Which condition defines vertical geometric irregularity in a building?

Vertical geometric irregularity in a building is primarily defined by the relationship between the horizontal dimensions of adjacent storeys. When the horizontal dimension of any storey exceeds 130% of that of an adjacent storey, it indicates a significant change in geometry that can affect the building's structural performance, particularly under lateral loads such as wind or seismic forces. This irregularity can lead to uneven distribution of forces throughout the building, potentially resulting in safety issues during extreme events.

The other choices do not reflect the criteria for vertical geometric irregularity. For instance, equal horizontal dimensions in adjacent storeys or uniform size throughout all storeys suggest a consistent geometry, which is typically preferable for structural stability. Similarly, the weight of the roof in comparison to the storeys below does not inherently indicate geometric irregularity but rather relates to load distribution and structural design considerations.