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CROSS Safety Report

Failure to maintain roof drainage during re-roof leads to ponding stability collapse

Report ID: 864 Published: 1 March 2020 Region: CROSS-US

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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

For contractors:

  • 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.

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:

1511.2: Structural and construction loads: Structural roof components shall be capable of supporting the roof covering systems and the material and equipment loads that will be encountered during the installation of the system.’

The Manual goes on to say:

‘The structural integrity of the roof assembly must be maintained during reroofing operations, including loading on the roof attributable to workers and material being present during this special period of time. The roof structure must be able to support all layers of roof-covering materials.’

Re-roofing project specifications frequently contain language such as:

‘Maintain roof drains in functioning condition to ensure roof drainage at the end of each workday. Prevent debris from entering or blocking roof drains and conductors.’


‘If roof drains are temporarily blocked or unserviceable due to roofing system removal or partial installation of new membrane etc. provide alternative drainage method to remove water and eliminate ponding.’

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.

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