to do so per local ordinances”
Two of those get fumbled constantly. The clamp has to bite clean metal — “remove paint or plating on steel pipe” is there because a clamp tightened onto black paint or galvanizing is a connection that measures fine on day one and does nothing later. And the bond lands on the rigid portion of the system, downstream of the meter, somewhere a person can actually reach it.
Black iron is not exempt from bonding in general — it is bonded as ordinary rigid metal piping under the electrical code. The difference is that yellow CSST carries an additional manufacturer requirement on top of that baseline.
Why the bonding requirement exists at all
This is the part where blogs reach for drama. The manufacturer’s own general warnings are more useful, and more measured:
“While Gastite/FlashShield+ provides significant advantages over more rigid gas delivery systems, its flexible design may make it more likely than steel pipe to be punctured by a nail or other sharp objects, or damaged by other extraordinary forces such as lightning strike, depending on the circumstances.”
“Properly bonding and grounding the Corrugated Stainless Steel Tubing (CSST) system may reduce the risk of damage and fire from a lightning strike.”
Note the verb: may reduce. The manufacturer does not claim bonding makes it immune, and neither will I. The physical reality is that CSST wall is thin compared to schedule 40 steel, so an electrical arc that a steel pipe would shrug off can perforate tubing.
There is a related routing rule in the same document that gets skipped more often than the bonding does: “Direct contact between electrically continuous metallic systems and Gastite yellow CSST is prohibited.” That means CSST resting against a metallic chimney liner, an appliance vent, metallic duct, or armored cable is a defect on its own — separate from whether you bonded the system.
#ad Buying for a shop? Amazon Business (opens in a new tab) accounts are free to create, with business-only prices and bulk quantity discounts.
If you work in the trade and want a note when a new guide goes up, leave your email below. No spam, no course funnel — written by a licensed Georgia plumber.
Capacity: the same number on the label is not the same pipe
This is the practical trap, and it is the one that turns into a callback when an appliance starves. CSST is sized by EHD (equivalent hydraulic diameter), not by the nominal pipe size stamped on steel. A “1/2 inch” CSST and a 1/2 inch black iron pipe are not interchangeable on a sizing chart.
Gastite’s guide happens to publish both tables under identical stated conditions — natural gas at 0.5 PSI or less, 0.5 inch WC pressure drop, 0.60 specific gravity — so this is an apples-to-apples read out of one document rather than two sources stitched together:
| Nominal size, 50 ft run | CSST (cubic feet per hour) | Steel IPS pipe (cubic feet per hour) |
|---|---|---|
| 1/2 inch | 42 | 73 |
| 3/4 inch | 87 | 151 |
| 1 inch | 191 | 285 |
At 1/2 inch, the CSST carries about 57 percent of what the steel pipe does over the same distance. Gastite’s steel table carries the note “Table reproduced from National Fuel Gas Code NFPA-54.”
So the honest framing is: CSST saves you labor, not size. If you swap a material without re-running the sizing, you can hand a tankless heater or a range a line that looked identical on paper and is not. Size the run for the appliance load, using the table for the material you are actually installing.
Where each one belongs
Reach for black iron when the pipe is exposed and can be struck — garage walls, mechanical rooms, along a basement ceiling where someone will eventually hang a bike on it. Also when you want the run to be trivially serviceable with ordinary tools; a threaded system can be broken down and re-made by anyone with a pair of pipe wrenches.
Reach for CSST when the run is long, concealed, and full of obstacles — the retrofit gas line to a kitchen island or a second-floor fireplace, where threading rigid pipe means opening walls you did not plan to open. Its own guide notes the flexibility “eliminates the repetitive measuring, cutting, threading and joint assembly that is common with installation of rigid steel piping systems,” and that it can move with the structure in seismic areas.
There is also a middle path worth knowing about: pressed steel fittings for gas, which get you rigid steel pipe without cutting threads. That is a different tradeoff from either material here, and I have written up how the press systems compare in MegaPress vs ProPress. If you go that route, the seal matters — press fittings for fuel gas use a different O-ring compound than water fittings, which I cover in what each press fitting seal is rated for.
What about galvanized?
“Black iron, not galvanized” is repeated as gospel, usually with the explanation that zinc flakes off and plugs burner orifices. I went looking for the code language behind that and want to be straight about what I found: none of the code pages I opened printed a prohibition on galvanized steel for fuel gas. The material sections I read name steel and wrought iron by schedule and standard without distinguishing the coating.
What the codes I opened do print, as adopted in those jurisdictions, is that “Steel, stainless steel, and wrought-iron pipe shall be at least Schedule 40,” that “Corrugated stainless steel tubing shall be listed in accordance with CSA LC 1,” and that “Cast-iron pipe shall not be used.” A separately published edition words the steel requirement as “Steel and wrought-iron pipe shall be at least of standard weight (Schedule 40) and shall comply with one of the following standards: 1. ASME B36.10 2. ASTM A53 3. ASTM A106.”
