How to choose the right connecting and fastening elements for the bathroom and plumbing
How to choose the right connecting and fastening elements for the bathroom and plumbing
A bathroom installation consists of dozens of small, seemingly inconspicuous elements – threaded connections, mounting brackets, connectors, seals, and mounting accessories. It is precisely these "insignificant details" that determine whether the entire system will be tight, stable, and trouble-free for decades, or whether you will become a regular customer of an emergency service. From practice, I know that most complaints in bathroom installations do not stem from a poor-quality boiler or battery – the problem is almost always in an incorrectly selected or improperly mounted connecting part.
This article will give you a comprehensive overview: what each connection and fastening system actually is, how to choose them, which dimensions and materials matter, and how to avoid mistakes that you will later pay for twice. We will focus on the bathroom and plumbing in the broadest sense – from expansion tanks through batteries to suspended toilet bowls and hidden installation frames.
Why the choice of connecting elements is really important
Connecting and fastening elements in the bathroom operate in an environment that is particularly demanding for metals and plastics: constant humidity, temperature fluctuations, aggressive cleaning agents, vibrations from pumps and boilers, and in the case of heating technology, also a medium pressure usually ranging between 1.5 and 3 bar. Add to that the fact that bathroom installations are mostly hidden behind tiles or drywall – and you will understand why a poor choice of element can lead to a very expensive failure without any warning.
In practice, I encounter several typical failure scenarios:
- Using a metric thread where a pipe thread should have been (or vice versa) – the connection looks tightened, but it leaks right after the first filling of the system.
- Underestimating the load capacity of the fastening element – the bracket or suspension system deforms when the tank or device is full of water.
- Mixing materials prone to galvanic corrosion – for example, galvanized steel directly on a copper pipe without a dielectric insert.
- Using a sealing material incompatible with the medium – common hemp thread on plastic threads or for drinking water without the appropriate certification.
Each of these problems can easily be avoided by making the right choice even before installation. Let's therefore systematically look at what you need to know.
Types of threads in plumbing installations: what do the letters G, R, M and Rp mean
This is an area where the most confusion – and also the most damage – arises. In the bathroom and plumbing technology, we encounter several standardized thread systems that are not interchangeable, although they may seem similar at first glance.
Pipe threads according to ISO 228 (designation G): These are cylindrical pipe threads with a 55° profile. Sealing here is not achieved by the geometry of the thread, but by an external sealing element – O-ring, flat sealing washer, or sealing tape/wool on the thread. Typical examples from the bathroom: valve connectors, air vents, pressure gauges. Most common sizes: G 1/2", G 3/4", G 1".
Tapered pipe threads (R / Rp): The designation R means a tapered external thread (taper male), Rp means a cylindrical internal thread intended for pairing with R. Sealing here is partially ensured by the geometry of the tapered shape, but it is still recommended to use a sealing tape. Typical use: taps, valves, expansion tanks – and precisely for this reason, it is important to understand what type of thread the connector actually has when using the mounting set for the expansion tank 3/4".
Metric threads (M): 60° profile, sharp peaks, size given in millimeters (M6, M8, M10...). They are mainly used for mechanical connections – mounting brackets, mounting fixtures to the wall, connecting load-bearing structures. A typical example: mounting bracket M8, which is used to anchor devices with an M8 thread – expansion tanks, boiler brackets, etc.
Combining these types in one assembly is common and correct – for example, an expansion tank has a connector in a pipe thread G 3/4" or R 3/4", but its mounting cap or bracket bolt is metric M8. Confusing these two systems – i.e., trying to fit a G 3/4" nut onto an M thread – is physically almost impossible, but experienced installers know that there are transition pieces, and one must be careful when using them.
Materials of connecting elements: what can withstand the bathroom and what cannot
The choice of material depends on three factors: environment (humidity, temperature, aggressiveness of the medium), mechanical load (weight, vibrations, pressure), and aesthetics (visible elements must match the interior).
Brass and nickel-plated brass
Brass (an alloy of copper and zinc) is still the gold standard for pressure fittings in bathroom installations. It is highly resistant to corrosion in a humid environment, easy to process, and compatible with most sealing materials. Nickel- or chrome-plated brass is also used for aesthetically exposed elements – rings, nuts visible behind taps. Disadvantage: higher weight and price compared to plastic, and when in contact with stainless steel or aluminum, the risk of galvanic corrosion – a dielectric insert or insulation should be used.
