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 Narrowing a vent pipe then taking it back to its original size
Author: Panic521 (VA)

I have a 3" primary vent pipe that runs from the sewer connection through the roof. Unfortunately, I have a gambrel roof and when the pipe gets to where the slope of the roof changes it no longer fits behind the wall. Whoever built the house "solved" this problem by boxing in that area (about 1 cubic foot) which is pretty darn ugly. I'd like to put a 1 1/2" wye below the problem point and run two 1 1/2" pipes up into the attic. 1 1/2" pipe will clear the gambrel bend. In the attic, I can then invert a second wye and connect the 1 1/2" pipes to the 3" pipe so it can continue on its merry way. Essentially, I'm not changing the amount of fluid that the stack can support, I'm just changing it's pathway. By the way, I'm a newbie to anything more complicated than hooking up a sink and connecting a toilet. I'd appreciate any comments you may provide.

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 Re: Narrowing a vent pipe then taking it back to its original size
Author: LI Guy (IN)

If you recall from math class, the area of a circle is the radius squared multiplied by pi. So for a 3" diameter pipe, the area of a cross section is the radius (1.5"winking smiley squared, multiplied by 3.14 = 7.07 sq in.

On a 1-1/2" pipe the area is 1.77 sq in, and two 1-1/2" pipes together have a cross section of 3.53 sq in.

so two 1-1/2" pipes can carry about half the air or water than a what a 3" pipe can carry (assuming same pressure and temperature).

The math says you would need four 1-1/2 pipes to match the volume of one 3". From a practical standpoint, it would probably work fine, but not sure if this type of thing is allowed by code, so will let the pros weigh in here.

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Not a plumber by trade but a fierce DIYer

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 Re: Narrowing a vent pipe then taking it back to its original size
Author: sum (FL)

If you have a 3" vent through the roof and if you have a blockage in your line inside the house and a plumber can't find a cleanout, he may go up to the roof to run a snake down. He will never expect that 3" vent will split into two or four 1.5" pipes and his full size auger head will be stuck 12 inches down. I think it's a bad idea because it messes with the serviceability of that pipe.

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 Re: Narrowing a vent pipe then taking it back to its original size
Author: bernabeu (SC)

that type of thing is NOT, repeat NOT, allowed by code

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Retired U.A. Local 1 & 638
"Measure Twice & Cut Once"

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 Re: Narrowing a vent pipe then taking it back to its original size
Author: jblanche (WI)

Can you add swing joints to change the slope?

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Links to the State of Wisconsin Plumbing Code:
[docs.legis.wisconsin.gov]
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I am not a plumber.
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