Roof Drainage Design and Analysis: Structural Collapses, Responsibility Matrix, and Recommendations

Consider the following scenario: An engineering firm lands a design contract for a large commercial building project. The owner is pushing for a fast-track schedule, such that the final building design will not actually be complete before construction begins. From the start, the structural engineering discipline becomes the critical path for the project. There is tremendous pressure to complete the design of the structural system so that construction materials can be ordered and fabricated in step with the schedule. The structural engineer must perform analyses that consider code-required, minimum design loading conditions. The roof design incorporates live loads, snow loads, dead loads, wind loads, and seismic loads. What about rain loads? Will the roof drainage system be designed to prevent excessive rainwater accumulation on the roof? What assurances does the structural engineer have that the subsequent design contribution of other disciplines (i.e., mechanical engineering and architectural) will not result in excessive rain loads? Recent roof structure collapses in the north-central region of the USA (referred to as the Midwest) have raised these questions and related concerns about the building design process. Is reform needed? This paper will address proper roof drainage design, a rigorous analysis technique for estimating pre-deflection ponding depth on roof systems, the responsibility matrix for roof design, and suggested protocols to ensure that the roof drainage system is appropriate for the roof's structural design.