Stainless steel (AISI 304 and 316)
Stainless steel is the material of choice for visible fastening elements in the bathroom (bolts, wall anchors, suspensions), because it does not corrode even under constant humidity. In plumbing installations for drinking water, it is mandatory to use stainless steel certified for contact with drinking water. AISI 316 (with added molybdenum) is more resistant to chlorides – relevant, for example, in pool areas or with softened water with a higher chloride content.
Zinc-coated steel
Cheap, but problematic. In the bathroom, zinc-coated steel is suitable only for hidden load-bearing structures – for example, mounting frames for suspended toilets, where it is permanently dry. Never use zinc-coated screws or nuts on visible surfaces or in permanently damp environments – the zinc layer oxidizes over the years and leaves brown stains.
Plastics (PP, PA, ABS)
Plastic fasteners (most commonly polypropylene or polyamide) are used in low-pressure connections, waste lines, and installations where galvanic corrosion is a risk. Disadvantage: lower mechanical strength, sensitivity to UV radiation and solvents in some cleaning agents.
Fixing equipment in the bathroom: brackets, anchors and mounting accessories
Mechanical fastening of sanitary equipment – water heaters, expansion tanks, sinks, toilets – is an area where errors may only become apparent after years. The equipment appears stable, but the bracket slowly loosens the anchoring until a critical situation occurs.
Wall anchoring: anchors, fasteners and base materials
The first question is always: what is the wall made of? Solid brick, hollow brick (e.g. Porotherm), aerated concrete (Ytong), concrete or drywall – each material requires a different type of anchoring element. From practice: I most often deal with cases where the installer chose an anchor "at a glance" into aerated concrete, and after a year the bracket with the water heater tilted by 10 centimeters.
- Solid brick / concrete: Standard expansion or threaded anchors (nylon or steel). For loads over 30 kg, I recommend chemical anchor adhesives – they achieve tensile values of up to 15–25 kN at a single anchoring point.
- Aerated concrete (Ytong, Siporex): Only special anchors for aerated concrete (e.g. with a threaded steel body), or chemical adhesive with a collar. A standard nylon anchor in aerated concrete carries only a fraction of the nominal load.
- Hollow brick (Porotherm): Expansion anchor with a bevel, which expands behind the hollow inside the brick. Without the correct type of anchor, the anchor does not hold at all.
- Drywall: For loads up to 5–8 kg, you can use molly screws or drywall anchors. For heavier equipment (water heaters, storage heaters), it is mandatory to anchor into the load-bearing SK frame or use an additional beam board.
Mounting accessories for threaded connections of equipment
Modern sanitary equipment – expansion tanks, small pressure vessels, flow heaters – are equipped with standardized threaded connections. In order to securely and aesthetically attach them to the wall, there are special mounting accessories that combine the function of a hanging element and a threaded adapter.
A typical example from practice: an expansion tank with a G 3/4" thread installed in a technical room. The tank is full of water and expansion medium, its weight can be up to 8–15 kg. Hanging it directly on a pipe is not the correct solution – a combined set must be used, which supports the tank mechanically from the wall and at the same time ensures a tight connection. For this purpose, the expansion tank mounting set 3/4" is used, which includes all the necessary elements including a bracket and seals.
For mechanical connection of equipment with an M8 thread (common with brackets, clips and mounting rails), it is crucial to use a correctly dimensioned mounting accessory. Mounting accessory M8 is intended precisely for these purposes – it ensures a firm, vibration-resistant connection without the risk of spontaneous loosening. More about when and why to use exactly M8 and not another size can be found in the related article Thread M8 vs. other sizes: which mounting accessory do I need.
Seals in plumbing connections: material, dimensions and correct application
The seal is as important in a plumbing installation as the thread itself. An improperly chosen seal is the cause of at least a third of the complaints from my experience as a contractor.
