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How to lower a drain arm

Posted by ecranny (My Page) on
Mon, Mar 28, 11 at 20:13

I am installing a new kitchen sink with a garbage disposal, but it turns out that the existing drain is too high, and I need to lower the drain arm so that the disposal unit does not fill with water. I think this is something I should be able to do myself, rather than hire a plumber, although I have not done anything like this before. I am looking for tips on how I should proceed - how to cut the existing pipes, what kind of 'T' fitting I need to get, and how I should do the installation. Any help would be greatly appreciated. Here is a photo showing the drain/vent pipe and the existing 'arm'. I should add that the new arm can be shorter, so that it will end up to the right of the water pipes, so there is no interference to worry about. Also, what would you do about the framing - it looks like I will need to cut a new notch below the existing one, but if there is some way to avoid cutting through the wood that would be great.


Follow-Up Postings:

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RE: How to lower a drain arm

As far as cutting the pipe, sawzall with a metal blade, try cutting on the threads where the pipe is thinnest.

To reconnect to that galvanized pipe you will use no-hub couplings of the appropriate size, typically two inch, but its hard to tell from the picture - looks like it may be 1 1/2".

There should be some re-framing done to fix that double (looks like jack/king stud from some opening) that being said. What if you turned the pipe below the floor using a no-hub coupling and pvc elbow, if necessary drill round holes through the floor joists, leave 2" of lumber above or below each hole, then run up the wall where you want the drain. Use a PVC ty to stub out for your drain at the height you want, turn with a pvc 90, run new pvc through the existing notches, use a pvc elbow and no hub coupling to reconnect to the existing vent.


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RE: How to lower a drain arm

One consideration: make sure the vent pipe is supported so it doesn't come crashing down when you cut it. I'd suggest a plumber to do the rough-in portion.

But I would first take a closer look as to whether it works at the current height with your sink. I've seen drains "about that high" with a GD and deep sink at it was "just enough" to drain the water. How far off are you?


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more info

Here's the basic idea (see link)

Here is a link that might be useful: cutting into cast iron pipe


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RE: How to lower a drain arm

"Here is a link that might be useful: cutting into cast iron pipe"

That is galvanized steel pipe, not cast iron.

It is much easier to cut.

Cast iron pipe does not screw together with threads, it uses lead & oakum packing in hubs, rubber donuts, or no hub connections.

Make sure the upper (vent) portion is supported.
If you cut the pipe without ensuring it is supported the upper section may come crashing down (all the way from the roof).
If it joins into other vents it is likely to be adequately supported by the other connections.

You are probably gping to have to cut the pipe twice.
Once at the top of the existing T, and again low enough for the new trap arm.

Two Fernco connectors can the be used to install a section of plastic pipe with a T at the desired height for the new trap arm.

The only stud that is weight bearing are the double stud to the right of the window.

Some tight fitting blocks of wood and a scab on each side will fill in the existing pipe notch.

Make the scabs extend down almost to the floor and then notch through all four pieces for the new trap arm.


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RE: How to lower a drain arm

The idea posted above by "Replacement Faucet" is a bad idea because you'd end up with a flat vent less than 42" above the floor and less than 6" above the flood level of the new sink.

Replace those galv. steel supply lines!!

This looks like an older house with 2x4 walls. It also looks like an exterior wall. I would really consider furring-out the wall with 2x2's to get the equivelent of a 2x6 wall. You'd then have more space in the wall for insulation, and you'd be notching less of the load bearing studs. By carefully scribing/ripping and shimming of the 2x2's you could end up with a perfectly plumb wall as well.


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How to lower a drain arm

I'd also consider replacing the steel elbow that's below the floor level with a PVC long sweep elbow. Also consider adding a clean-out on the vertical drain below the vent. No telling how those old small steel drain pipes are going to react once you start cramming ground-up food, bones, and whatever else you toss into the new garbage disposal.


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RE: How to lower a drain arm

..that's galvanized steel...no cast iron.

Yup, my oversight. (I pasted that link too quickly)


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