I want to install a toilet on the second floor of our house. We have a very small room upstairs in the center of the house which is ideal for an upstairs 1/2 bath. The main 4" vent is on the gable end of the house about 26 feet away from where I want to install the new toilet. My first thought was that I could simply go straight up through the wall behind the toilet into the attic and then across the attic to connect to the main vent 26 feet away. But then I read about "critical distance" for venting purposes. Will my original plan work? Or do I need to go straight up through the roof from my new toilet and have a second vent pipe poking throught the roof?

I guess what has me confused is "Where is the critical distance measured from?" If I have a 4" vent coming out of the roof, can I run horizontally from that vent in the attic to connect to my fixture? Is the distance measured horizontally from my trap that point where the vent stack rises vertically through the roof? Or is it measured horizontally from my trap to a point where I can connect to the venting system as a whole?


Thanks in advance for your help!

""Where is the critical distance measured from?" If I have a 4" vent coming out of the roof, can I run horizontally from that vent in the attic to connect to my fixture? Is the distance measured horizontally from my trap that point where the vent stack rises vertically through the roof? Or is it measured horizontally from my trap to a point where I can connect to the venting system as a whole?"
It's measured from the trap to the vent as a whole.
By 1/2 bath I'm assuming that you'll be installing a lavatory. Right?
If so the lavatory vent can do up in the attic and revent back into the 4" main vent. The toilet will be wet vented by the lav vent. Critical distance doesn't come into play unless there's no opening to the outside air,(a vent).
Since your lavatory trap is right next to access to outside air,( the vent) it doesn't make any difference how long the horizontal line is to connect to that outside air. Good luck, Tom

Thank you Tom very much for the information! Yes, there will be a lavatory and a toilet. So let me make sure I understand. I will vent the lavatory into the attic connecting to the 4" stack, and vent the toilet into the same (probably 2 inch) vent pipe rising from the lavatory. Is that correct?

Thanks again!

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Originally Posted by mwarney

Thank you Tom very much for the information! Yes, there will be a lavatory and a toilet. So let me make sure I understand. I will vent the lavatory into the attic connecting to the 4" stack, and vent the toilet into the same (probably 2 inch) vent pipe rising from the lavatory. Is that correct?

Thanks again!

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Not quite! You will have a 3" toilet drain line going to the 4" stack. You'll pick uo the lavatory with a 3 X 2" fitting and elbo up 18" to the center of a 2 X 2 x 1 1/2" sanitary tee from the floor. The top of the tee will continue up to the attic where it will revent back into the main vent with a 4X2" inverted tee or wye. The toilet will not require a dedicated vent as it will be wet vented by the lavatory. Good luck, Tom

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Originally Posted by speedball1

Not quite! You will have a 3" toilet drain line going to the 4" stack. You'll pick uo the lavatory with a 3 X 2" fitting and elbo up 18" to the center of a 2 X 2 x 1 1/2" sanitary tee from the floor. The top of the tee will continue up to the attic where it will revent back into the main vent with a 4X2" inverted tee or wye. The toilet will not require a dedicated vent as it will be wet vented by the lavatory. good luck, Tom

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Thank you soooo much! Very well explained... I can see it cear as a picture now!