Probably one of the most common leaks in a home is a leaking sink drain. So yes, that awful mess under your kitchen sink or, vanity where the wood is all funky and rotted isn’t just happening at your home. You aren’t the only home that has some kind of pan sitting under there catching drips. Have you ever had the drain pipes under the sink just fall apart when you bump them putting something away under the sink? I have seen amazing ways off propping up drain pipes to prevent them from coming apart. Sometimes it’s a worn out or, rotted out pipe causing the problem but, most of the time it is just a bad installation. Not just DIYer’s handyman work is responsible for these leaks and loose drains, quite a few plumbers aren’t very good at installing tubular drains either. I will tell you some of the most common errors people make when hooking up tubular drains under a sink and how to avoid them so that your drain has no leaks and won’t fall apart if you look at it wrong.
To have a good drain installation you must first start with quality materials. This is not a debate about tubular plastic or, tubular brass as both are good materials and will serve you well. Instead it is to tell you about products often cleverly marketed as easy to use that are pure junk, if these parts are used will be the Achilles Heel of your drain. The infamous flex pipe or, flex trap which looks like and accordion and can be stretched or bent into various configurations is one of these products to avoid. Another is the flexible rubber trap with the two hose clamps that secure it in place. The flex connectors have a problem where debris can be caught in the accordion folds and give off horrible odors if used between the water seal of the p-trap and the sink drain. The problem with flexible connectors is the drain is connected rigidly at the sink drain and where the p-trap wall outlet connects at the wall, the pipes in between should have solid connections giving each piece of tubing used solid support from the two sides. This support is what makes the drain strong so it doesn’t fall apart. The flexible rubber traps also have a tendency to kink when not lined up well enough also. If you use either of these products your drain installation is doomed from the start, there is no substitute for doing it right.
Another common mistake is mixing components between plastic and metal. It is acceptable to transition from metal tubular to plastic tubular at a slip joint connection I try to avoid it except in the case of using metal basket strainer assemblies and metal lavatory sink pop-up assembles then transitioning to plastic tubular. I consider metal sink drain assemblies to be a superior choice over plastic. A rule of thumb I usually follow is if the drain is hidden in a cabinet or vanity I use plastic tubular and if a drain is exposed like on a wall hung sink or, pedestal lavatory sink I use chrome plated brass tubular or whatever other finish is desired. This is only for cost and appearance in these cases. The plastic pipe is inexpensive and is very durable when properly installed. While the plated brass costs much more than plastic but has the looks you want to see on an exposed drain. The problems come when people mix parts such as metal nuts on plastic threads and plastic nuts on metal threads, this often results in damaged threads and a loose connection that leaks. Another common mistake I have seen is mixing components of a part. P-traps come in two parts, the bend and the wall outlet. Where the two pieces of the p-trap connect different methods of sealing are used for brass and plastic. Plastic p-traps use a bevel connection and the chrome ones use a rubber washer held in place with a flange compressing against a flat surface. If you combine these 2 different ways of sealing it will leak.
Zinc slip joint nuts are another commonly sold item that should be avoided. In theory they should be fine to use but the reality is even the smallest amount of leakage will turn into a big leak when the zinc slip joint nut corrodes and falls apart. It will not be the simple matter of tightening the connection like a brass nut often would be. The brass tube may have to be replaced as well if the zinc slip joint nut will not come off the brass. Stick with using brass or chrome plated brass slip joint nuts on tubular brass they are vastly superior to the zinc slip joint nuts.
The slip joint connections should not be installed with Teflon tape or, thread sealant. The actual sealing is done by the slip joint washer. The washer is compressed into a bevel by the slip joint nut. This seals it against the tube and the slip joint hub and it also grips the tube holding it in position. If any thread sealant was to get in this area it could serve as a lubricant allowing the joint to slip apart. Some thread sealants may also have an adverse effect on the plastic. My personal preference is to use the plastic poly slip joint washers on plastic tubular and to use rubber slip joint washers on the tubular brass. I feel that rubber grips the chrome plated tubular brass much better than the poly slip joint washers holding the connection in place better so things don’t fall apart. Slip joint nuts if they are plastic should be hand tight with just a little extra with a pair of channel locks, metal slip joint nuts should be tightened enough that the connection locks in place and doesn’t slide.
This next part is the Holy Grail of putting a tubular drain together. Granted the tips I gave above are important, but, if you are not going to do this part right you might as well do everything wrong. Without getting this part right you will have leaks and or, a drain that falls apart. It is absolutely critical that the tubes are cut long enough so that they almost bottom out in the hub and all the tubes should come together in a relaxed state. What I mean by a relaxed state is that there is no bending needed to make the parts stay together. The connections should almost want to stay together even if the slip joint nut was not tight. Only by having all the tubes cut to the proper lengths and having everything plumb can this be accomplished.
The best way to get the connections right is to use the fact that the p-trap is the part that can move virtually anywhere. By varying the wall outlet length and swiveling the p-trap you have quite a range of motion. Start at the wall and make sure the trap can be aimed at where the drain from the sink is going to land. You can use a bend coming out of the wall to aim the drain in the right direction if needed. I usually put the trap in its approximate place coming out of the wall first leaving the trap loose enough to swivel. The connections at the wall and the sink are rigid and the variable is moving the trap so I want to work from both the wall and the sink towards the trap. After getting the trap roughly in place I start at the sink and work down to the trap. Using the trap set in place as a guide to get lengths of the tube cut correctly. I want them to have the maximum insertion into every hub without bottoming out. If the tube bottoms out it is too long and may stress another connection. If a tube is short and barely into the hub the support the tube gets from the hub will be lacking, there will be flexing at that joint and it may come apart or leak. If there are any horizontal tubes like in the case of a double bowl kitchen sink I like to have them up as high as possible, this maximizes under sink storage and lessens the probability of damage occurring. When I get all the tubes in place coming down to the p-trap, I cut the tube to the proper length for the slip joint connection to the trap. I then take apart the trap swivel connection and put the trap in place loosely. I then work on the trap height and the wall outlet length so the beveled swivel connection stays together without any stress. I then put on the swivel connection nut and tighten all the connections. When you put the drain together in this manner the drain wants to stay together and even with vibration from a disposer and an occasional knock, the drain remains together and leak free.