Sealing Materials for Threaded Connections – PTFE Tape, Hemp Fiber or Gasket
Sealing Materials for Threaded Connections – PTFE Tape, Hemp Fiber or Gasket?
When an installer or a skilled DIY handyman finishes a threaded connection and turns on the water, one of two scenarios usually occurs: either the joint stays dry and secure, or it starts to drip. And in most cases when it drips, the problem isn't the thread itself – the problem is the sealing material, or rather its absence, incorrect choice, or improper application. Over years of working with heating technology, I've seen dozens of cases where someone installed quality 1/2x16 screw fittings with male thread and yet, after starting up the system, everything leaked – simply because the sealant was applied incorrectly or an unsuitable material was chosen for the given type of medium.
This article focuses precisely on this topic: how sealing materials work on threaded connections, what the differences are between PTFE tape, hemp fiber, and rubber gaskets, where each material is suitable and where it absolutely isn't, and what exactly to do so that the joint lasts for years without leaking. We also cover the topic of proper installation in a separate article Installing a Screw Fitting on a Pipe Step by Step and the causes of leaks in the article Causes of Water Leaks in Threaded Connections.
Why do we even need a sealant on threaded connections?
A standard pipe thread – whether BSP (British Standard Pipe), metric, or NPT (National Pipe Taper) – is not designed to form a watertight joint on its own. The thread is primarily a mechanical connection that holds two parts together. Microscopic gaps always remain between the thread surfaces – not because the thread was manufactured carelessly, but because producing a geometrically perfect thread without any gaps would be technically extremely costly and, in practice, unnecessary, since much simpler solutions exist.
These microscopic gaps are enough for water, steam, or gas to flow through under pressure. The sealing material therefore serves to fill all these gaps while creating a solid, elastic barrier that prevents the medium from leaking. A proper sealing material must be compatible with the medium (water, gas, steam, oil), resistant to temperature and pressure, and must not degrade or harden over time to the point of losing its sealing ability.
PTFE tape – the king of modern seals?
PTFE tape, correctly called PTFE (polytetrafluoroethylene) tape, is today probably the most widespread sealing material for threaded connections. You can find it in every sanitary supply store, it's cheap, easily available, and at first glance very simple to use. But it's precisely that apparent simplicity that causes many people to apply it incorrectly.
How PTFE tape works and its properties
PTFE is a chemically inert substance, meaning it reacts with practically nothing – not with water, most acids, bases, oils, or most industrial media. It is resistant to temperatures from -200 °C to +260 °C (even more with special types), which makes it suitable for most applications in standard heating and water installations. It has excellent chemical resistance and, when applied correctly, creates a reliable seal.
It works mechanically: the wound tape compresses between the thread surfaces as the joint is tightened, filling the gaps. Unlike hemp fiber or rubber gaskets, it doesn't require any auxiliary agents (pastes, sealant), it seals sufficiently on its own.
Several types of PTFE tape are available on the market:
- Standard white PTFE tape (thickness approx. 0.075 mm) – suitable for cold and hot water, low-pressure applications. Width usually 12 mm or 19 mm.
- Thicker PTFE tape (0.1 – 0.15 mm) – better for larger threads or worn threads, where more material is needed.
- Yellow PTFE tape for gas – specially thickened, resistant to propane, butane, and natural gas. NEVER mix it with regular white tape on gas pipelines!
- Expanded PTFE tape – softer material, even better compressibility, also suitable for non-standard threads.
Correct application of PTFE tape – step by step
The most common mistake: the tape wound in the wrong direction, too few layers, or on the contrary too many. The tape should always be wound in the direction of the thread – i.e., so that it tightens further, rather than unwinding, as it is screwed in. Specifically: when you hold the male thread with the tip facing away from you, wind the tape clockwise.
The number of layers depends on the tape thickness and thread size. For standard installation threads 1/2" and 3/4", 3 – 5 layers of standard tape are sufficient. For larger threads or worn threads, 5 – 8 layers. The tape starts right at the beginning of the thread (not at the very end) and is wound from the end of the thread inward – so the first layer is at the end, the last one closer to the fitting body.
A practical example from a job: A customer had a leaking problem on a 1/2x20 screw fitting with male thread, which he had installed himself. After disassembly, it turned out he had wound the tape "from body to tip" – i.e., in the opposite direction. Result: when tightening, the tape unwound and practically nothing remained on the thread. All that was needed was to disassemble the joint, clean it, and apply the tape correctly.
Hemp fiber (flax fiber) – a traditional sealant that still makes sense
Hemp fiber, also called flax fiber or simply "tow", is the oldest and yet still very reliable method of sealing threaded connections. In the past, it was used almost exclusively; nowadays PTFE tape has replaced it in common applications, but in many situations it is still irreplaceable.
