3 Best Practices on How to Properly Assemble uPVC Fittings

3 Best Practices on How to Properly Assemble uPVC Fittings

3 Best Practices on How to Properly Assemble uPVC Fittings

When it comes to leak-free installations, uPVC plastic piping and threaded fittings are the most reliable and widely used solutions favoured by many. The issues that some people encounter with them, such as stress cracking with threaded fittings, can all be easily avoided by following the proper installation guidelines. In the following sections, we outline the dos and don’ts when installing threaded connections with uPVC pipes and fittings to ensure a smoother and stress-free assembly process.

1. Don’t overtighten joints by turning one more time just to be sure; do finger tighten for one or two more times and no more

Threaded Male or Male Pipe Thread (MPT) uPVC fittings use threads with increasing diameter, while Female Pipe Thread fittings have a decreasing one. Since these threads are tapered, any extra turns can strain the female part and potentially cause it to split. Smaller diameter fittings are subject to greater strain, making them more prone to stress cracking. Thread sealants can act as lubrication, which makes it easier to over-torque.

The more proper way to install threaded uPVC Schedule 40 or 80 fittings is to use one’s fingers to tighten them one or two more times and no more. Two turns past finger tightening, in addition to the stress of the system pressure, ensures the assembly is within the material’s tensile strength.

2. Don’t lubricate the joints with sealant tape, pipe or paste dope; do use sealants specifically designed for threaded joints

PTFE tape’s intended use is not for plastic pipes and fittings but for metal ones. However, PTFE paste and pipe dope are suited for use with both metal and plastic. Metal-to-metal fitting joints are much more difficult to tighten, unlike plastic, due to the harder physical surface and will chafe if no PTFE tape or pipe dope is used. In short, wrapping PTFE tape around uPVC male threads can increase strain.

When working with threaded plastic fittings, it is recommended to opt for sealants specifically designed for them. The non-hardening compound gets forced into the areas where leaks could potentially arise due to pressure and sealing them completely. When choosing a sealing compound, make sure they are compatible with plastics, as some may contain ingredients like oils that could damage them. Lastly, the sealing compound must not provide excessive lubrication that encourages overtightening.

3. Don’t assume that the stronger Schedule 80 fittings can eliminate the issue of splitting threaded connections due to overtightening; do use Schedule 80 uPVC nipples with both Schedule 40 and 80 fittings

Although Schedule 80 fittings are much stronger with higher pressure ratings than Schedule 40 fittings, this only applies when comparing systems using components that are solvent-welded together. The thicker walls of female Schedule 80 threaded fittings do not help resolve the issues of overtightening as the strain and stress levels remain the same. Moreover, both schedule 40 and 80 fittings have the same performance in the cracking aspect.

Pipe nipples are only available in schedule 80 uPVC. Cutting threads into schedule 40 uPVC pipes compromise its strength as it results in a thin wall section. Therefore, both schedule 40 and 80 fittings are used with schedule 80 uPVC pipe nipples. Although it is a rule of thumb to use the same schedule for pipes and fittings, this generally only refers to systems that use solvent weld fittings.


uPVC fittings are now the common and go-to solution for many plumbing applications. If assembled correctly, one can expect them to perform extremely well. Keep these best practices in mind to avoid the unnecessary extra costs and headaches that arise from improper installation.

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