Dangerous!  These units slide on a composite deck like they are on ice.  They may keep the umbrella from tipping, but would not keep a wind from blowing the unit across the deck.  Also, this was the base I was told to get to go with the umbrella I bought, but only three of the units can be fitted in.  The foot pedal for the rotating base blocks the fourth.    DO NOT BUY THIS.  I would give it zero stars if I could.