How hemp fiber works
Hemp fiber alone doesn't seal – it achieves a seal only in combination with a sealing paste (most often linseed oil varnish, a special installation paste, or oil). The fiber mechanically fills the gaps in the thread, and the paste penetrates the fibers, hardens, and seals any remaining gaps. The advantage of hemp fiber is its uneven, fibrous structure – it can fill even larger and irregular gaps, which is an advantage, for example, with older or slightly worn threads.
Hemp fiber is traditionally combined with:
- Boiled linseed oil varnish – a classic, proven for decades. Hardens in air, resistant to water. Not suitable for gas.
- Special installation pastes (e.g. Loctite 55, Henkel Tangit Uni-Lock, etc.) – a more modern alternative, faster drying, higher reliability.
- White engineering paste – also suitable for higher pressures and temperatures.
When hemp fiber outperforms PTFE tape
There are several situations where experienced installers prefer hemp fiber over PTFE:
- Old or worn thread – the fiber is thicker and more compressible, able to fill larger gaps.
- Larger threads (1" and above) – on large threads, it's much harder to apply PTFE tape correctly; hemp fiber is more reliable.
- Joints that won't be regularly disassembled – hemp fiber with paste creates a stronger, more durable joint. Once cured, it's much harder to take apart.
- Cast iron and steel fittings – where the material's microporosity requires greater penetration of the sealant.
Disadvantages of hemp fiber: Moisture can cause swelling and deformation of the joint over long-term exposure. With some pastes, there is a risk of corrosion. Application is less convenient than PTFE tape. Not suitable for gas without a special gas paste. And above all – hemp fiber is not suitable for plastic threads, where the pressure of the wound fiber may damage the thread.
Correct application of hemp fiber
The fiber is wound similarly to PTFE tape – in the direction of the thread. First, the thread is coated with paste, then the wound fiber is pressed into the thread profile with a finger. The amount of fiber must be such that the thread offers noticeable resistance when tightening, but doesn't crack the fittings. A common mistake: too little fiber – the joint doesn't create enough pressure and leaks. Too much fiber: the thread can't be tightened to the correct position and the plastic fitting cracks, or the fiber tears unevenly.
Rubber gaskets, O-rings, and flat seals – when they replace thread sealing
Rubber gaskets form an entirely different category. They are not a material that is wound or inserted into the thread – these seals seal on a surface, not in the thread. They're used in combination with joints where one surface rests against another (so-called face sealing), or in the form of O-rings, which seal radially.
Flat rubber gaskets (surface seals)
Typical use: hoses, clamps, flanges, faucets. In the world of threaded connections, such as those solved when installing, for example, a 3/4x20 screw fitting with male thread or when connecting to wall outlets, we may encounter a flat gasket in joints where fittings sit face to face. In such cases, it is a serious mistake to combine a face rubber gasket with PTFE tape or hemp fiber – the rubber gasket must sit cleanly on the surface; any other material in the thread disrupts this function and the joint may push out the gasket under increased pressure.
O-rings
O-rings are rubber rings with a round cross-section that seal in grooves. In threaded connections, they occur, for example, in quick couplings, some types of air vents, consumption meters, etc. They are made from various materials – EPDM (most common, for water and heating up to 130 °C), NBR (for oil and fuels), FKM/Viton (for aggressive media and high temperatures).
Special sealants – paste and putty
In addition to solid seals, there are also liquid or paste-like sealants – anaerobic sealants (Loctite 542, 243, etc.), which cure only inside the thread without access to oxygen. These products are excellent for metal threads, where high joint strength with temperature resistance is needed. Paste sealants (e.g. Tangit, Unipak) are applied directly to the thread and can be used alone or in combination with hemp fiber.
Specifics of sealing joints on PEX and PP plastic pipes
Most screw fittings in modern heating and water installations are made on plastic pipes – most often PEX (cross-linked polyethylene) or PP (polypropylene). These pipes have different sealing requirements than metal piping.
For plastic threads, the basic rule applies: never use hemp fiber. Plastic threads are finer and less resistant to pressure loading. When tightened, hemp fiber exerts pressure on the thread from outside and can damage or tear it. PTFE tape is the clear choice for plastic threads – it's thin, distributes evenly, and doesn't exert excessive pressure.
The same applies to joints between plastic fittings and metal threads (e.g. a brass fitting) – PTFE tape or special paste, not hemp fiber.
A special situation arises with the universal simple wall outlet holder for water and universal double wall outlet holder for water, where the threaded connection is made between the brass threaded part and the plastic pipe or the inter-ceiling line. Here it is always appropriate to use PTFE tape, possibly in combination with sealing paste, but hemp fiber has no place in this type of joint. You can read about when to choose a simple and when a double holder in a separate article Universal Wall Outlet Holder for Water – Simple vs. Double and When to Use Which.
