Dennis, a guard is required on an open sided walking surface with a change in elevation of greater than 30″. The guard is required to be 36″ for residential construction and 42″ for non-residential construction. See Virginia Residential Code section 312.1.1 and Virginia Construction Code 1013.2 respectively…
One may also evaluate whether the open side is an accessible walking surface or potentially navigable terrain.
Sean P. Farrell