Architectural / Project Development and Documentation · roof coordination
Roof overflow: coordinate water depth and the membrane transition
Combine overflow crest elevation and hydraulic head, compare a supplied structural limit, and preserve the drainage-control path through the parapet.
Reviewed for the current ReasonSpan™ exam profile · Updated October 11, 2026By the ReasonSpan™ editorial team · Our content standards
Two interfaces must be solved together
A roof's secondary overflow crest is 2 in above the low point. The selected opening requires 1.5 in of hydraulic head at the stipulated flow. The structural engineer's project water-depth limit is 3 in. Assume the crest and structural limit use the same datum and the hydraulic estimate includes the relevant opening condition. Determine whether this proposed overflow arrangement stays within the supplied limit.
The predicted depth at that flow is 2 + 1.5 = 3.5 in, exceeding the specified limit by 0.5 in. Crest height alone does not describe the water level during flow. Increasing the nominal opening width only resolves the conflict if a revised hydraulic calculation demonstrates the required head and all other criteria.
Resolve the performance and enclosure details
Coordinate an approved opening/elevation revision with the responsible structural and plumbing designers. Then trace the roof membrane into and around the overflow opening so the revised detail preserves a continuous drainage path. A capacity change cannot replace a missing waterproof transition, and sealant on the exterior face alone does not demonstrate concealed continuity.
These supplied depths are exercise criteria, not universal roof-drainage code requirements. Actual design must address rainfall criteria, blocked-primary conditions, ponding behavior, stability, local requirements and compatible manufacturer details.
Test a proposed revision
A revised opening reduces required hydraulic head to 0.75 in with the crest still at 2 in. Predicted depth is now 2.75 in, giving 0.25 in of arithmetic margin against the supplied 3-in limit. That result resolves this depth comparison; it does not independently verify flow capacity or membrane workmanship.
These are original instructional givens, separate from timed mocks. State assumptions before calculating and check the source edition required for your exam or project.
Sources and further reading
Exam requirements can change. Confirm current details with the organization that administers your exam and use the cited sources for the preparation principles summarized here.