Temperature and pressure – limits of individual seals
The choice of sealant must take operating conditions into account. The following overview will help you understand the limits of commonly available materials:
| Sealant material | Max. temperature | Max. pressure (common install.) | Suitable media |
|---|---|---|---|
| White PTFE tape | up to +260 °C | up to 10 bar | water, steam, air, most liquids |
| Yellow PTFE tape (gas) | up to +200 °C | up to 5 bar | natural gas, propane, butane |
| Hemp fiber + linseed varnish | up to +120 °C | up to 10 bar | water, heating, steam up to 120 °C |
| Hemp fiber + (special) paste | up to +140 °C | up to 16 bar | water, heating, gas (with gas paste) |
| EPDM rubber (O-ring/gasket) | up to +130 °C | up to 10 bar | water, heating, drinking water |
| Anaerobic sealant (Loctite) | up to +150 °C | up to 20 bar | metal threads – water, heating, hydraulics |
Important note: for installations with heating media above 90 °C (high-temperature heating, steam systems), it's always necessary to verify whether the manufacturer has certified the specific product for those conditions. Standard white PTFE tape can technically withstand higher temperatures, but with repeated temperature cycles (cold – hot – cold), it may lose elasticity faster than at a stable temperature.
Most common mistakes when applying sealing material and how to fix them
From experience, I know that 80% of leaks in threaded connections aren't caused by a bad product, but by an installation mistake. Here is an overview of the most commonly seen problems:
1. Wrong winding direction of PTFE tape
As mentioned above – the tape must be wound in the direction of the thread, not against it. If wound in the opposite direction, the tape unwinds during tightening and falls out of the thread. Result: zero sealing. Solution: disassembly, cleaning, new tape, correct direction.
2. Too little or too much sealing material
Too little: gaps remain unfilled, the joint leaks. Too much: PTFE tape tears, hemp fiber prevents the thread from tightening to the correct position, fittings may crack. For common 1/2" and 3/4" threads, the rule of 3-5 layers of 0.075 mm PTFE tape applies, or a corresponding amount of hemp fiber (roughly five to six wraps with a finger).
3. Greasy or wet threads during application
PTFE tape on a greasy or wet surface doesn't hold properly – it slips and doesn't seat. Hemp fiber with linseed varnish doesn't cure well on a wet surface. The thread must be clean and dry. When repairing old joints, the thread should be degreased and thoroughly dried (where possible).
4. Backing off the joint after sealing
A tightened joint should not be turned back – any counter-rotation disrupts the seal. This is especially important with PTFE tape. If the fitting is in the wrong position, it must be disassembled, the thread cleaned, and the process started over with new sealant. Never back off more than 1/4 turn!
5. Combining incompatible materials
For example, a face rubber gasket plus PTFE tape in the thread – this doesn't create a better seal, but on the contrary can damage the joint. Or PTFE tape plus hemp fiber at the same time – completely unnecessary and counterproductive. One material, applied correctly, is always better than two incorrectly combined ones.
We cover the causes of leaks in more depth in the article Causes of Water Leaks in Threaded Connections – Most Common Mistakes and Their Repair, where you'll also find a procedure for diagnosing exactly where the leak is coming from.
Sealing for different thread types – BSP, NPT, metric
In European installation practice, we mainly encounter three types of threads:
- BSP – British Standard Pipe (G thread) – a parallel thread, used throughout Europe including Slovakia. Designated G 1/2", G 3/4", etc. This thread doesn't seal on its own in the thread – the seal must be either in the thread (PTFE, hemp) or on the face (rubber gasket).
- BSPT – British Standard Pipe Taper (R thread) – a tapered thread, where the thread itself wedges as it is tightened and contributes to sealing. Despite this, applying PTFE tape or hemp fiber is still recommended. Designated R 1/2", R 3/4".
- NPT – National Pipe Taper – an American tapered thread, common on imported products. The taper is different from BSPT – these threads must not be combined! Sealing is the same as for BSPT.
For tapered threads (BSPT, NPT), fewer layers of PTFE tape (2-3) are usually sufficient, since the thread's wedging effect itself helps with sealing. For parallel threads (BSP/G), more layers are needed (4-6), since the thread itself doesn't seal. We also cover thread analysis in the article Male vs. Female Thread on Screw Fittings – What's the Difference and When to Use Which and in the article Screw Fitting Dimensions – How to Correctly Measure and Match 1/2" and 3/4" to 16 and 20 mm Pipes.
Practical tips from field experience
A few things I've learned from real practice and from fixing other people's jobs:
- Always check the thread before sealing – if the thread is mechanically damaged (broken, torn out), no sealant will help. The thread must be replaced.
