Except as provided in section 27-555 of this article, the dead load shall be the actual weight of the building materials or construction assemblies to be supported, computed from the unit weights given in reference standard RS 9-1. Where unit weights are not established in reference standard RS 9-1, the actual weights may be determined by analysis or from data in manufacturers' drawings or catalogs. Unit weights less than those given in reference standard RS 9-1 may be used only with approval of the commissioner.