Context: DIY shelf with steel threaded pipes and fittings. (NOT plumbing! -- I guess such a construct does not make much sense there.)

Consider e.g. this example, if you look at it from the side, it has some "closed loops", where I define closed loop as: T-connector #1 -- pipe A -- T-connector #2 -- pipe B -- T-connector #3 -- pipe C -- T connector #4 -- pipe D -- T-connector #1. See also schema below:

#1 ├------------┤#2
   |            |
 D |            | B
   |            |
#4 ├------------┤#3

Question is: what is the way of building such a "loop"?

Of course, it is trivial to build one semi-loop with only 3 pipes (T-connector #1, pipe A, T-connector #2, pipe B, T-connector #3, pipe C, T-connector #4). But then, how do you insert pipe D?

Another idea would be to build first the sections T#1 - D - T#4 and T#2 - B - T#3 separately. And then somehow screw in pipes A and C simultaneously. However, (unless I'm mistaken) if you tighten pipe A in connector #1, then you are loosening it in connector #2 and vice-versa.

So it seems impossible to build such a loop, but obviously there is a way, as the example in the link (and many others) demonstrate.

  • Depending on the weight you expect to place on it, maybe consider PVC pipes instead. Usually glued on, slip on fittings.
    – crip659
    Jul 25, 2021 at 13:58
  • it is clearer if you call them gas pipes
    – jsotola
    Jul 25, 2021 at 17:52
  • Is it possible that all the T's along the front aren't actually threaded and might have a hidden set screw in them?
    – user113627
    Jul 25, 2021 at 22:25
  • @jsotola: I can change it. So you are saying these kind of pipes are used for gas and not for water?
    – Attilio
    Jul 26, 2021 at 7:38
  • 4
    Not being used to threaded pipe, it took me a while to realise what the problem was.
    – SQB
    Jul 26, 2021 at 8:27

9 Answers 9


The link shows a hidden trick. The front section and the rear sections are built separately, there are no closed loops in each piece. Then the short horizontal pieces are cut in half and screwed into the tees. The front and rear sections are joined by sliding the cut pieces into larger diameter horizontals. After a dry fit, the front and rear can be glued together with a small amount of adhesive. If you can find tees with smaller thread on the crossbar, then you can use a smaller half-sleeve which would be covered by the correct size horizontal pipe. It is easy once you find the right parts.

  • Well spotted in that blurry photo.
    – Jasen
    Jul 26, 2021 at 0:59
  • Nice, I think I get it. So there are small pipe "stubs" coming out from the T's, and those stubs are covered by a larger pipe, glued onto the stubs. Correct? Also, for my understanding: 1. what kind of glue is good for this purpose? 2. what is "dry fit"? 3. in this way, you won't be able to disassemble the shelf ever again, right? (Because it is glued together.)
    – Attilio
    Jul 26, 2021 at 7:37
  • 3
    @Attilio "dry fit" is connecting or placing parts together before using glue or means to bond them together to check how snug or aligned they are..
    – user113627
    Jul 26, 2021 at 13:03


Classically, for threaded pipes, you use a union, or two unions.

Modernly, it's trivial with push-to-connect fittings.

It's also classically trivial with soldered connections. Sometimes brazed rather than soldered, depending on the "industrial" nature of the process.

(for the example shown in the link, given no need to hold water, probably the side connection pipes are loose in the threads from having been partially unscrewed to make the connection; or else the threads have been drilled out on one side.)

For considerable investment in tooling, you could use left-handed and right handed threads on opposite ends of a pipe nipple so that it can be screwed into both ends at the same time to make tight connections. For a manufacturer who's found a market for things like you show in the linked picture, that could pan out; for a DIY project the costs would not make much sense.

  • > for the example shown in the link ... side connection pipes are loose in the threads Do you mean they first created the two separate sections 1D4, 2B3, then screwed in A and C into 1D4 and then they half-screwed it out from 1D4 to half screw in into 2B3?
    – Attilio
    Jul 25, 2021 at 14:20
  • 1
    OP is talking about building a shelf, so "industrial water pipes" likely means galvanized or black iron pipe. Unions and sharkbite fittings are not very... decorative.
    – Z4-tier
    Jul 26, 2021 at 0:56
  • It would also be easy with compression fittings, which are common here (UK) on our common water pipe but not, I believe in the US. There are also similar fittings for industrial pipes.
    – Chris H
    Jul 26, 2021 at 10:59

Using iron pipe with threaded fittings and ends, the "trick" used is to put inline joints at B and D so the pipework can be taken apart.

This is also done in real systems so that changes and repairs can be made easily. This is the type of connection, called an inline joint or coupling :

threaded pipe coupling

And it has a flat rubber type washer which is compressed by the nut. There are various sizes available.