PTFE tape (Teflon)
The most common sealing material for pipe threads. Apply in the direction of the thread (in the tightening direction), 3–5 layers. Advantage: chemically inert, resistant to high temperatures (up to 260 °C), does not harden. Disadvantage: for conical threads under higher pressure (over 6 bar), it is more reliable to use a combined sealing compound. Note: PTFE tape is not suitable where the standard requires a re-attachable connection – after unscrewing, the tape is damaged and a new one must be applied.
Flax rope with sealing compound
A traditional method, still valid and reliable for metal threads. Flax rope (only certified for potable water if it is a water supply system!) is wound in the direction of the thread, then covered with sealing compound. The combination ensures excellent tightness even after decades. Not suitable for plastic threads – the rope with compound can mechanically damage the plastic thread when over-tightened.
O-rings and flat seals
For cylindrical pipe threads (G), sealing is achieved with a flat rubber seal under the nut or an O-ring in the recess. Critical parameter: the dimensions must fit precisely – the O-ring must not be even a millimeter smaller (leakage) or larger (inability to close). Common dimensions in plumbing: G 1/2" → O-ring 18×2 mm, G 3/4" → O-ring 24×2 mm or 26×2 mm (depends on the valve manufacturer). Always check the technical documentation of the product.
Adhesives and sealing compounds
For HT waste PVC pipes and fittings, PVC adhesives based on solvents are used. The joint is permanent and very strong after hardening. Never mix adhesives from different manufacturers and always respect the curing time (usually 24 hours at room temperature) before pressure loading.
Specific situations from practice: when standard solutions are not enough
Vibrations from pump or boiler
In the case where the mounted device is a source of vibrations (circulation pump, compressor, boiler with combustion fan), an anti-vibration mounting must be used. A rigid connection transfers vibrations to the wall and gradually loosens the anchoring and threads. Solutions include rubber gaskets, anti-vibration pads under the bracket, or flexible hose couplings in the piping before and after the device.
Expansion in high-temperature piping
Heating pipes expand when heated – a copper pipe DN 22 expands by 1 mm per meter of length at 70 °C compared to the state at 20 °C. If the clamps on the pipe are mounted too tightly without expansion clearance, the pipe will bulge or the joint will loosen. Every third clamp should be a "fixed point" (fixed), the rest "sliding points" (allowing movement).
Electrochemical corrosion with mixed materials
If you install a copper pipe into a steel bracket without insulation, or a galvanized fitting directly to a brass valve, a galvanic cell is created. Result: intense corrosion of the weaker metal – usually zinc or aluminum. In practice, this looks like a joint that has disintegrated after 3–5 years, although both parts look new from the outside. Solution: dielectric couplings (plastic insert between metals), or use the same material throughout the entire joint.
Dimensions and standards: what you find in documentation and on the market
Orientation in dimensions is one of the most commonly overlooked areas. Here are the key dimensions you will encounter most often in bathrooms and plumbing systems:
Pipe threads G (ISO 228): G 3/8", G 1/2", G 3/4", G 1", G 1 1/4", G 1 1/2", G 2". In practice, G 1/2" is dominant for faucet connections and corner valves, G 3/4" for expansion tanks, air vents and larger valves, G 1" and larger for manifolds and distributors.
Metric threads (ISO 261): M4, M5, M6, M8, M10, M12. In plumbing, the most common are M6 (mounting lighter sanitary fixtures, toilet seat installation), M8 (expansion tanks, boiler consoles, valve frames), M10 and M12 (heavier suspended systems, load-bearing structures). The M8 thread has a pitch of 1.25 mm and a core diameter of 6.647 mm – these parameters are key in calculating the joint's load capacity. A more detailed comparison can be found in the article M8 Thread vs. Other Dimensions: What Mounting Accessories Do I Need?.
Drain pipe diameters: DN 32 (basin, bidet), DN 40 (bathtub, shower), DN 50 (toilet in some systems), DN 100 / DN 110 (waste branch for toilet). The connecting range here includes various types of rubber seals, plastic neck joints and inspection pieces.
For those who want to go deeper into the topic, we recommend the article What Thread and Diameter to Choose for an Expansion Tank – there you will find a detailed table of dimension pairings according to the tank volume and system pressure.
