Back to back bathroom DWV layout

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Jonathon Zetterholm

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New house, back to back bathrooms, underslab plumbing. I am a contractor and have a basic knowledge of a lot of things. However this is my first major DIY plumbing project for my own personal home. Will the attached sketch work for back to back bathrooms? All dry vents, all double fixture fittings, (no san tee crosses). Any help/advice/constructive criticism would be greatly appreciated.
image.jpeg


Change wording to "double fixture fitting" added by Terry

 
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Reach4

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I am not a pro, but as I understand it, you don't want a double wye to vent fixtures. You want a double fixture fitting for lavs It allows the vent air to flow up. You want double sanitary tee-wye for toilets. Not sure about their characteristics. See page 2 column 2 of https://terrylove.com/pdf/cst474cefg-specs.pdf for a description of that fitting.
 
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Terry

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abs_fixture_cross.jpg


Double fixture fitting, good for tubs, shower and lavs.
Not good for toilets.
For toilets both Kohler and TOTO recommend a double wye for back to back toilets.

TOTO

back_to_back.jpg


Kohler
back_to_back_kohler.jpg


Kohler
 

Terry

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A double fixture cross will empty the opposing bowl when flushing.
Flush one toilet five times, and there is almost no water left in the other bathroom.
For toilets, you need a fitting that doesn't allow the waste water to skip across the fitting.
Toilets can be vented within six feet, and can be vented below the flange. They depend on siphon to work well anyway.

A lav or shower is vented to break up the siphon. Very different.
 

Jonathon Zetterholm

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Awesome! So stick with a double wye for the toilets. Change the sinks and showers to a double fixture fitting.

- Is dry venting out the top of a double fixture fitting (or double wye for toilets) okay?
- Is my vent and soil sizing okay? 2" for combined shower, 3" soil and 2" vent for combined toilets, and 2" for combined sinks.
 

Terry

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The toilets can vent with 2"
The lavs and showers can vent with 1.5"
Waste on combined lavs 2" and 2" on combined showers.
 

Jonathon Zetterholm

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The toilets can vent with 2"
The lavs and showers can vent with 1.5"
Waste on combined lavs 2" and 2" on combined showers.
Great! Can you confirm whether I am okay venting out the top of the double fixture fittings (or double wye for the toilets)? Some box store employees have told me I can't do that and each fixture/trap needs its own vent.
 
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