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So, this is me reaching back aways in memory. Walls with plumbing through them are held to different standards to other walls. Walls with plumbing use 2 by 6s. Builders and architects try to optimize by making as few walls as possible walls with plumbing because they are more expensive. Your regular walls use 2 by 4s. So yes, some walls have plumbing running through them, but not all. Any architect and contractor wil…
I've never seen a wall framed with 2x6s because it had plumbing in it. It's all 2x4 regardless, here—you almost never see a 2x6 wall. Is that a California thing?
My California home is all 2x4s everywhere and there's plenty of plumbing in most walls.