So treat “no galvanized for gas” as a local rule and a shop convention rather than a universal code citation — some jurisdictions and utilities absolutely do restrict it, and section numbers move between editions. Confirm with your local authority having jurisdiction before you buy pipe. If you are dealing with galvanized on the water side instead, that is a genuinely different conversation and it is covered here.
Joints, sealant, and the small stuff that fails
On threaded black iron, the joint compound is not optional and not interchangeable with the water-side product. Gas-rated PTFE tape is the yellow one, and it is thicker than the white tape used for water — a detail covered in the plumbing thread tape guide. Using white tape on gas threads is the single most common DIY defect I see on this material.
On CSST, the failure modes are mechanical rather than chemical: tubing routed against a metallic vent, an unprotected run passing too close to a stud edge without a striker plate, or a bond clamp landed on painted pipe. None of those leak on the day of the install, which is exactly why they get signed off.
Permits and who is allowed to do this
Fuel gas work is licensed, permitted work in most jurisdictions, and CSST specifically carries manufacturer training expectations on top of that. Gastite’s guide states plainly that direct bonding is “to be performed by a person qualified to do so per local ordinances.” This is not a topic where a homeowner should be piecing it together from a video. Pull the permit, get the inspection, and if a shop tells you a gas line does not need one, that is information about the shop.
FAQ
Does all CSST have to be bonded?
No — the requirement depends on the product. Gastite’s guide states that direct bonding of its yellow CSST is required for all gas-piping systems incorporating it, whether or not the connected gas equipment is electrically powered, while its jacketed FlashShield+ product has no additional manufacturer bonding requirement and is bonded in the same manner as rigid metal piping. Check the installation instructions for the exact product you bought, and follow local code where it is stricter.
Is 1/2 inch CSST the same as 1/2 inch black iron pipe?
No. Under identical published conditions of 0.5 PSI or less and 0.5 inch WC pressure drop, a 50 foot run of 1/2 inch CSST is rated at 42 cubic feet per hour while 1/2 inch steel pipe is rated at 73. CSST is sized by equivalent hydraulic diameter rather than nominal pipe size, so you must size the run from the table for the material you are actually installing.
What size bonding wire does CSST need?
Gastite’s installation guide specifies that the bonding conductor is #6 AWG copper minimum or equivalent, and not exceeding 75 feet in length, attached with a bonding clamp listed to UL 467. The clamp must make metallic contact, which means removing paint or plating on steel pipe first.
Can CSST touch a metal vent or duct?
No. Gastite’s guide states that direct contact between electrically continuous metallic systems and its yellow CSST is prohibited, and directs installers to maintain as much isolation and separation as reasonably possible from metallic chimney liners, appliance vents, metallic ducting and piping, and jacketed electrical cables.
Is black iron better than CSST?
Neither is better in general. Black iron resists impact and is easy to service with ordinary wrenches, which is why it remains the default for exposed runs in garages and mechanical rooms. CSST installs far faster through finished framing and can run continuously without intermediate joints. The manufacturer notes that CSST’s flexible design may make it more likely than steel pipe to be punctured by a nail or damaged by a lightning strike, which is the tradeoff you are accepting for the speed.
Sources
- Gastite / Titeflex Corporation, Gastite & FlashShield+ Design & Installation Guide, January 2020 — Section 1.1 General User Warnings and Section 4.10 Electrical Bonding, and the capacity tables in Section 7 (Table 7-1, CSST natural gas; Table 7-23, steel IPS pipe, noted as “reproduced from National Fuel Gas Code NFPA-54”).
- UpCodes, gas piping materials sections as adopted — “Steel, stainless steel, and wrought-iron pipe shall be at least Schedule 40,” “Corrugated stainless steel tubing shall be listed in accordance with CSA LC 1,” and “Cast-iron pipe shall not be used” (Hawaii Plumbing Code 2021, Section 1208.6); and “Steel and wrought-iron pipe shall be at least of standard weight (Schedule 40) and shall comply with one of the following standards: 1. ASME B36.10 2. ASTM A53 3. ASTM A106” (Texas, 2018 adoption, Section 1208.6.3.1).
- Referenced within those documents and named here only as the sources name them, without any claim about their contents: NFPA 70 (National Electrical Code) Article 250.104, ANSI LC 1 / CSA 6.26, and UL 467. Section numbers move between code editions — confirm the current adopted edition with your local authority having jurisdiction.
Written by Isaac Daniels, licensed Georgia plumber #JP507711. Gas piping is permitted, licensed work in most jurisdictions; this article is reference material, not a substitute for a permit and an inspection.
Fix the 12 Most Common Plumbing Problems Yourself — Free Plumber’s Handbook
Step-by-step fixes for the 12 most common problems — and exactly when to call a pro. Free and instant, from a licensed plumber. Save hundreds on calls you didn’t need.