Most tile manufacturers know the COF of their tile, you may have to ask. Many list it in their prooduct catalogs. A 4 would be plenty.
www.daltile.com
www.crossvilleceramics.com
Dave Gobis CTC,CSI
Executive Director
Ceramic Tile Education Foundation
www.tileschool.org
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If you use a anti-slip tile with a nonslip coating. In a few years the non-slip coating will come off and you will need to replace the tile. It is better to look for antislip porcelain tile that has a rough or diamond shapes pattern on the surface. This will last you much longer and you will lower maintainance cost
Edmond M Samander
Ceramica Nova