Returns the value of TransomThickness attribute. This attribute represents thickness (width in plane parallel to door leaf) of the transom (if provided - that is, if the TransomOffset attribute is set), which divides the door leaf from a glazing (or window) above. If the TransomThickness is set to zero (and the TransomOffset set to a positive length), then the door is divided vertically into a leaf and transom window area without a physical frame.

Returns the value of TransomThickness attribute.