@John Underwood I'm probably going to specify them for general blockages in future leases, but I'm not sure what's standard.

Check the outlets on the French drain and look for blockage (leaves and debris) along the drain as well.

The inspection found out there was sewage problems with blockage.

You may find roots or just a simple blockage.

We attempt to evaluate drainpipes by flushing every drain that has an active fixture whileobserving its draw and watching for blockages or slow drains,but this is not a conclusive test and only a video-scan of themain line would confirm its actual condition.

Make sure your tenant isn’t using flushable wipes, they can lead to pipe blockages. 

Short of some sort of lock-out, or physical blockage of occupancy, I am struggling to come up with any alternatives and thought I'd tap into the community's collective wisdom.Thank-you in advance.   

A kind lady at Lowe's (where I had gone to buy Drain-O) suggested I use vinegar and baking soda to loosen up any gunk, then use the cheap plastic "snake" to get the blockage out.

You could test air temp before and after the blockage and then decide. 

I am getting it scoped tomorrow to see where the blockage is exactly but we think it is probably a root ball in the pipe (caused by nearby tree).