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CROSS Safety Report

Roof collapse of warehouse structure under rainwater ponding

Report ID: 969 Published: 10 May 2021 Region: CROSS-US


Overview

This report highlights the structural risk caused by rainwater ponding on relatively flat roofs.

It highlights the collapse of a flat roof warehouse structure.

It demonstrates how ponding was caused by inadequate roof drain design exacerbated by changes to parapet scupper screens.

Key Learning Outcomes

For structural design engineers:

  • Although drainage design is normally not part of the structural engineer’s scope of work, it is good practice to coordinate roof drainage with other members of the design team, especially on relatively flat roofs

  • Understand how the performance of roof drainage that may be designed by other disciplines affects rainwater structural design loads

  • While typically not required by building codes, it is advisable to indicate the assumed rainwater design loads on the structural design drawings

For architects, mechanical engineers, and other designers of roof drainage systems:

  • Roof drainage design and roof rainwater design loads require careful coordination amongst disciplines

For contractors:

  • Consult the drainage design professional of record before modifying roof drainage systems

For building owners and managers:

  • Consult a qualified drainage design professional before modifying roof drainage systems when the building is in service

  • Maintain drains, scuppers, and plumbing to be free of obstructions

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Structural system

The project was designed and constructed in 1985 at a cost of $5M. The structure is a concrete tilt up wall building that is typical of warehouse construction in Texas, both then and now. The building is rectangular in plan, 748 ft (228 m) long (E/W) by 330 ft (101 m) wide (N/S). The roof structure was designed to slope at 1/4 in/ft (21 mm/m) to the north and south walls from an east-west ridge that is located 150 ft (46 m) north of the south wall. The design live load was 20 psf (0.98 kPa). The concrete tilt up wall panels are 12 in (30 cm) thick, nominally 24 ft (7.3 m) wide, and extend 37 ft (11 m) from drilled pier foundations to the top of parapet, about 13 in (33 cm) above the roof. The column grid is 30 ft (9.1 m) (N/S) by 36 ft (11 m) (E/W). In the north-south direction, steel roof deck is supported by 16H6 open-web steel joists spaced at 6 ft (1.8 m) on center. In the east-west direction, the joists are supported by 40G 6N 6.3K joist girders spanning between 6.5 in (17 cm) diameter pipe columns. The drainage system was not addressed by the Structural Engineer of Record (SEOR), since it was not within the structural scope of work.

Partial roof collapse

In May 1989 the building experienced a severe rainstorm. Meteorologists estimated a peak rainfall intensity of 4.0 in/hr (10 cm/hr) at the building site. During the storm, three roof bays collapsed adjacent to the south wall and pulled four wall panels inward , crushing the rack area where electronics were stored.

During the storm, three roof bays collapsed adjacent to the south wall and pulled four wall panels inward

The floor of the warehouse was covered with several inches of water, which caused many tall stacks of boxed appliances to topple. Ultimately, the owner filed a $26 million insurance claim, including $1 million of building damage and $25 million of inventory loss. The SEOR learned of the collapse two days after it occurred and rushed to the site. The air in Texas is dusty, and the dust settles on building roofs. When rainwater accumulates, the dust floats and leaves behind high water marks on building parapets. The SEOR observed and documented consistent watermarks indicating that at least 12 in (30 cm) of water had accumulated on the roof at the parapets, more than three times the design live load.

Drainage system

Codes then and now require primary and secondary roof drainage, with scuppers restricted to secondary drainage. Despite the code requirements the architect provided only scuppers spaced at 48 ft (15 m) on center along the north and south parapet walls. Each scupper is 9.5 in (24 cm) wide by 4.25 in (11 cm) high, with an invert elevation of 2 in (5 cm) above the roof. The roofing system is a rubber membrane under 1.5 in (3.8 cm) of river rock ballast. To prevent the ballast from washing through the scuppers, the architect specified hardware cloth gravel guards spanning across all scupper openings. In some locations, the contractor or owner apparently substituted a steel plate with a matrix of drilled holes in lieu of hardware cloth. Along the south parapet wall, there were a total of sixteen scuppers to drain an 112,200 sf (10,424 m2) watershed. Simple analysis using weir equations indicates that the scuppers were wholly inadequate, even before considering any blockage by the gravel guards.

Aftermath

Due to the size of the claim, extensive testing of the structural and drainage systems was undertaken. Ultimately, all parties concluded that the failure was due to the drainage and not to the structure. Repairs included the removal and replacement of eight bays of roof framing and six concrete tilt-up wall panels. No changes were made to the original structural design. New intermediate scuppers were retrofit in the north and south parapet walls. Each new scupper was approximately 30 in (76 cm) wide. This effectively increased the roof drainage capacity by more than 400%.

Lessons learned

Even though it is normally beyond their scope of work, structural engineers should pay attention to roof drainage, especially on relatively flat roofs. This is not limited to traditional considerations of ponding. A structural engineer should also verify that there are separate primary and secondary drains, properly sized and meeting code requirements. Why? If there is a roof collapse, the SEOR will almost always be named as a party in any resulting litigation.

Expert Panel Comments

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The principal cause of the roof collapse was inadequate roof drainage, a not uncommon source of problems, particularly in light roof structures.  Flow restriction from modification of the primary scuppers by the installation steel plates with drilled holes compromised the already inadequate original design. The lack of adequate primary and secondary drainage was a code violation and critical factor in the failure.

The principal cause of the roof collapse was inadequate roof drainage, a not uncommon source of problems, particularly in light roof structures

Design of drainage systems typically is not the primary responsibility of the structural engineer. Drainage is generally designed by a mechanical engineer (particularly in the case of roof-mounted drains) or the architect (common for parapet scuppers).

The awareness and involvement of a structural engineer in roof drainage design has been the subject of much recent discussion and appears to be an evolving issue. For example, the American Society of Civil Engineers Standard ASCE/SEI 7-16, Commentary, Para. C8.2 notes ‘Roof drainage is a structural, architectural, and mechanical (plumbing) issue. The type and location of secondary drains and the hydraulic head above their inlets at the design flow must be known in order to determine rain loads. Design team coordination is particularly important when establishing rain loads.’ No similar statement exists in the predecessor to ASCE 7 current at the time of the building’s design, ANSI A58.1-82. The author of Roof Drainage Design and Analysis: Structural Collapses, Responsibility Matrix, and Recommendations, published in 2005, suggests ‘The subject of roof drainage design should be a required item of discussion at the design review meetings between architects, mechanical engineers, and structural engineers.’

Drainage considerations can impact rainwater loads, so the structural engineer should consider the implications of the expected drainage system performance on roof loads . For example, roofs must be designed for the weight of rainwater assuming the primary drains are blocked. For the structural engineer, the rain load includes the static (depth of water to the invert) and hydraulic head (depth of water above the drain invert). Ponding load due to deflection of the structure is in addition to the rain load, and that is determined by the structural engineer for the specified system.

Drainage considerations can impact rainwater loads, so the structural engineer should consider the implications of the expected drainage system performance on roof loads

On most projects, roof drainage design and roof rainwater design loads require careful coordination amongst disciplines. While at present the International Building Code does not explicitly require that the structural design drawings indicate the assumed rainwater design loads, some consider this to be advisable practice.

Drains and scuppers should not be modified by an owner or contractor without the consultation of qualified professionals. Building owners and managers should maintain drains, scuppers, and plumbing to be free of obstructions.

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