I am turning a huge room above my garage turned into two bedrooms and a bath. Thought this would be a piece of cake...

I would like to heat the floor in the bathroom, including the doorless walkin shower. The plumber says I have to have a whole new zone for this (already have hot water baseboard in the room on it's own zone) so when the electrician came I asked about the heated mats, can do but not in the shower floor. Is this true? Can't use an electric floor mat under the tile in the shower?

Nancy