dwv system for new 2 sink installation

[handsrusinkc]

I am taking out a sink and putting in 2 sinks in my bathroom on the opposite wall. Luckily I have a crawl-space below and an attic above.
I bought some good plumbing books, but they don't cover pipe sizes

I would like to know how to run the DWV system The sink drains will be run separately, because there is a set of drawers between the sinks.

I will run the sink drains to a pvc pipe in the wall that will go up to the attic (for venting) and down through the floor for drainage.

1) what size diameter should this pipe be?

2) When I join these two DRAIN pipes together in the crawl space, should they "become" a pipe of bigger diameter? If so, how big? can they join with a "T" or does it need to be more of a "Y" so it flows better?
(I know the rule about 1/4"drop per foot of linear/lateral travel.)

3) In the attic for venting, can I use a "T" here or is a "Y" better? How big should the pipe be that I end up putting out through the roof (It will be the two sink vents combined into one pipe for a vent)?


your help is greatly appreciated

[e-plumber]

1) what size diameter should this pipe be?
>1-1/2" is adequate for 2 lavatory basins<
2) When I join these two DRAIN pipes together in the crawl space, should they "become" a pipe of bigger diameter? If so, how big? can they join with a "T" or does it need to be more of a "Y" so it flows better?
(I know the rule about 1/4"drop per foot of linear/lateral travel.)
>If the pipe is vertical, a sanitary tee acceptable. If horizontal, you need to use a wye<
3) In the attic for venting, can I use a "T" here or is a "Y" better? How big should the pipe be that I end up putting out through the roof (It will be the two sink vents combined into one pipe for a vent)?
>A sanitary tee is acceptable. The pipe passing through the roof may need to be increased for frost over protection if you're in a cold climate, check the local plumbing code<

e-plumber

[handsrusinkc]

when I join the 2 pipes together, each 1 1/2 inch diameter, they will be heading horizontally to a larger drainpipe.

Can I join these two pipes together to make a 1 1/2 inch pipe, or should I upsize to a 2inch pipe for this horizontal run?

[hj]

Apparently my previous reply disappeared. The drawer unit is immaterial because the pipe will be in the wall. Install a 2" drain approximately in the center between the two sinks. Insert a cleanout in the vertical pipe underneath a 2 x 1 1/2 x 1 1/2 x 1 1/2 fixture cross and then run 1 1/2" pipes horizontally to each sink with an elbow on the end. Then install a 1 1/2" vent through the roof or to an existing vent.

[e-plumber]

Originally Posted by handsrusinkc

when I join the 2 pipes together, each 1 1/2 inch diameter, they will be heading horizontally to a larger drainpipe.

Can I join these two pipes together to make a 1 1/2 inch pipe, or should I upsize to a 2inch pipe for this horizontal run?

I do not disagree with over sizing in a case like this but according to plumbing code in this area, an 1-1/2" drain line for (2) lavatory basins is adequate to pass an inspection with flying colors. Although a 2" drain line will probably not clog up as quickly and to be honest, I would most likely install it this way (2") in my own home.

[handsrusinkc]

two sinks: I don't think I understand what you are offering here, but...

My setup is that of two sinks with a vanity with drawers between them. I can't just connect the two sinks together under the countertop. This is why I decided to run two separate drains, one from each sink and connect them together in the crawlspace. By the way, I'm installing this in my 50 y.o. house and trying not to tear out too much drywall.

Thanks for the help

[LonnythePlumber]

The drains are in the wall and the outlets stub out into the vanity space. You're not forgetting that you need a vent are you?

[froddan]

I am doing a similar project, and my vertical drain is 1.5, and I would like to connect two sinks to it.
I would connect a cross pipe, but I haven't been able to find one that is 1.5x1.5x1.5x1.5
Only 2x1.5x1.5

Will this still be adequate? Pass an inspection for two lavatory sinks?