>> That's because people who build the stairs works with that order
>> of tolerance (mm).
> 
> So... what the hell do you do if you build some stairs, and the sement 
> sinks by 0.4 mm while drying? (A perfectly likely outcome.) Then each 
> step might be 0.4 mm out...! No noes! o_O

Who secures stairs in place by resting them on unset concrete? :-S

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