If you need collisions then do a copymesh, flipmesh and addmesh. The will double your triangles though.
http://www.blitzbasic.com/b3ddocs/command.php?name=copymesh
http://www.blitzbasic.com/b3ddocs/command.php?name=flipmesh
http://www.blitzbasic.com/b3ddocs/command.php?name=addmesh
http://www.blitzbasic.com/b3ddocs/command.php?name=copymesh
http://www.blitzbasic.com/b3ddocs/command.php?name=flipmesh
http://www.blitzbasic.com/b3ddocs/command.php?name=addmesh