BS 5385 states.
7.1.9 Tolerances
7.1.9.1 Finished tile surfaces
The surface should be true such that, when checked with a 2 m
straightedge with 3 mm thick feet at each end, the straightedge
should not be obstructed by the tiles and no gap should be greater
than 6 mm.
There are permissible manufacturing tolerances for ceramic tiles
defined in BS EN 14411; certain types of tiles, e.g. extruded or large
format, might have permissible surface flatness irregularities that
cannot satisfactorily be accommodated within the surface flatness
tolerance permitted to the tile installer; this should be taken into
account when evaluating the achievable flatness of a wall floor tiling
installation.
NOTE Where adhesives are used, this degree of accuracy can be
achieved only when the background surface is equally true.
7.1.9.2 Across joints
There should be no appreciable difference in level across joints
(commonly called “lipping”) and the maximum deviation between
tile surfaces either side of a joint, including movement joints, should
be as follows.
a) Joints less than 6 mm wide, 1 mm.
b) Joints 6 mm or more wide, 2 mm.
