Common Copper Pipe Soldering Mistakes

Knoji reviews products and up-and-coming brands we think you'll love. In certain cases, we may receive a commission from brands mentioned in our guides. Learn more.
A list of tips to help you make watertight solder joints in copper pipes and fittings.

Many homeowners attempt plumbing repairs from time to time. This article covers some of the most common mistakes made when soldering copper pipes to brass or copper fittings.

Proper Cleaning

Copper pipe and copper or brass fittings must be clean for the solder to create a watertight bond. The outer surface of copper pipe must be cleaned with emery cloth until the protective coating or oxidation is removed. The inner surface of fittings and valves and the outer surface of copper pipe should look like a new penny. It is also important to remember not to touch the cleaned surfaces with your hands or fingers. Dirt or the natural acids that are on your fingers can prevent the flux from flowing properly and hence the solder from properly bonding the pipe and fitting together.

Removing Burs

The inside of a copper pipe needs to be deburred with a pipe deburring tool. Most pipe cutters include a deburring tool as part of the pipe cutter assembly. If your tubing cutting does not have a deburring attachment you can purchase a separate deburring tool. Removing the burs from a cut pipe allows the pipe to slide easily into fittings and prevents solder from flowing into the pipe and away from the fitting.

Applying Flux

A thin layer of flux needs to be applied to the outside end of the copper pipe and the inside of the copper or brass pipe fitting. Without flux, solder will not flow into the fitting to form a bond; the solder will melt and adhere to the copper unevenly. Too much flux will cause the fitting to become weak overtime and fail.

Never use your finger to apply the flux as the natural acids on our fingers will have a chemical reaction with the flux causing it not to do its job and you can cut your finger if there are burs remaining on the edge of the pipe.

Note: Flux is harmful if it gets into your eyes or if it is ingested. Always protect your eyes with safety goggles.

Proper Seating

Copper pipes must be inserted into fittings to the proper depth; this is referred to as mating. Fittings and valves typically have a ridge or stop to stop the pipe at the proper depth. Some fittings called slip couplings do not have a ridge so that the fitting can slide down the pipe when there is not enough room move the pipe.

Bad seating of pipes into fitting

Good seating of pipes into fitting

Cutting Copper Pipe

Copper pipe should be cut with a pipe cutter not a hacksaw. Hacksaws tend to deform the pipe and create a ragged edge that makes it difficult to insert the pipe into the fittings. The deformed end will also make it difficult to form a tight solder joint.

The blade on a pipe cutter must be sharp in order to create a smooth cut without distorting the copper pipe. If you do a lot of soldering or plumbing work, you may want to purchase a good quality pipe cutter that allows you to replace the cutting wheel.

Draining the Pipe

Pipes must be drained of all water before soldering. If the water is too close to the pipe it will boil away and form steam which cools the pipe and prevents the solder from forming a bond with the pipe. Water is a heat sink that will absorb the heat from the torch.

Water can be drained away from the solder joint by shutting off the water to the pipe and opening up a faucet at the lowest point to drain out the water. In some cases it may be necessary to angle the pipe up to stop the water from draining into the solder joint.

Note: Ensure that appliances that require water for safe operation, such as boilers are turned off when the water is turned off and has been drained.

Proper Flame Position

To create a good solder joint it is necessary to place the tip of the flame at the proper position. This will heat up the fitting quickly and evenly. The longer it takes to heat up the fitting and pipe, the better the chance for the flux to flow away from the joints and prevent the solder from adhering to the copper.

Heat the joint with your propane torch, moving the cone back and forth to heat it evenly. Hold the solder against the joint on the side opposite the flame until it melts and flows into the joint. The joint should appear full on all sides.

Heavy brass fittings or valves require five to six times as much heat to raise the joint to the solder melting point. Heat the joint from several sides if possible. Always leave valves at least partially open when soldering them.

Heating the Flux Directly

Do not allow the flame from the torch to melt the solder. The solder must melt when it comes in contact with the joint between the copper pipe and the fitting. Applying heat directly to the solder will not allow the solder to bond to the copper pipe and copper or brass fittings.

A good test is to remove the flame from the pipe and touch the solder to the joint. If the solder melts without the flame on the pipe, the joint is at the correct temperature to take the solder. If it does not melt apply more heat to the pipe and fitting and test again.

Feeding the Solder into the Joint

Be careful not to apply too much solder into the fitting, called overfeeding. About 1/2 in. of solder wire is enough for a 1/2-in. copper joint, 3/4 in. of solder for a 3/4-in. copper joint. Overfeeding can clog the tube, cause the joint to leak overtime, and shows poor workmanship. Remember to use lead-free solder in your home. If you have old, lead solder you should dispose of it in a hazardous waste pick-up location in your town.

Heat the Pipe Evenly

It is important that the pipe and the fitting have an even heat distribution from the torch. If for some reason the torch is only heating one side of the copper pipe, the solder will not flow all the way around the joint and you will not have a sealed joint.

Mechanical Fasteners

After the solder has filled the joint, allow it to cool for a few minutes. You can wipe off any excess flux from around the joint with a slightly damp cloth. Solder joints are not designed to take weight and stress. Pipe straps should be installed on the pipe to hold the pipe in position and take stress off the soldered joint. You should use copper pipe straps instead of steel to hold the pipe in position and prevent galvanic corrosion. Galvanic corrosion occurs when two dissimilar metals come in contact with one another.

Copper pipe strap

Remember to temporarily hang a flame protector cloth or a steel plate over wood and other flammable materials when soldering nearby and always keep a fire extinguisher or bucket of water nearby in case you start a fire.


Blossom S
Posted on Feb 21, 2011
carol roach
Posted on Feb 20, 2011
Jerry Walch
Posted on Feb 20, 2011
Rana Sinha
Posted on Feb 20, 2011