Rick nailed it - usually it is the lending institution which requires it.

Bathrooms and kitchens used to be required to have an impervious surface, but that is gone now (I think - would have to look that up to be 100% sure, right now only 99% sure).

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Jerry Peck, Construction / Litigation Consultant
Construction Litigation Consultants, LLC ( www.ConstructionLitigationConsultants.com )