Load-bearing walls initially supported the original Singer Building at 149 Broadway, while the Bourne Building annex at 85–89 Liberty Street had an internal steel skeleton. The original Singer Building was altered between 1906 and 1908 to use a steel skeleton. The entire building used 850 steel columns. The columns were constructed in two-story segments, except on the basements, first floor, and 14th through 16th floors. Rafters supported the mansard roof of the base, excluding the tower. Millikin Brothers Inc. was the structural steel supplier for the project.