This website: http://www.therabbithouse.com/indoor/rabbit-cages-cubes.asp
has a really good NIC guide, including stuff about flooring On there they say "Shelves MUST be solid so if you use grids cover them with wood, lino, carpet etc. A good option is hardboard covered with lino/tiles to create an easy to clean surface. You can add rubber or straw matting for extra grip."