How to Sweat (solder) Copper Pipe





Introduction: How to Sweat (solder) Copper Pipe

About: I'm an engineer with a need to build things, anything really. My current project is an aluminum bike trailer. Welding aluminum is rather hard and I don't have the hang of it yet.

I had a plumbing problem needing to be addressed (people: stop mixing copper with cast iron piping!) and learned how to sweat (solder) copper piping. Why do they call it sweating instead of soldering? I have no idea, since I'm not a plumber, but I've done a decent enough job at sweating the copper that I have no leaks in my basement (anymore).

I don't know everything, as you'll see I'm a little short on details, good tips, good practices, background info, or plumbing code (did I mention I'm not a plumber). But hopefully it's good enough to get you started on your next project. Have fun!

Step 1: Start With a Freshly Cut Copper Pipe.

Depending how bad you cut the pipe (the best thing I had at the time was an angle grinder), you'll end up with a crappy looking end with burrs.

Step 2: Deburr the Pipe End.

I used the angle grinder to remove the burrs on the outside of the pipe, and then used a multi-step drill bit to remove the burrs on the inside.

Step 3: Sand the Outside

Use some plumbers fabric to clean the outside of the pipe. You could try sandpaper, but the plumbers fabric is really abrasive, works well, and it's cheap.

Step 4: Ream the Inside.

Ream the inside of the pipe with a pipe cleaner. Make sure you buy the appropriately sized cleaner for your pipe.

Step 5: Apply Flux

Buy a flux on the with a wide temperature range, especially if you're a newbie like me. I used the Oatey No.95 flux. Apply it on the outside of the pipe. Non-disclaimer: I am not affiliated with Oatey by any means.

Step 6: Install Fitting, Then Apply Heat and Solder :)

Finally, the fun part (the part with fire).

After installing the fitting (note: ream the fitting with a pipe cleaner too!), start applying heat. Move the torch around to "evenly" heat the mating area. After a subjectively short amount of time, try applying the solder to pipe. Don't just put the solder in the flame for it to melt, you want the pipe to melt the solder.

Once the solder melts around the joint and starts pool at the bottom, you're finished (it takes very little solder to do this). Do a quick check around pipe to make sure solder flowed all around it.

After it's cooled, clean off the leftover flux with soap and water. Then hope you have no leaks.



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    27 Discussions

    no no no! all these power tools are more likely to cause damage to the soft copper pipe than spending $10 for a pipe cutter, available at any hardware store:
    even if it seems like you have good cuts/smooth surfaces with these crazy power tools, i would wager that the soldered seal you make is more likely to fail than if cut cleanly with a pipe cutter.
    also, don't clamp the soft copper pipe with vise grips as shown; again, you are likely to deform the pipe/push it out of round.
    the pipe really is softer than you might imagine. for this reason, always cut half an inch off the end of new pipe, because odds are it has been dented slightly out of round somewhere in the supply chain scheme.
    i would also like to emphasize that it's imperative that both pipe and fitting are sanded/brushed until they are super shiny - they need to be for full adhesion of the solder.
    the "subjectively short amount of time" is actually precipitated by a change in flame color, so you will have some visual warning. the flame turns kind of greenish, at which point you will know it's time to load the solder. you'll get the idea after half a dozen times.
    (i worked as a plumber's mate for a year.)

    17 replies

    Thanks for the warnings, I'm glad to have a real plumber respond. Since this project, I have purchased a pipe cutter like the one you've shown. But of course, since I bought the proper tool for the job, I haven't had the chance to use it. I'm planning to remodel my kitchen, so I'm sure it'll get used this year. I should have given more warning about being careful with clamping. I was really careful not to put much pressure on the pipe w/ the vise grips. But I'll take your words of caution and find a better way to hold the pipes down while soldering. What do you use for this? I'm lucky enough that these joints haven't leaked ...yet.

    These work great on the outside of the pipe, and inside of the fitting.


    i'll usually just sit something handy that's heavyish that won't melt or burn on top of the pipe - a wrench or piece of 2x4 works fine. longer pieces of pipe tend not to move around so much, so they don't need much. the other thing is that you're often soldering while pipes are in their finished place, so they're held by the fittings on the existing pipe. have fun doing your kitchen - it's pretty rewarding being able to something like this yourself, and it gets easier every time you do it.

