Adding a dual sink

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havasu

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Right now, I have one sink in my master bath. I would like to expand to a dual sink cabinet. Since I have new copper lines, adding the hot and cold water supply would be easy. My concern is the drain lines. Does each sink need a separate vent? Could I just add a "P" trap and connect to the existing waste line with a "Y" pipe? Any advice would be apprecated.
 
The flow rates are so low on lavatory sinks it will likely not be an issue to just add a wye fitting. If you do have venting issues, installing an air admittance valve (studor vent) would be simple enough.

This method likely will not comply with local codes, but I won't tell if you won't. :D
 
So, in other words, the only legal solution would be to tie into the existing vent pipe up in the attic, or would it be necessary to add an entirely new vent pipe up through the roof?
 
havasu, it looks like I am one step behind you on home improvements... :) Changing from a single to double sinks is on our short-list of to-do's around here too... I was unaware that sinks has individual vents to the roof...

I thought you could just run a Y from just above the p-trap on sink 1, to reach over to sink 2... Is that what would be against code?
 
BRad704,

If you lived closer, we could help each other and learn by our own mistakes!
 
I'll install your wood shelves, if you put down the epoxy floor in my garage... fair enough? :)

I think this forum needs a "General Home Maintenance and Repairs" section...
 
No, what you would want to do would be to put a wye fitting on the drain arm that comes through the back of your cabinet, and then reattach the existing lav drain to the end, the new one to the side. Or vice versa if that would be easier. What would be against code, even the way I am describing, is that you have 2 fixtures connecting to one drain arm. There are ways to do this in a manner that complies with code, but this way will work and the "right" way would be a lot more work.
 
No, what you would want to do would be to put a wye fitting on the drain arm that comes through the back of your cabinet, and then reattach the existing lav drain to the end, the new one to the side. Or vice versa if that would be easier. What would be against code, even the way I am describing, is that you have 2 fixtures connecting to one drain arm. There are ways to do this in a manner that complies with code, but this way will work and the "right" way would be a lot more work.

So then IS there any realistic (and within code) way to add an extra sink to an upstairs exterior walled bathroom?
 
Several caveats in my answer to that. Firstly, I am not sure if your jurisdiction uses the same code that my area does. Second, the code edition that I am most familiar with was in effect 10 or so years ago. I have read the current code cover to cover, and in most ways it is very similar, but I can't quote wording from memory, so....
Third, I am not looking at your specific situation, so I have to make some assumptions, as follows. I am assuming that there is a vent in the wall behind your lavatory, that the trap arm drains into that stack, and that from there the vent either goes out the roof or revents back into another vent stack. And, that IF the lav stack is a wet vent for the rest of the bathroom, that it is 2" or larger at least to the point that the lav drains into it.

You would need to remove the vanity cabinet. Cut out the wall behind it to expose the drain arm and the stack. If the stack is between the existing lavatory and the proposed lavatory, replace the existing sanitary tee with a sanitary cross, 1 1/2" each way. If the stack is to one side or the other of either the existing or the new, replace the 2"x1 1/2" sanitary tee with a 2" sanitary tee. If between, route 1 1/2" drain pipe to the approximate center both lavatories. If to one side, use 2" to the first drain, then use a 2"x1 1/2"x1 1/2" sanitary tee on it's side to stub out for that drain, then use 1 1/2" to the second lavatory. While you are at it, run the water piping over to the new lavatory as well.

Now, if you have to penetrate any of the studs with 2" pipe, even if you drill the proper size hole, and even in a nominal 2"x6' wall, you will have to double that stud. In a 2"x4" nominal wall, you will have to double the studs that you drill through even if the pipe is 1 1/2". And if you have to double up the studs, you can do that to no more than two consecutive studs.

Off of the top of my head, and following the code that I know best, that is what is required to bring this up to code. MOST inspectors that I have dealt with would probably OK the wye under the existing lav, because they are sentient, logical creatures. Some inspectors aren't.
 
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I'm sorry, but that explanation got me dizzy! By chance are there any pictures of your description? I did understand the stud drilling, but would a non-load bearing wall need dual support as well? I guess Brad's dual sink job is on the outside, and my dual sink is on an interior wall.
 
I'm sorry, but that explanation got me dizzy! By chance are there any pictures of your description? I did understand the stud drilling, but would a non-load bearing wall need dual support as well? I guess Brad's dual sink job is on the outside, and my dual sink is on an interior wall.

The code that I am familiar with states that a load bearing stud can be drilled to no more than 40% of it's thickness. Since most exterior walls are load bearing, I assume that such is the case for Brad's situation. Non load bearing walls can be drilled to 60% of their thickness, as MIGHT be the case in your situation. Find out before you break out your drill, as in some cases interior walls are used to hold up a lot of weight.

As far as the sizing and arrangement of the drain piping goes, try to look at it this way. The drain pipe leading from the stack to the trap underneath the lav needs to do two things. Take away the water that is drained out, and allow air to get back to the trap. If the pipe is too small, it doesn't have the room necessary to do both, and the system will malfunction. So, if you add second source of drain water to the pipe, you need to increase the size of the pipe so that the drain pipe can still do both things. But, if the stack is between the two drains, the water is routed to the stack by two separate horizontal pipes, that can both be of a smaller size than if both drains ran through the same horizontal pipe. Make sense?

And no, I don't have pictures, and even if I did, my connection would take a month to upload them.
 
I think I would survive by doing what I drew up below. We would never be in a situation where we'd both be blasting the water at the same time anyway... So its not really additional volume load, its just hitting the trap from a different source sometimes...

dual sinks.JPG
 
That will work. Similar to the way some double bowl kitchen sinks are piped. Some things to consider, though.

You very well might not have the space neccesary to do this. If the drain arm comes out of the wall too high, there will not be enough room between the top of the trap and the bottom of the drain body to install the wye fitting. If there is enough room, then you have to find a fitting to turn 90 degrees out of the drain body of the new lavatory. There is a compression 90 made that would do this, but I wouldn't reccomend that you use it. With the expansion and contraction cycles caused by hot drain water warming up the pipe, and then cooling down, there is a potential for the fitting to slip on the tailpiece of the lav drain, dumping a whole lot of water into your cabinet.
 

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