Joining pipes is one of the most vital aspects of completing any plumbing job, whether it's new or a rehabilitation. The pipe joints must withstand the pressure from each line going through the joint. To employ the best pipe joining method, workers must understand the different options and which is the best choice for a given job. (One major project is replacing the old lead pipes in your home. Learn more in How to Tell If Your Home Has Lead Pipes and How to Replace Them.)

Threaded

If you have a low-pressure system, using a simple threaded joining method is best. The threaded joint is exactly what it sounds like: One component has internal threads while the connecting piece has external threads. All manner of piping is available with threads: PVC, copper, cast iron, and GI pipes. They have a diameter range of 6 mm to 300 mm.

This joining method works only with low-pressure and low-temperature flows. Higher temperatures and pressures could cause the joints to expand and leak.

Stab-Type Fittings

These coupling methods are used for pipe diameters with a 2-inch or smaller diameter. While there are several styles of stab fittings, they all have a similar design that includes an elastomer seal, a gripping device for effective pressure sealing and a self-contained stiffener.

With a stab-type fitting, the union between pipe ends is stronger than the pipe itself, making it ideal for polyethylene pipes, especially heating, ventilation and air conditioning applications. Stab-type fittings must be ASTM International D2513 Category I rated to ensure a complete seal and full pullout restraint.

Soldered

One of the more common types of pipe joining methods is the heat fusion method. There are several ways to fuse by heat, with soldering being one of the first choices for many plumbers. Solder melts using temperatures below 840 degrees C.

When soldering, the skilled technician should use a paste which keeps the fittings from oxidizing. This pipe joining method is used for copper and copper alloy pipes. It is best for pipes used at a lower temperature such as home water lines.

Brazed

Brazing is the second type of heat fusion. This heating method differs from soldering by using temperatures above 840 degrees C to melt the filler material. Like soldering, workers typically use brazing methods on copper or copper alloy pipes.

The filler material used to seal the connection between the two pipes often consists mainly of tin. However, since tin is a weaker metal, it is combined with copper, bismuth, silver or nickel to strengthen the filler and help it maintain structural integrity. Brazed joints work well for pipes transporting material of moderate temperatures.

Welded

There are two main types of welds used to connect pipes: the butt weld joint and the socket weld joint. Both methods require a skilled worker to install the fittings.

Butt Weld

A butt weld is the most common type of pipe joining welding method. Workers use this method to join two pipes of the same diameter. Generally, these joints are most prevalent in commercial or industrial pipe systems.

Joints sealed with a butt weld are not accessible for maintenance as they are a fixed joint. However, these welds provide additional strength in the joint and allow the seal to resist high pressure if sealed with filler only. Welds using an internal backing ring to reduce the amount of filler and lower the overall cost of the project may not tolerate the same pressure and may crack under heavy stress.

While welding is used for copper pipes, butt fusion is used for connecting polyethylene and high-density polyethylene pipes. Instead of using a filler to combine the two pieces, workers heat the pipe pieces by holding them against a heated plate until there is a proper melt. Once the melt is achieved, they remove the plate, push the two pieces together and hold them in place until they cool, forming one solid pipe piece. (For more on butt fusion, see Butt Fusion & Polyethylene Pipe: How to Effectively Use This Trenchless Method.)

Socket Weld

Plumbing experts tend to use a socket weld when they determine there is a higher chance of leaking from the welded joints. Instead of butting the two pieces together, one portion is inserted into the other and then welded together. This weld works best with pipes of two different diameters. Pipelines with the same diameter can also use a socket weld with fittings employed.

As with butt welding, socket welds are used with copper or other metal pipes. This method can also be used with high-density polyethylene pipe, which is called a socket fusion. As with butt fusion, the process heats the outer surface of the tube and then the interior of the fitting. Once achieving the proper melt on the two surfaces, workers join the parts by inserting the line into the joint. The two pieces solidify together as the surfaces cool.

Saddle Fusion

While the saddle fusion technique is not technically a weld, it is like the socket and butt fusions performed on high-density polyethylene pipes. Workers use this method to install a saddle fitting into the line seamlessly. As with other fusion methods, both the base of the fitting and the surface of the pipeline are melted with a heating plate. Once the appropriate melt occurs, they join the two pieces and allow them to cool while applying force to solidify the parts as one.

When appropriately performed, each of these pipe joining methods should connect the lines without any leaks. Testing should occur once the joints are connected to ensure a correct seal before project completion.