After testing, when attempting to walk on the grate (or what used to be the grate in the original tile), that is where I have issues. I get stuck, and can't seem to walk away from the center of the tile.
Upon closer examination, you have a layer of faces underneath the very topmost layer, which is apparently causing the "stuck" issue.
To set your walk material: Select the faces you want to set, scroll down the rollout panel until you find "Surface Properties". In the Material group you will see "Set ID". Use the up and down arrows to change the Material ID of your selected faces.
MDA