replying to Donn, Iggy wrote:
I think you've got to rip open the garage wall or ceiling (wherever the pipe is)
and investigate that sag. This will let you see what the plumbing actually is
and if it even looks professional or proper or something of a patchwork of
methods and materials.
Your home should be too young for Lead Drains, but that stuff is notorious for
sagging and there could be a section of it used for whatever reason. However,
you should be full cast iron and you may simply have a poorly done joint or a
joint that was unsupported and has slowly fallen or settled out-of-level.
If shoddy material or work is found, then you may have other issues contributing
to the overall problem...you may not have proper venting and especially for
anything that's more than 5-feet from the vent stack pipes. S-traps or
double-bendy pipes under sinks or other places usually indicate the lack of a
One common contributor is the pipe immediately under the toilets. The cast iron
there is bathed in both water and air and rusts itself shut. The toilets need to
be removed and the rust chiseled off to return the pipes to full diameter (you
must vacuum-out the chiseled rust).