    No, you want the pipe to be rough, on the ends, not the whole pipe. Think about it, when you paint something you sand it to rough it up, so that the paint will stick. I have been a plumber for 20 years, so I would know. If you sand it shiny, it will have no surface to grab onto.

    Umm... I'm a licensed plumber as well (as is my father) and I think you're mis-explaining the purpose of this step. You need to CLEAN the inside of all fittings and the tips of the pipes that will be going into the fittings with an emory cloth or coarse sandpaper (for folks who aren't plumbers) and the net result should be that the metal will appear "shiny" (as opposed to the dull oxidized copper look that pipes get once they've been installed for some time) but you do not want to actually POLISH the metal. it's an important distinction because getting to the "shiny" stage is a useful rule of thumb for someone who is trying to clean fittings or pipes. Ideally it should be something close to the lighter reddish-orange copper color that you seen in newer fittings and pipe, but you should also see lines/scratches from the coarse sanding. It's also a useful habit to get into even with new (i.e. shiny) pipes and fittings because those can have non-obvious gunk on them that could negatively affect the quality of the soldered joint. As a rule of thumb I lightly sand even brand new fittings (and of course I thoroughly clean older materials) just in case (I'd rather spen a little extra effort then than a lot of extra effort later tearing a hole in my wall to fix a leak).

    hmmm - have been very successful using scotchbrite on smaller joints (15mm - 22mm) (1/2" and 3/4") and +- 60 roughness sandpaper on the larger sizes. Primarily the use is to get rid of oxidation (which the flux should assist with). Not sure why you're reaming the inside of the pipe - maybe because you've distorted the pipe by cutting with the angle grinder, but your inner tube bore won't interfere in the joint - just get rid of the burr and you should be ready to go (assuming the pipe hasn't heavily lost shape in cutting).

    The reason you ream or deburr the inside of the pipe is basically to allow for a "smooth", uninterrupted flow of the fluid (remember that gases are fluid in a gaseous phase) through the pipe. If you have burrs on the inside of the pipe where you have cut it, you are going to have eddies forming around those burrs resulting in what in aerodynamics/hydrodynamics/fluid dynamics is referred to as drag. Drag ultimately reduces the effiency of the flow of the fluid in the joint which in general plumbing is not a big issue, however in piping where you have a less dense medium (fluids in a gaseous phase) or where you require super accurate pressure at the end point (as in aircraft hydraulic systems) this becomes critical.

    I reamed it to get rid of any oxidation/dirt/grease on the inside of the pipe. There was no appreciable distortion using the grinder.

    can't really ream all that deep can you (tubes supplied in 5.5m lengths installed as is on some sites)? The manufacturers tube bore should be free from grease if it meets international specifications, if not complain (if you're worried cos the pipe is old and has lain in your garage so long then pushing through a cloth should remove any dirt inside) . Also oxidation is a good thing on the inside of your pipe - it shouldn't affect your joint (because the joint is outside the tube) or the flow of material in the least (oxidation is bound to happen when the copper comes in contact with whatever you're conducting i.e. water, oil or air). In fact some studies show that oxide layers help protect metals from corrosion

    Ream = enlarge tube end opening; de-burr.

    As a general rule, you sand the inside of the fittings, not the inside of the pipe.

    Also I'd be curious to know if those studies you mention specifically deal with copper pipe. I know there are several metals (often alloys iirc) that count on developing a thin layer of oxidation in order to extend their useful life when used in various objects, but oxidation of copper pipe will eventually lead to it failing so you generally want your copper to be as non-oxidized as possible. Technically cleaning a pipe or fitting prior to soldering won't prevent oxidation, but it can reveal if there is enough oxidation such that the item in question is no longer serviceable.

    ok I won't quote a study but have a look at this book "The use of copper as a plumbing material" by the Deutches Kupfer-Institut (German copper institute) which should be available from your local copper development association. on page 30 section 5.2.1. "When a potable water installation with copper pipes is put into operation for the first time, the reaction of copper with oxygen dissolved in the water leads initially to the formation of a protective layer of copper(I) oxide (Cu2O) on the surface of the copper..." a few paragraphs later... "In some types of water only the dark brown protective layer develops. It is this layer, however, which is decisive for the good corrosion behaviour of copper. Some copper tube manufacturers have guaranteed their product for 50 years of service (they dug up some tube in the pyramids over 2000 years old and still usable) I doubt you'll find a piece of tube lying around the house that's not usable. I've heard that a similar protective layer forms on galvanized steel waterpipe installations (we have some old pipes here in South Africa) but can't quote any reference so I'd be speculating

