Bad odor in units

I have a small single story commercial/retail building that has rooftop parking. Many of the units report a sewer smell from time to time and I haven't been able to solve this problem. Each unit has their own tenant improvements which include restrooms. Each unit vents up to the bottom of the rooftop concrete deck and takes a 90 degree turn to the back of the building where the vapors escape.

I'm wondering if there is such a device where I can insert an exhaust (fart fan) fan inline to help suck out the vapors. Does such a devise exist? Is this safe considering I'm dealing with vapors? This can be inserted inside each unit or even outside.

Thanks for your assistance!!!!

One of the reasons I find for the odors, is floor drains, if they do not have trap primers attached and are not used much then the water in the P=Traps can dry out and provide a path for the odors to escape. On the Bars I try and talk them into dumping a 5 Gal bucket of hot water down each drain as part of their weekly clean up procedure.
Michael

Michael,
That's the first thought I had too.
Floor drains are trouble in commercial buildings to be sure. That is the first thing he should check, and then get back to us.

The problem some people make with floor drains on buildings with improper venting is they dump 5 gallons of water down it and it just siphons out. More is not always better. My office at work was an old hotel that got converted and I often pour a cup of water into the floor drain to stop the smell. In the kitchenette down in the basement there is a constant sound of gurgling at the sink trap as it too is not properly vented. The roof vent was not done right as it is a wet vent and it freezes in Winter.

Are You saying the vent pipes do not extend 10 feet above the parking deck or above the roof if the parking area is covered? If that is so, extend the vents
George