How to Sweat Copper Pipe

Of all the myriad plumbing mysteries confounding the DIY homeowner, sweating (soldering) copper pipe seems to be foremost among them. It needn’t be so. Once the process is understood, the steps are actually quite simple. Following is an explanation of that process.

Sweating is the name given to the act of joining copper pipe by way of the introduction of solder into the pipe joint. The solder itself is a combination of tin and lead that is easily melted with the application of heat, allowing it to flow into the joint. Once the solder cools, the joint is then fused and watertight.

sweating copper pipes

The flowing process is aided by the use of flux, which is essentially a form of acid that etches the copper to attain an extremely high level of cleanliness. The solder “chooses” this super-clean metal to adhere to.

The heat, in the case of sweating copper pipe, is applied by the use of a (generally) propane torch. The pros use other gas combinations, but the DIY homeowner – usually working on 1/2″ and 3/4″ pipe, at most – is well served by a standard propane torch found in any home center or hardware store.

Sweating copper pipe begins with a preliminary cleaning of the pipe and fittings with either an abrasive paper product (usually emery cloth) or stiff wire brushes dedicated to the task. These brushes, and the emery cloth, are available in the plumbing section of the store.

Burnish the outside of the pipe and the inside of the fitting until bright and shiny. On new material, this just takes a few strokes, but on existing pipe or old fittings that have oxidized, the job is just a bit more time consuming. Clean the pipe end for a distance equal to twice the depth of the fitting socket. Once done, do not touch the burnished metal again – the oil from your fingers can contaminate the joint.

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Apply the flux with a flux brush, both available in the plumbing section as well, on the burnished end of the pipe and inside the fitting socket. Apply evenly all around that length of pipe that will fit into the socket and around the entire interior surface of the socket itself. Don’t be stingy with the flux, as any excess will simply boil off. Make sure you get flux specifically formulated for soldering copper pipe. There are various types of soldering flux for differing uses. Assemble the fluxed pieces, making sure the pipe “bottoms out” in the socket. Now you are ready to solder the joint.

Solder for sweating copper pipe comes in varying combinations of tin and lead, from 50/50 to 95/5 (tin to lead). If you are doing residential plumbing or repairs, check the building codes for the proper ratio for your particular area. The solder you will buy will be called “Solid Wire Solder”, meaning it does not contain flux – or any other compounds – in its core. Some solders, for other uses, do. Again, this is available in the plumbing department.

First unroll about 10 to 12 inches of the solder, bending a slight hook in the end, toward you. Then ignite the torch per the manufacturer’s instructions, adjusting the flame to about a medium-high burn. Hold the torch in your dominant hand. In the other, hold the solder roll. Direct the flame to the fitting side of the joint, holding it close enough so that the flame splits around it. When the copper starts to turn color – you’ll actually see the heat signature “flowing” along the fitting – touch the end of the solder to the joint, on the back side, opposite from the torch, allowing an inch or so of the solder to flow into the joint. You will see the solder appear on your side of the joint almost instantly. Take the solder away, then the torch flame.

And that’s it! After the joint cools, wipe off the excess flux with a dampened rag, and you are done. You can now sweat copper pipe. As with all work around your home, you must exercise caution with the flame, handle the chemicals with care – the flux is caustic – and always, always wear appropriate eye protection. Happy Plumbing!