I have the same frost line here too. A lot of times we strap the pipe under the joists. Branch lines will tee of the top into the bays. The floor insulation is looped under the mains so the pipes are to the warm side. treat your tub/shower traps the same.
This is not guaranteed not to freeze though. Home owner needs to be proactive- keep vents and such stuff closed in the winter.
I suppose the real reason plumbers pipe like this is its all about money- get in, get out. On to the next one
drilling holes take time!
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