I have a tooth with a crown. when i floss vertically i feel that there is a sizeable ledge between the crown and the tooth root. a problem?

See your dentist: Whenever there is a overhang, space or gap between the edge of a crown and a natural tooth structure, that's an open margin. This can cause decay to occur around or under the restoration and also fum inflammation. If this is the case, crown replacement is recommended. See your dentist for evaluation and x-rays.

Yes: There should be smooth, almost undetectable transition between tooth and crown. This would indicate a problem that needs to be addressed. See you dentist asap before the problem is even greater. It could be a number of things including decay, loose crown, overhang of margin, abrasion of root surface, calculus, etc.