What is a "locus" when referring to house plumbing pipes?
The only thing that comes to mind (and it's not related to wooden house construction at all) when thinking of pipes and the word locus is that in German, "Lokus" is a term used for a toilet (perhaps it's a little nicer to say than toilet). That would fit your description of "locus pipes" - the waste pipes (and maybe supply) for the toilets.

Source: I'm a native German speaker. Also see the German Wikipedia entry for "Toilette"