The structural roofplate thickness for any built object is the median distance between the upper and lower surfaces of the basic load-bearing slab used in the roof construction. The structural roofplate may rest over the roof's framework, in which case the framework members are not included in the thickness. In other cases the slab may cover its span without supplemental framework, or it may be reinforced internally; in this case the reinforcements are included in the thickness. Any interior finish, dropped ceiling, or exterior roofing veneer is not counted in this measurement.

This measurement is always less than or equal to the finished roof thickness.

The value is a median across the whole object and not a figure taken from one place. A tower normally has thicker walls low down than high up, so a number read off a single detail drawing answers a different question than this field asks.

Where a building has two clearly different constructions, such as a podium and a tower, each is described on its own record rather than averaged into one value.