Max drain lengths without reventing

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Whale

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I need to change from my existing cast iron drain pipes in the unfinished basement to plastic. They have not been up to code anyway.
I would like to not have to revent bathroom fixtures which are next to the main drain/vent stack. Here is a chart that I have about distances. I have a few questions about using the measurements.
When measuring the horizontal lengths of the pipe to use the chart:
1. Is 45 degree drain pipe coming from a vertical main stack used in the measurement in the horizontal measurement of the pipe? or only when the pipe turns horizonal.
2. The bath tub P-trap is in the basement. When measuring the horizontal length of the pipe from the main stack to the tub do you measure to the from the beginning of the P-trap or to the vertical pipe coming down from the tub?
 

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wwhitney

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Your chart is old and wrong.

The rule for a trap (other than a WC) is that the trap arm has to be vented within one pipe diameter of fall of the trap outlet. That means the trap arm has to stay horizontal. And for the IPC (in force where you are), that also means that if the trap arm is falling at exactly 1/4" per foot, your trap arm length (measured along the run of the pipe) is limited to 4' per 1" of trap size. I.e. 6' for a 1-1/2" trap and 8' for a 2" trap. But if you put extra fall on your trap arm, the trap arm allowable length would be reduced. I.e. at 1/2" per foot, you get half the allowable length.

Now, the stack can be your dry vent connection if you connect your trap arm to it via a san-tee, and if there are no fixtures draining in from above. If you have fixtures on a 2nd floor draining into the stack, then it is not usable to dry vent your first floor fixtures; you need a separate vent riser running alongside the stack.

If there are no fixtures on a floor above, then presumably your lav will hit your stack above the floor. Your tub and WC can still be vented by the stack, with a few limitations: any non bathroom fixtures (which couldn't be vented by the stack) would have to hit the stack below the bathroom fixtures. And the WC has to hit the stack below the tub and lav.

Cheers, Wayne
 

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Your chart is old and wrong.

The rule for a trap (other than a WC) is that the trap arm has to be vented within one pipe diameter of fall of the trap outlet. That means the trap arm has to stay horizontal. And for the IPC (in force where you are), that also means that if the trap arm is falling at exactly 1/4" per foot, your trap arm length (measured along the run of the pipe) is limited to 4' per 1" of trap size. I.e. 6' for a 1-1/2" trap and 8' for a 2" trap. But if you put extra fall on your trap arm, the trap arm allowable length would be reduced. I.e. at 1/2" per foot, you get half the allowable length.

Now, the stack can be your dry vent connection if you connect your trap arm to it via a san-tee, and if there are no fixtures draining in from above. If you have fixtures on a 2nd floor draining into the stack, then it is not usable to dry vent your first floor fixtures; you need a separate vent riser running alongside the stack.

If there are no fixtures on a floor above, then presumably your lav will hit your stack above the floor. Your tub and WC can still be vented by the stack, with a few limitations: any non bathroom fixtures (which couldn't be vented by the stack) would have to hit the stack below the bathroom fixtures. And the WC has to hit the stack below the tub and lav.

Cheers, Wayne
thanks Wayne, This is my situation. "
"If there are no fixtures on a floor above, then presumably your lav will hit your stack above the floor. Your tub and WC can still be vented by the stack, with a few limitations: any non bathroom fixtures (which couldn't be vented by the stack) would have to hit the stack below the bathroom fixtures. And the WC has to hit the stack below the tub and lav."

So the WC definitely needs to hit stack below the bathroom fixtures. (it is above the other fixtures right now, Has it ever been code this way? house built 1924)

Does it matter which is hits the stack the highest, sink or tub?
 

wwhitney

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So the WC definitely needs to hit stack below the bathroom fixtures. (it is above the other fixtures right now, Has it ever been code this way? house built 1924)
Only if you want the stack to vent the other bathroom fixtures. If the stack is dry venting the WC (because the WC is the highest fixture draining into the whole stack), then you'd need to separately vent the lav and the tub. That would be in the form of a vertical dry vent takeoff on the trap arm before the stack, with the vent rising to a distance at least 6" above the flood rim levels of the fixtures before reconnecting to the dry vent portion of the stack.

And you could dry vent the lav that way, and use the lav drain to wet vent the tub, in which case the tub wouldn't need a separate dry vent takeoff. The wye where the horizontal lav drain joins the horizontal tub drain (before the stack) would be the vent takeoff, and so the resulting trap arm would need to meet the limits I mentioned in my first post.
Does it matter which is hits the stack the highest, sink or tub?
No.