  • What's the name for this fitting?
    – user113627
    Jul 25, 2021 at 22:26
  • @GWarner inline joint or coupling...
    – Solar Mike
    Jul 26, 2021 at 4:04
  • sounds interesting: could you please expand on how this in-line joint works? (did not find it with a quick search). If I understand correctly, the threaded part (visible on the picture) would go e.g. into T#4, right? And the end of pipe D would go into the part which is semi-hidden in the picture? But what is in that part, and how does it get fixed to the end of the pipe?
    – Attilio
    Jul 26, 2021 at 7:44
  • 1
    @Attilio any decent plumbing store has them - some have exploded diagrams. It has 3 pieces and works as I explained.
    – Solar Mike
    Jul 26, 2021 at 7:46
  • 1
    @Paul_Pedant Also called "union" in the US.
    – Theodore
    Jul 26, 2021 at 17:42

If you want to make a loop, or repair a hole you drilled in a pipe without undoing all of it, there are unions, but those are large, and not legal to use inside walls (or so I’ve been told). The other clean option is a left/right nipple and a left/right coupling.


These are fittings with left handed threads so when you turn the nipple, it tightens both sides at the same time like a turnbuckle.

  • The rules against using them inside walls is obviously not applicable to this case since these pipes aren't being used for plumbing. Jul 26, 2021 at 16:36
  • Right, I was just explaining why they exist when a union is easier to find and use.
    – JPhi1618
    Jul 26, 2021 at 16:37

An alternative to threaded plumbing connections is to use T's and elbows (and corners) with set screws. Called 'structural pipe fittings' they actually have a threaded screw perpendicular to the pipe, and its tightened to keep pipes in place. I suspect they don't have the weight capacity of threaded or bonded (cemented) pipe.structural T with set screws][1]

source: lowes.com (entire webpage of structural fittings)

They are commonly available in a variety of metal types with a black or silver finish. They certainly won't look as 'industrial' as common black iron gas pipe though.

  • Just for my understanding: would this still be suited for its original purpose, i.e. transporting gas? Or is this already a gas-pipe looking element made on purpose for hobbists who want to build furniture out of it?
    – Attilio
    Jul 26, 2021 at 16:53
  • 2
    @Attilio - The large holes where the pipes are inserted are not threaded internally and do not provide any "seal" and are likely oversized to allow for easier assembly. So, they would not be suitable for transporting any sort of gas or liquid. It's strictly for building things like shelving, storage, furniture, etc. Jul 26, 2021 at 17:27
  • @Attilio The name of the fitting describes it's use as structural.
    – user113627
    Jul 26, 2021 at 20:30
  • They can be found in numerous sizes for different diameters of pipe. I used a stainless steel variety when I made a framework to hold up a canvas tent.
    – user113627
    Jul 30, 2021 at 14:01

If you are looking to pipe this as a closed loop, such as a configuration for a heating loop, all you need to do is start with one long pipe and add an elbow at one end, next is a nipple, then the next elbow, then the next long pipe and so on. this will give you a closed loop. IF this is not what you want, please explain what you want the end product to be and what you will be using it for.

  • Se OP: I'm trying to build a shelf out of industrial pipes, and this would be the frame.
    – Attilio
    Jul 26, 2021 at 7:46

There is also a slimline union method. You can get a straight iron (gas barrel) connector which is completely threaded on the inside. You cut (parallel) threads on one of the pipe ends to be connected so you can wind a locknut, and connector, completely onto it.

You thread the normal amount (tapered) of the end of the other pipe to be connected, apply tape or sealant, and back the connector off the pipe it is on, and onto the other pipe, until it seals on the taper.

You then tape and seal around the other end of the connector, and back the locknut up to it enough to seal that end too. It's not a great seal because the thread isn't tapered, but iron pipe fills any gaps with rust in a day or two.


Most threaded pipes an fittings for water use are tapered so that the fit gets tighter as you thread the pieces together.

If you get pipes that have straight threads for "A" and "C", you can first thread one end in farther than you need to, and then as you thread in the opposite end, it will unscrew, but not all the way.

Unfortunately, pipes and fittings with straight threads will probably be more expensive and harder to obtain than tapered.

Comparison of threads (from Parker Hannifin blog.) Comparison of threads

  • How good are you at welding?
    – Gil
    Jul 26, 2021 at 18:36
  • @Gil Haven't welded anything in 30 years. Soldering, sure.
    – Theodore
    Jul 26, 2021 at 19:55

You cheat

You make a saw cut halfway down the least observable part of the pipe. Now the 2 halves of that pipe can rotate independently.

How do you keep them physically aligned? Install inside it, a pipe or rod whose OD is slightly smaller than the ID of the pipe. This sleight-of-hand pipe won't have any strength in tension, so don't depend on it for that. I suppose you could drill holes and drive shear pins through it if you needed that.

  • The thing is, I want something that can bear lot of weight, I'm not interested too much in aesthetics. In that case, I'd rather simply omit the third pipe.
    – Attilio
    Jul 27, 2021 at 7:19

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