Rerouting a vent

areeferFebruary 6, 2011

I'm converting a tub to a shower stall. After I demolished the cast iron tub I discovered that the vent didn't go straight into the floor. About a foot off the floor, the vent angles off at 45 degrees into the floor to the drain. This was hidden by the tub front apron.

The vent is galvanized steel above the 45 and copper below. I want to cut the galvanized pipe above the 45, continue the vent straight into the floor and then continue it horizontally to the drain.

Can the vent be run horizontally (with a slope) to the drain? Or do I need to attach the vent with a 90 into the top of the drain run?


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There are no horizontal vent lines under the floor. All vent lines MUST RISE Vertically until they reach an elevation 6" higher than the flood level rim of the highest fixture served by that vent.

    Bookmark   February 7, 2011 at 6:59AM
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Thanks lazypup.

But I'm confused with Pattern 1 in Peter Hemp's "Plumbing a House" (pg 109):

Pattern 1

Here the vent runs horizontal to the vertical portion of the vent.

Is this incorrect?


    Bookmark   February 7, 2011 at 8:54AM
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I looked at that reference and I even copied the ISBN number and traced the book. I could not find out when that book was published but I did confirm that it is currently out of print, even though there are some used copies available.

The layout as illustrated in Pattern #1 is totally wrong.

Not only does the code prohibit a horizontal vent line under the floor as shown, it also prohibits using a sanitary tee on a horizontal line.

The concern is that if we have horizontal dry vent below the flood level rim of the fixture water & effluent could get backflowed into the pipe and the effluent could then be deposited forming a clog in the vent line.

Both the IRC & UPC require a 2" line for a shower drain.

Under the IRC a 2" line may run 8ft from the trap weir to the vent opening.
Under the UPC a 2" line may run 5ft.

There is a way that you can attach a vent under the floor. Install a vertical vent line at the lavatory drain location. You then tee the lavatory arm off that vent riser and continue the riser down below the floor and over to the shower drain, connecting the line from the lavatory to the side of the shower drain line by means of a Wye & 1/8bend. In this manner as liquids drain from the lavatory they would drop down the riser then go through the horizontal pipe to the shower drain line thus continually washing the line. In this configuration the line from the tee behind the lavatory waste arm to the shower is properly defined as a "Combined Waste & Vent".

    Bookmark   February 8, 2011 at 8:42AM
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The book was originally published in 1994. I have the 1998 revision. I guess that's what I get for picking it up on sale!

Here's a picture of my situation. The drain continues to the toilet to the left.

How do I continue the vent straight into the floor and hook up the shower drain?


    Bookmark   February 8, 2011 at 4:11PM
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To avoid any horizontal vent run, I think my only option is to extend the drain line to the vent wall and tee in the vent from above. But that would mean that the drain from the shower would need to run into the back of tee. Which would make the shower drain turn 180 degrees to get to the drain.

Is that ok?

    Bookmark   February 11, 2011 at 8:47AM
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The first post is correct, careful what you read on the Internet

    Bookmark   March 5, 2011 at 10:55PM
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In plumbing any pipe which rises at an angle equal to or greater than 45 deg is said to be a "Vertical Offset" and any pipe that rises at an angle less then 45deg is said to be a "horizontal offset".

In the photo the vent line is connected on the downstream side of the WC waste arm and rises vertically at a 45deg angle therefore it meets code as a vertical line.

If you extend the line from the W.C. waste arm to the wall then turn up there is no fixture discharging into that section of line therefore the extension from the W.C. waste arm to the wall would be a horizontal vent, which is below the flood level rim of the fixture and it would be prohibited.

There is one option that may be open to you.

You could continue the from the W.C. fixture arm to the wall then turn upwards, then connect the drain from your lavtory to the vertical riser at the same height at the lavatory fixture arm. In that manner the vertical riser from the lavatory drain and the horizontal from the vertical riser to the W.C. fixture arm would be a "Combined Waste & Vent". But be careful here. If the length of the drain from the lavatory to the vertical riser exceeds the total developed length allowed for the lavatory fixture arm you would need to add a vent on the lavatory line,rising vertically until it is 6" above the flood level rim of the lavatory then turn horizontal to the stack.

    Bookmark   March 6, 2011 at 7:41AM
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Another forum suggested that I could bend the shower drain into the back of the vent. I extended the drain to the vent and attached it with a wye/45 combo. The shower drain connects with a 180 into the back of the wye.

It's only dry fit until I figure out which shower pan I'm going to use.

From Bathroom

    Bookmark   March 6, 2011 at 11:29AM
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