Securing Roof Garden Furniture

Hi All-

I am working on my first roof terrace garden.  We want to use moveable furniture, but the client is concerned about it blowing off the roof and injuring someone below.  There are hundreds of rooftop gardens in the world, how is this concern typically handled? 

Some ideas we have come up with:

-lightweight furniture that someone can move inside in case of a storm (con: people aren’t able to move it inside fast enough, or a storm brings winds unexpectedly)

-very heavy furniture that is not easily blown around (con: harder for people to move the chairs to an arrangement they like, very high winds can move incredible weights, heavy furniture is usually more expensive)

-tether the furniture with an eye-bold/cable system which allows for some movement, but won’t blow off the roof (con: can be unsightly)

Any advice on how others have handled this is appreciated!