If there are faces with normal reversed, select those faces, then edit polygon --> normal --> reverse, u may need to play with the options if some faces are hard to select.
Another way to correct the normal in some cases is edit polygon
--> Conform. This makes all the face normals pointing the same side (outward or inward) of their corresponding faces, check it with Display --> polygon components -->normal, if all normals are reversed, reverse the whole thing.
Set to face makes ur vertex normal the same with its face normal, it will harden your model's edges but it will not reverse the face normal. But I find it useful to correct the improper direction of the vertex normal which might be the problem of your floor.
Please be sure that u don't confuse face normal with vertex normal.