When to convert from 2" to 1.5"

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jeffvdm

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I have a shower reno in progress and my existing shower drain line is 1.5" abs. The new drain (kerdi) is to be connected to 2". I have access to the p-trap and a couple feet of the drain line following the p-trap. I was curious at what point do I convert the 2" line to connect to my existing 1.5" abs. Before or after the p-trap?
 
It is a bad idea to reduce the pipe size and generally is not allowed under any of the codes. You really should replace the entire line with 2". Follow it back until the 1.5" line expands to 2" or greater and that whole section should be replaced with the 2".
 
Thanks nukeman for your response.
It's a second floor shower, so there is limited access to change the line to 2".

Thanks Terry for your response.
I'm sure there will be enough space between by 2x8 floor joists for the drain followed by a chunk of 2" followed by a bushing and then the trap?
 
You should never go from larger to smaller. It's fine to go the other way. Is this the same shower that is going to have all sorts of heads? If so, you are asking for all sorts of problems cutting back the drain size, and as noted, will not meet code anywhere.
 
If you are just running a single head shower, then 1.5" is allowed by code. You should use a reducing bushing right at the shower drain fitting.
 
1 1/2" is a bad idea for a shower drain, but if you are locked into it, then just insert a bushing into the drain, assuming it is a glue joint drain.
 
Thanks all for your replies.
I'm stuck with the 1 1/2" drain. It's just a simple, one shower head setup. The shower that was previously there had no issues draining with the existing plumbing.
I'll add the reducing bushing right at the kerdi drain.
Thanks again.
 
I was fortunate enough to use the existing shower plumbing, as I was able to cut the 2" pipe down to sufficient length for my drain. However, when I dry fit the drain on pipe, it goes on with little effort at all. Should I be worried, or should I want that nice snug fit that actually requires some effort?
Thanks.
 
I've only installed one Kerdi drain, and, like you, found that it isn't as much of an interference fit as I was accustomed to. Just make sure you use a liberal amount of cement. I think that they may do this intentionally, as they don't want the fitting (the drain) to move once you glue it on, as it must stay in intimate contact with the mortar bed. A more 'normal' fitting would have the tendency to push back, since it is tighter, and that would result in the drain sticking up slightly if you didn't hold it long enough, which, ultimately, could lead to a failure in the long-term. IOW, glue it well, and don't worry!
 
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