Cheers, Wayne
 

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Only if you want the stack to vent the other bathroom fixtures. If the stack is dry venting the WC (because the WC is the highest fixture draining into the whole stack), then you'd need to separately vent the lav and the tub. That would be in the form of a vertical dry vent takeoff on the trap arm before the stack, with the vent rising to a distance at least 6" above the flood rim levels of the fixtures before reconnecting to the dry vent portion of the stack.

And you could dry vent the lav that way, and use the lav drain to wet vent the tub, in which case the tub wouldn't need a separate dry vent takeoff. The wye where the horizontal lav drain joins the horizontal tub drain (before the stack) would be the vent takeoff, and so the resulting trap arm would need to meet the limits I mentioned in my first post.

No.

Cheers, WayneW
To wet vent the tub from the dry vented lav: the trap arm for the tub would be the measurement from the tub P-trap to the Wye before the main stack?
The horizonal lav drain, after the dry vent is tied in, is draining at 1/4 inch per foot. The horizontal tub drain is draining at a 1/4" per foot and running higher then the lav drain. Where the tub drain will drop down to the Lav Wye, can that drop down at a 45 degree angle with added length of pipe? Would the trap arm be measured from the tub P-trap to where the drain falls 45 degrees or including the 45 degree fall of pipe?
 

wwhitney

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The horizonal lav drain, after the dry vent is tied in, is draining at 1/4 inch per foot. The horizontal tub drain is draining at a 1/4" per foot and running higher then the lav drain.
Stop there. The lav drain's vent take off has to be at the elevation of the trap. Usually a san-tee in the wall behind the lav trap. That's a consequence of the second sentence of the first post I made: "within one pipe diameter of fall".

Now after that lav dry vent takeoff occurs, your lav drain can turn down (which it will with a san-tee), enter the floor system, and turn horizontal within the floor system. Then if you want to use the lav drain to wet vent the tub, the tub trap and that horizontal lav drain need to be at approximately the same elevation in the floor system. Because again that tub trap needs to be vented "within one pipe diameter of fall".

So no, the tub drain can't turn down at a 45 before it joins the lav with a wye, if you want the lav to wet vent the tub. [You could instead dry vent the tub drain before it turns down.] It needs to join via a wye in which all the openings are horizontal (2% slope minimum, and pretty close to 2%).

Cheers, Wayne
 

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Stop there. The lav drain's vent take off has to be at the elevation of the trap. Usually a san-tee in the wall behind the lav trap. That's a consequence of the second sentence of the first post I made: "within one pipe diameter of fall".

Now after that lav dry vent takeoff occurs, your lav drain can turn down (which it will with a san-tee), enter the floor system, and turn horizontal within the floor system. Then if you want to use the lav drain to wet vent the tub, the tub trap and that horizontal lav drain need to be at approximately the same elevation in the floor system. Because again that tub trap needs to be vented "within one pipe diameter of fall".

So no, the tub drain can't turn down at a 45 before it joins the lav with a wye, if you want the lav to wet vent the tub. [You could instead dry vent the tub drain before it turns down.] It needs to join via a wye in which all the openings are horizontal (2% slope minimum, and pretty close to 2%).

Cheers, Wayne
thanks for clarifying Wayne. I think I understand now. The Wye needs to be on it's side and the trap arm needs to be able to connect to the wye within on pipe diameter.

If I wanted to get back to the original configuration of all 3 fixtures draining and venting into the main stack: sink 1st, tub 2nd and WC 3rd and not need additional venting: Where the 3 inch WC pipe connects to the main stack could I use a sanitary tee with a 2" side outlet. The washing machine is on the other side of the wall from the lav. Could I place the trap for the washing machine in the floor space, the ceiling of the unfinished basement, and run the 2" horizontal trap arm to the take off in the WC sanitary T so as not to need a dedicated vent for the washing machine?
 

wwhitney

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If I wanted to get back to the original configuration of all 3 fixtures draining and venting into the main stack: sink 1st, tub 2nd and WC 3rd and not need additional venting: Where the 3 inch WC pipe connects to the main stack could I use a sanitary tee with a 2" side outlet.
The tub could come into that side outlet, but not the washing machine.

Washing machine standpipe traps are typically above the floor. I believe the IPC would allow the trap below the floor, but it puts a maximum height on the standpipe above the trap. [30" IIRC, but I'm not confident.] While the washer manual will give you a minimum height above the floor for the top of the standpipe. Depending on the two values, below the floor may or may not be possible for the trap.

Even if it is possible, you can't wet vent the standpipe trap with bathroom fixtures above it. If you only had the WC and standpipe on the stack, and nothing above, then the double san-tee would work OK for dry venting both of them.

Cheers, Wayne
 
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