Adding a dual sink

**havasu** · 02-04-2010, 05:30 PM

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.

**Reedwalker** · 02-04-2010, 07:10 PM

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.

**havasu** · 02-04-2010, 07:29 PM

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?

BRad704 · 02-04-2010, 08:02 PM

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?

**havasu** · 02-04-2010, 08:08 PM

BRad704,

If you lived closer, we could help each other and learn by our own mistakes!

BRad704 · 02-04-2010, 08:24 PM

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...

**phishfood** · 02-04-2010, 09:38 PM

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.

BRad704 · 02-04-2010, 09:51 PM

Quote:

Originally Posted by phishfood

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?

**phishfood** · 02-05-2010, 01:06 AM

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.

**havasu** · 02-05-2010, 01:16 AM

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.

02-05-2010, 01:06 AM	#9
phishfood Senior Member Join Date: Jan 2010 Location: Florida Posts: 406	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. Last edited by phishfood; 02-05-2010 at 01:09 AM.

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02-04-2010, 07:10 PM	#2
Reedwalker Senior Member Join Date: Jan 2010 Location: Detroit, MI Posts: 156	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.

02-04-2010, 07:29 PM	#3
havasu On permanent vacation.... Join Date: Jan 2010 Location: So. Cal & Lake Havasu Posts: 821	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?

02-04-2010, 08:02 PM	#4
BRad704 Moderation Join Date: Jan 2010 Posts: 305	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?

02-04-2010, 08:08 PM	#5
havasu On permanent vacation.... Join Date: Jan 2010 Location: So. Cal & Lake Havasu Posts: 821	BRad704, If you lived closer, we could help each other and learn by our own mistakes!

02-04-2010, 08:24 PM	#6
BRad704 Moderation Join Date: Jan 2010 Posts: 305	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...

02-04-2010, 09:38 PM	#7
phishfood Senior Member Join Date: Jan 2010 Location: Florida Posts: 406	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.

02-05-2010, 01:16 AM	#10
havasu On permanent vacation.... Join Date: Jan 2010 Location: So. Cal & Lake Havasu Posts: 821	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.