Under the 2009 International Residential Code, there are new definitions of Stair and Stairway. The new Stair definition is that it would have one or more risers and the new definition of Stairway is that it has one or more flights of stairs, interior or exterior, including landings and platforms. These new definitions also match what is already written into the IBC.
In addition, there is new language added under stairway design to require all tolerance requirements of treads and risers to be measured exclusive of carpets, rugs, or runners. One such tolerance requirement deals with how much one tread or riser can vary from one another. The maximum tolerance that the greatest tread depth within any flight of stairs can exceed the smallest tread depth is 3/8″. The same tolerance is required for riser heights also and while the tolerance is not changing, it is the material to which the measurement is to be taken from is now irrespective of carpets, rugs, or runners. This is a signifigant difference that contractors will need to take into account as they construct stairs.
