CROSS Safety Report
Failure to maintain roof drainage during re-roof leads to ponding stability collapse
This report is over 2 years old
A roof collapsed while the building’s roofing membrane was being replaced.
The failure was caused by inappropriate stockpiling of construction debris during reroofing, which obstructed the roof’s drainage scheme.
Key Learning Outcomes
During reroofing operations, be aware of the roof’s drainage scheme and maintain necessary drainage at all times
Comply with industry standards such as the NRCA Roofing Manual
Use care not to overload the roof deck and structure with construction equipment and debris
For building owners and managers:
Contracts for reroofing operations should have specific requirements for maintaining necessary drainage throughout the reroofing operation and incorporate industry standards such as the NRCA Roofing Manual by reference
Consider engaging a professional roofing consultant to prepare contracts and specifications for reroofing operations, if you do not have such expertise in house
Consider engaging an independent professional to observe reroofing operations while they are in progress
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An existing building was being re-roofed, which involved the removal of existing roofing materials from atop the structural roof deck. During this process, construction debris was stockpiled along the low side of the roof and was periodically removed off the roof to a dumpster. During an afternoon, a rainstorm developed which dropped approximately 1/2 inch [1.3cm] of rain at the project site.
Unfortunately, the stockpiled construction debris along the low side of the roof acted as a ‘dam’, preventing rainwater from running off the roof. This rainwater built up on the roof, leading to a ponding-stability collapse of the roof framing at the lowermost bay. This collapse could have been averted by simply periodically breaking the ‘dam’ of roofing debris along the low side of the building or by stockpiling debris at the high side of the roof instead. Roofing contractors need to be aware of how existing drainage performs and work to maintain the operability of roof drains and overflow drains during the re-roofing process.
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Expert Panel Comments
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Roof collapses due to unintended overloads are commonplace. Some, like this case, are due to drainage impediment. Others are due to overloads from construction materials or debris storage, snow piles from plowing, dirt piles from amenity deck construction, and heavy trucks parked in unapproved areas. The NRCA Roofing Manual, 2019, Chapter 9, gives considerable guidance on reroofing operations, including the importance of understanding existing conditions, evaluation of drainage, and loads that must be considered. The Manual quotes the following from the International Building Code - 2018:
The Manual goes on to say:
Re-roofing project specifications frequently contain language such as:
This case highlights the duties of a contractor to not overload the existing structure and to ensure proper drainage is maintained during re-roofing operations. The latter requires a careful understanding of how the roof surface is sloped and intended to drain. For projects of the scale and consequence that warrant written specifications, language requiring these duties is helpful. Independent inspection of the progress of the work also can reduce risk.