Practical Tips for Installation: What I Know from the Field
Theory is important, but practice brings situations that no standard explicitly considers. Here are tips I would give to anyone tackling a bathroom installation:
- Always check the thread before purchasing a valve: Older residential buildings sometimes have non-standard dimensions or heavily worn threads. If you are buying a replacement part, take the old part physically to the store or measure the diameter and pitch precisely.
- Do not overtighten plastic threads: Plastic connections (e.g., corner valves made of injection-molded plastic) have limits – usually 15–20 Nm. Over-tightening will cause the thread to crack from the inside, and the problem will only become apparent during a pressure test, not immediately.
- Photographic documentation before embedding: If you are hiding the installation, photograph the entire route including the location of joints, dimensions and types of components. This will save hours of searching in the event of a future failure or renovation.
- Pressure test before tiling: Never start tiling or drywalling before performing a pressure test on the entire system. Standard test: 1.5 times the operating pressure for 30 minutes.
- Check the load capacity of the console with the full device: Always calculate with the weight of the device when it is full of water, not when it is empty. An expansion tank of 12 liters weighs about 1.5 kg when empty, and up to 14 kg when full (water + pressure medium).
- Anti-corrosion protection for cut edges: If you have cut or drilled into galvanized steel, paint the cut edge with a zinc spray. Otherwise, pitting corrosion will start to develop on this surface immediately.
A deeper look at typical mistakes and how to avoid them can be found in the article Common Mistakes in the Installation of Connecting Components in the Bathroom and How to Avoid Them. If you are dealing with a leaking joint, I recommend reading the article Why a Joint Leaks at an Expansion Tank: Causes and Solutions.
Expansion tanks and their mounting kits: a comprehensive view
An expansion tank is a device that is almost always found in bathroom and heating installations – it compensates for the thermal expansion of water in a closed system and maintains pressure within a safe range. Its correct mounting is a matter that surprisingly receives little attention.
Expansion tanks are available in various volumes (from 2 liters for small apartment circuits up to 200 liters or more for large systems). Their connection is typically G 3/4" (cylindrical external thread) or R 3/4" (conical). They are mounted on a wall or ceiling either using an integrated bracket (tanks with a flat back panel) or using an external mounting kit.
Mounting kit for an expansion tank 3/4" is designed for this type of device – it includes a bracket, seals and connecting material, so the installer has everything in one package without the need to improvise with parts from different sources. This is important because an incorrectly assembled mounting kit may have a lower load capacity than required by a fully filled tank. A detailed step-by-step installation procedure can be found in the article Installation of Mounting Components for Expansion Tanks and Sanitary Fixtures Step by Step.
Another aspect: tank location. Most diaphragm expansion tanks are designed for vertical installation (connection at the bottom). Horizontal installation is only possible if the manufacturer explicitly allows it in the technical documentation – otherwise, the diaphragm will deform over time and the tank will lose functionality.
Comparison of types of mounting systems in plumbing
The market offers several different approaches to mounting sanitary fixtures and piping. There is no one universal solution – each has its optimal use:
- Fixed steel brackets (L-profile, U-profile): Strong, cheap, suitable for heavy devices (boilers, storage heaters). Disadvantage: fixed position, difficult to adjust after installation.
- Adjustable hanging systems (sliding nut tracks): Flexible positioning, suitable for distribution pipes, manifolds. Require wall anchoring at least every 1.5–2 m.
- Sanitary mounting frames (for suspended toilets, basins): Steel or aluminum frames anchored to the floor and wall, then covered with drywall. They transfer the load directly into the floor structure, not the wall. Recommended minimum load capacity of the frame for a toilet: 400 kg (standard EN 97).
- Clamp and bracket systems for pipes: Plastic or steel, in various diameters. Important: the correct internal diameter of the bracket for the given pipe (for Cu 22 mm you need a 22 mm bracket, not 20 or 25 mm).
A comparison of the different approaches in terms of cost, durability and ease of installation can be found in the article Comparison of Types of Mounting Systems for Plumbing.
Maintenance and inspection: what not to forget
Connecting and mounting components are not forgotten forever after installation. The recommended inspection frequency depends on the type of device and the intensity of operation, but in practice it applies:
- Once a year, a visual inspection of visible joints (looking for mineral deposits around the threads, which is a sign of micro-leaks, rust stains, cracks in the seals).
