The best possible fixing would be to fix into the actual studs that make up the wall if you can locate them as this will provide the strongest fixing. Failing this, you will need to fix into the plasterboard (assuming the wall is made up of this).
Secondly, the type of fixing you use will determine how large a hole you will need to drill. The best fixings to use will be either Redidrive (3mm pilot hole and 8-10mm hole needed to insert the fixing itself) or a Nylon Toggle (8mm hole to insert toggle into). More information on fixings can be found here: http://www.diydoctor.org.uk/projects/fi ... rboard.htm.
It’s best to not try and drill a large hole in one go, start small (3mm hole) and work your way up to the required size (depending on type of drill bit you are using). Be careful not to drill through the tile into the plasterboard behind as this will affect the strength of the overall fixing.
Take your time drilling the hole as drills can slip and mark tile and tiles can crack easily. Also, do not use the hammer action on your drill!! Also, add water as you drill to cool.
With the hole drilled you can then insert your chosen fixing into the plasterboard behind. Make use the fixing goes right through the tile into the wall behind. If the fixing is within the confines of the tile and is of the type that expands outwards as you screw in a screw, it may expand to the point that it pushes on the tile from inside the hole and the tile will crack.
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