Plumbing Jointing Compound and Your Water System

Do you know if and/or what jointing compound is being used on your domestic water system, and the pitfalls if not used correctly, along with, is it helping or harming the water system's safety?

Plumbing jointing compounds (or pastes as they are also known as) have been a trusted part of water system installation practice for decades, which is applied to the threads of compression fittings to create a watertight seal. These jointing compounds remain widely used in domestic hot and cold water systems, commercial buildings, and healthcare environments.

As standards evolve and our awareness of water safety improves, so too must our approach to these jointing compounds, over recent years there have been changes to WRAS (Water Regulations Advisory Scheme) testing and certification for these products, alongside some concerns about potential bacterial contamination issues linked to compound overuse, meaning that installers and project/estate managers must now think more carefully than ever about product selection and application.

This blog explains what has changed and why knowing exactly what jointing compound is being used on your domestic hot and cold water system matters for compliance, performance, and public health.

WRAS Testing: From 85°C to 23°C What’s Changed? 

For many years, WRAS approval testing for jointing compounds was carried out at up to 85°C, a temperature chosen to reflect typical domestic hot water systems. This assured installers that a product approved by WRAS was safe for contact with potable water in both cold and hot systems. However, WRAS’s testing laboratory has now revised its testing methodology, reducing the test temperature to 23°C. This shift aligns with European standard testing protocols, which traditionally use room temperature conditions when assessing whether any substances may migrate into drinking water.

Q - Does this mean WRAS approved compounds are no longer suitable for hot water?

A - Not necessarily, as the key distinctions are:

• WRAS approval confirms water safety at room temperature
  
  
• The manufacturer must confirm temperature suitability for hot water (however, it may refer back to the WRAS approval).

Taking the above on board, this means installers must check the technical data sheet and not just rely on a WRAS badge to confirm the maximum recommended operating temperature.

Why the change matters

Some water systems, such as those in healthcare, operate under strict control requirements/regimes defined in documents such as HTM:04-01.

Using the wrong product in a healthcare setting could:

• breach compliance requirements
  
  
• compromise water quality
  
  
• create risk for vulnerable patients.

So, the WRAS adjustment isn’t a limitation, but a gentle reminder that approvals and temperature ratings must be checked together.

Other Relevant Approvals: BS 6920 / KIWA / NSF.

WRAS is not the only certification to review or consider when choosing jointing compounds:

Products supported by several certifications have the potential to provide stronger assurance of long-term safety, especially where uncompromised water quality in high-risk areas is paramount.

The Hidden Issue: Overusing Jointing Compounds

Jointing compounds can work well when applied correctly, but overuse/application can be a common and costly mistake, which can lead to serious water quality problems.

What goes wrong when too much compound is applied:

• It has the potential to be pushed into the pipework during tightening
  
  
• The excess travels through the system
  
  
• It can accumulate in valves, strainers, aerators, or filters
  
  
• Where the compound is captured, it can provide an area for biofilm formation, which can lead to microbial growth/contamination
  
  
• Restriction in flow where the compound has settled in valves, strainers, aerators, or filters.

In severe cases, this can:

• Reduce operational performance
  
  
• Cause valve malfunctions
  
  
• Increase maintenance for cleaning
  
  
• Compromise overall water quality, leading to adverse microbiological sample results.

Best Practice Application (if required as a last resort)

A safe and hygienic installation requires:

• A thin, even application only on male threads
  
  
• Avoiding smearing compound into fittings or pipes
  
  
• Post-installation flushing (with no restrictions i.e., filters, strainers in place) to clear any residue
  
  
• Selecting only the amount of compound required to complete the task.

If the compound squeezes out when tightening a fitting, it is evident that too much compound has been used.

Why Some Experts Recommend Reducing or Avoiding Use:

Due to the potential water quality concerns, it has become apparent in project specifications that the use of jointing compounds is to be avoided wherever possible, especially in water systems systems containing:

• Thermostatic Mixing Valves / Taps (TMVs / TMTs)
  
  
• Fine strainers (e.g., inline Y-strainers)
  
  
• Tap filters/aerators.

Alternatives include, but not limited to:

• PTFE (Teflon) tape or cord
  
  
• Pre-formed sealing washers (Non EDPM).

The above options create an effective seal without introducing excessive debris into the domestic water systems, with the following results:

• Reduced risk of bacterial growth
  
  
• Longer system lifespan
  
  
• Simplified compliance
  
  
• Lower maintenance and callouts.

Summary:

Jointing compounds may remain an important part of plumbing practice, but their use must be smarter and better informed than previous. The WRAS test temperature change from 85°C to 23°C does not prohibit hot water use; instead, it emphasises the responsibility of installers to verify suitability and prioritise system hygiene.

By ensuring the correct product choice, using minimal quantities, and exploring safer alternatives where appropriate, we can protect both the water system performance and public health, along with the safety of the end user from any potential infection issues.

Feel free to reach out if you have any questions about the issues mentioned above or if you would like to consult with one of our experts on water hygiene.

Editors Note: The information provided in this blog is correct as of the date of original publication - November 2025

© Water Hygiene Centre 2025