    If the oxide forms a crystal lattice that is the same size and shape as the metal's natural lattice structure, the oxide layer will protect the metal from further oxidation and corrosion. These metals are often referred to as "self-healing" because a scratch in the oxide layer will just form a new oxide layer on the exposed metal and prevent corrosion. I know that titanium is one of the "self healing" metals. I think zinc is as well. Iron and most iron alloys (including steel) are not self-healing because the oxide (rust) lattice is a different shape than the metal. The oxide sticks to itself better than it sticks to the metal substrate, which is why rust flakes off iron and steel. The steel in galvanized steel isn't self-healing, but the zinc coating may be. Note that galvanized pipes are usually only galvanized on the outside - they will continue to rust over time on the inside if oxygenated water circulates through them. Incidentally - fire sprinkler systems can get away with using non galvanized iron pipe because the water inside is in a closed system. Whatever oxygen is in the water when the pipes are filled will bond with the iron, but the oxidation will stop once all the free oxygen is used up. As long as the system is sealed to prevent new oxygen from getting in, the iron pipe won't rust on the inside.

    Thanks - must admit I didn't understand why the protective oxide layer would work on galvanized tubes (didn't think about the zinc duh) but after reading your response it makes sense... I'm pretty sure the galvanized tubes in SA are galvanized on the inside - am an electrical conduit fitting manufacturer (and copper tube manufacturer) and we make some of our products from the conduit (ok different sizes) that is made locally and from the certificates of conformance I've had so far it looks like they're making their tubes from already hotdip galvanized sheets. Of course the weld will always be a weak point rustwise - they have sophisticated spraying techniques but don't see how they'd spray the weld on the inside of the tube... that said, having cut the tubes open I've never noticed a non galvanized point ... will have to go chop some open tomorrow at work and get back to you! Very interesting about iron pipe in the firesprinklers

    as far as I'm aware, the primary concern for removing oxidation on the pipe when soldering is because solder will not flow on an oxidised surface (much like cleaning a surface to ensure paint or glue will hold)

    Technically, in this situation ream = enlarge tube end opening; de-burr.

    That said, cleaning out the inside of the fittings to remove oxidation/dirt/grease is definitely important, but you don't really clean the inside of the pipe.

    Your average run of the mill pipe cutter can slightly deform very edge of the pipe as it cuts through. You ream it largely to make sure that there aren't any slivers of copper pipe left over inside the pipe from the cutting process plus it also helps you make sure that you haven't reduced the inside diameter of the pipe after cutting it. Usually it's not a big deal whether you do or don't, but if you want to do it right, having a reaming tool and running it through the inside of the pipe is the way to go.

    A good rule of thumb for the amount of solder needed for each joint comes from the pipe diameter. If I am soldering a single joint in 1/2" pipe, I will form a bend of about 1/2" length at the end of my solder "string". Then I make sure to feed that much solder to the heated joint - and no more. If excess pools or drips outside the joint, I try to whipe it off with a rag or paper towel before it solidifies. If I can't get it whiped off in time, I either leave it and don't worry about that unsightliness or grab a file (after fully cooled) and file it down. I've also found that if I am soldering a horizontal joint, I only need to apply the solder to the top of the joint. It will both wick into the joint and travel all around it. There is really no need to try to evenly apply the solder all the way around the joint. If it's hot enough, it will travel and fill just fine. Although it can happen, I rarely have leakers. I have never tried applying more flux in an effort to clean the joint after soldering. Don't know if this will work or not, but using the same steel wool I use to prep the parts for sweaing in the first place works well in shining up the joint after cooling.

    im a plumbing apprentice , i wouldve used wire wool to shine the ends off and inside the fitting and used a pipe cutter to cut the pipe, NOT A blowtorch as this will waste time because youll have to sand of the the parts that bong out otherwise the pipe will not go in the fitting thats if its going in a fitting if not i recommend a pipe bender this will save alot of money becuase you will no longer have to buy as many elbow joints but if its just for the one occasion just stick with the fittings