- Every 2–3 years, check the tightness of the expansion tank connection and check the pre-pressure of the membrane.
- After each incident (pressure relief valve opened, thermometer showed an anomaly), a complete inspection of all joints in the circuit.
Detailed instructions for regular maintenance and inspection can be found in the article Maintenance and Inspection of Mounting Components in Plumbing Installations.
Most frequently asked questions (FAQ)
What is the difference between G 3/4" and R 3/4" thread?
Both have the same diameter and pitch (14 threads per inch), but G 3/4" is cylindrical (parallel) – the diameter is the same along the entire length, sealing is achieved by an external sealing element. R 3/4" is conical (taper) – the diameter gradually narrows towards the tip, which partially contributes to sealing by the geometry itself. Connecting a cylindrical internal thread Rp with a conical external thread R creates a standard combination according to EN 10226-1 – this is common and correct. However, be careful when combining two conical elements or two cylindrical ones without sealing.
Can I mount a suspended toilet bowl only to drywall?
No. A suspended toilet bowl and its mounting frame must be anchored to a load-bearing structure – the floor or a masonry wall. Drywall serves only as a cover panel, not as a load-bearing element. The frame is anchored at a minimum of four points: two into the floor (floor legs of the frame) and two into the wall or ceiling structure. The load in real use (100 kg person + dynamic load) easily exceeds 200 kg of force, which no drywall mounting system can withstand.
How many layers of PTFE tape is enough?
Typically 3–5 layers for pipe fittings G 1/2" to G 1". More important than the number of layers is the direction of application (always in the tightening direction of the thread, i.e. clockwise when viewed from the tip of the thread) and even coverage without gaps or air bubbles. For larger diameters (G 1 1/4" and above) I recommend combining PTFE with sealing paste or using hemp rope.
Why do I need to use a special anchor for aerated concrete – isn't a regular nylon one enough?
Aerated concrete has a spongy structure with large cavities. A regular nylon expansion anchor expands into these cavities during tightening, but does not find sufficient solid support – the resulting tensile strength is only a fraction of the nominal value (typically 20–30 % of the value for a solid brick). A special anchor for aerated concrete has a longer shank and a different expansion mechanism that evenly loads a larger volume of material. For critical applications (water heater, water tank) I recommend chemical anchor adhesive with a perforated insert – it achieves significantly higher values even in aerated concrete.
Can I use the same type of gasket for potable water and for heating medium?
It depends on the gasket material. NBR (nitrile rubber) is suitable for both heating medium and potable water, but for potable water the gasket must be certified according to the relevant standard (e.g. DVGW W 270 for contact with potable water). EPDM is an excellent choice for water and steam, but it cannot withstand contact with oil products or glycol in circuits with antifreeze mixtures. For circuits with antifreeze mixtures (glycol) PTFE or FKM (Viton) is the most suitable.
How can I tell that a joint is starting to leak before water reaches outside the hidden installation?
The most reliable indicator is an unexplained pressure drop in a closed system – if the pressure in the heating circuit repeatedly drops without an obvious cause (not through the air vent), it is a strong indication of a micro-leak in the joint. In potable water systems, moisture sensors installed behind the drywall can help – today they are available for a few euros and can detect moisture long before it becomes visible on the wall. A physical inspection of the pipe behind the wall can be done with a borescope camera through an inspection opening.
Conclusion: Choosing good connecting elements is an investment, not an expense
Connecting and fastening elements make up a small fraction of the total cost of a bathroom installation – but a poor choice can lead to damage that many times exceeds the total installation cost. When chosen correctly, these elements will go unnoticed for decades. When chosen incorrectly – they will call you on a Saturday morning with water dripping through the ceiling.
Key rules for correct selection: distinguish the type of thread (pipe vs. metric), choose a material suitable for the environment and medium, verify the load capacity for the actual load including the full equipment, never forget the gasket and its compatibility with the medium, and always anchor into the load-bearing material, not just the surface layer. In case of doubts, consult the manufacturer's technical documentation or read our Knowledge Center articles – for example Common questions about connecting and fastening systems for bathrooms and sanitation.
Do you have a question about this topic?
Having trouble deciding or dealing with a specific situation in your home? Write to us – we are happy to help.