Like I stated in a previous post, here in Michigan, drains in outbuildings are a no, when the drain, drains directly outside. If it's tied into a sanitary system, that's ok. Interestingly, storm water can drain undergrouind and exit at a lower level. Grey water can't.
In my situation, I can't have any slope to the floor because I'm setting machinery on it. The floor was laser levelled during the pouring phase and power trowelled with an epoxy top coat.
My contractor thought I was wacky when I specified 8" of 6 bag mix until he looked in the other building and saw what I was going to set in the new building. It's important that the floor is stable and level as possible. When working in tenths, every little bit helps.
The building I added on to has floor drains (prior to the Michigan Law) with appropriate slope. It's a PITA to accurately level machinery in 2 planes with a floor that slopes.
If I was one of my motorcycles in the new shop in the winter with the PEX system running, a floor squeege to push the water out the door and 20 minutes later the floor is dry and clean.....
In retrospect, I should have run another zone under the parking pad and driveway to the building with a temperature controlled servo valve to keep the snow and ice melted off. It don't take a whole lot of heat/bru's to melt snow and ice.