- When repairing, always completely replace the sealant – tightening isn't enough. Old sealant (fiber or PTFE) is no longer able to reseal after the joint has been disassembled.
- Tighten plastic fittings by hand and then at most 1/4 – 1/2 turn with a wrench – you can use more force with brass or steel, but not with plastic. Cracking plastic threads is one of the most common unnecessary damages during installation.
- Don't use WD-40 or lubricating oils as a substitute for sealant – they are lubricants, not sealing materials. They might briefly "mask" a small leak, but within a week or two they break down and the leak gets worse.
- PTFE tape has a shelf life – old, brittle tape breaks and doesn't seal. If the tape cracks while winding, it's time to buy new tape.
- For threads larger than 1", make sure linseed varnish or paste is available – hemp fiber alone without paste isn't enough on large threads.
Frequently Asked Questions (FAQ)
Can I use PTFE tape on a gas connection?
Don't use regular white PTFE tape on gas pipelines – not because PTFE reacts with gas, but because white tape isn't dense or resistant enough for gas flow under pressure. For gas pipelines, there is a special yellow (or possibly pink or white marked "for gas") PTFE tape with higher density. Always check that the tape is certified for gas pipelines. Even better, for gas, use a special anaerobic paste or hemp fiber with a paste certified for gas – and have this done by a professionally qualified person.
What should I do if the joint still leaks even after resealing?
If the joint still leaks after proper disassembly, cleaning, and new sealing, look for a mechanical problem – a damaged thread, a cracked fitting, or an incorrect thread combination (for example, a BSP female and NPT male won't connect watertight even with a seal). In such a case, the damaged component must be replaced. Never repeat a third or fourth attempt with the same damaged thread – you're just wasting time and material.
How many layers of PTFE tape are correct for a 3/4" thread?
For a parallel G 3/4" (BSP) thread, I recommend 4 – 6 layers of standard white PTFE tape 0.075 mm thick. For a tapered R 3/4" thread, 3 – 4 layers are sufficient. If you have thicker tape (0.1 mm), use one or two fewer layers. When in doubt, err on the side of one more layer – excess tape gets compressed, missing tape doesn't help. You can also find specific thread dimensions in the article Screw Fitting Dimensions – How to Correctly Measure and Match 1/2" and 3/4" to 16 and 20 mm Pipes.
Is hemp fiber suitable for drinking water installations?
Yes, hemp fiber is suitable for drinking water, but only in combination with a paste certified for drinking water (food grade, or WRAS/KTW certification). Regular linseed varnish may release substances into drinking water that aren't approved for food-contact purposes. If you're unsure about the paste's certification, it's better to use PTFE tape for drinking water – it's chemically inert and safe for drinking water.
Can I combine PTFE tape and hemp fiber on one thread?
No – this is unnecessary and can be counterproductive. Each material works differently, and combining them won't provide "double" sealing. On the contrary: the fiber may prevent proper compression of the PTFE tape and vice versa. Choose one material suitable for the given application and apply it correctly.
How long must I wait after sealing with hemp fiber and linseed varnish before turning on the water?
It depends on the paste used and the temperature. Traditional (boiled linseed) varnish cures on the surface within 4 – 12 hours at room temperature, with full curing taking 24 – 72 hours. Modern installation pastes (e.g. Tangit, Unipak) allow the joint to be loaded after 30 – 60 minutes at room temperature. Always read the paste manufacturer's instructions and observe the curing time – loading the joint with pressure too early can damage the seal before it has properly cured.
Conclusion – choosing the right sealant is the basis of a reliable installation
The sealing material for threaded connections is a seemingly trivial detail, but in practice it determines whether an installation will be trouble-free for decades or whether you'll be returning to the same joint every two years with a bucket and towels. The basic rules are simple: PTFE tape for plastic threads and common installations, hemp fiber with paste for metal threads and larger dimensions, rubber gaskets exclusively where the seal works on a surface. For gas, always a specially certified solution.
It's equally important to verify that the fittings themselves are correctly sized – for example, when working with 1/2" and 3/4" threads and 16 and 20 mm pipes, the article How to Choose the Right Screw Fitting for Pipes – Diameter, Thread, and Material will help you get oriented. And if you're interested in the complete installation procedure from start to finish, check out the article Installing a Screw Fitting on a Pipe Step by Step, and for long-term reliability also Maintenance and Inspection of Threaded Connections – When to Tighten, Replace, or Treat a Joint.
Threaded connections are reliable and, with correct installation and appropriate sealing, will last the entire lifetime of the installation without problems. The key is always the same: the right material, correct application, and enough patience to do it properly the first time.
Do you have a question about this topic?
Can't decide or dealing with a specific situation in your home? Write to us - we'll be happy to help.