Wind-damaged Roof Systems

Wind damage to roof systems is often catastrophic, placing the building users at a life-safety risk, resulting in interior and furnishing damage and suspension of interior operations, loss of revenues, legal ramifications and great costs to repair. Because of my 30 years of experience in the design of roof systems and forensic investigation, I’m often called upon as an expert witness after wind events. In this article, I’ll review a couple wind-event roof failures, the causes of the failures and how they could have been prevented. I’ll also provide recommendations for failure prevention in the design process for new roof systems, as well as for existing roof systems.

1. The concrete roof deck panels deflected more than 3/4 inch, which the design architect should have accounted for if a thorough field investigation was undertaken.

1. The concrete roof deck panels deflected more than 3/4 inch, which the design architect should have accounted for if a thorough field investigation
was undertaken.

The Perfect Storm

How can it be that when roof systems are to be designed for code-required wind-uplift resistance that so many fail in winds well below the design parameters and/or warranty coverage? The answer could be design-related, material or installation; typically, it involves all three.

Architects and some roof system designers are often not as knowledgeable about roof systems as they should be, have little empirical evidence in how all the components work together as a system, and move beyond their abilities (a violation of their standard of care) when designing roofs where specific detailing is required. In addition, manufacturers are all too often
bringing new products to the marketplace that have not been properly vetted in the field and their long-term performance is truly unknown. Unfortunately, the roofing contractor cannot escape any of this. The lack of proper specification and contract document review; failure to review product data, including installation guidelines for new products; poor project oversight and management; and pressure from general contractors often result in installations that are subpar. The result is a “perfect storm” of design, materials and installation that fail under stress.

Consider the following case studies that I have been involved in as a forensic or “expert” witness when litigation was involved.

Coastal Facility

A large aged warehouse along the eastern seaboard was in need of a new roof system. Because the interior was not conditioned, thermal insulation was not required. The existing roof was an asphalt built-up with aggregate surfacing on high-density fiberboard on precast concrete panels 24-inches wide on a steel structure. The northern portion of the building had overhead doors that were seldom closed. On the interior, an aedicule structure (a building within a building) was constructed approximately 65-feet south of the overhead door, which had a ceiling level 5-feet below the roof deck.

2. The thin, flexible 1/2-inchthick high-density board was found to have little, if any, contact with the full-coverage spray-foam adhesive, making uplift extremely easy.

2. The thin, flexible 1/2-inch-thick high-density board was found to have little, if any, contact with the full-coverage spray-foam adhesive, making uplift extremely easy.

The architect who designed the replacement roof system called for the existing BUR roof to be removed down to the precast concrete roof panels. Then a new 1/2-inch 4- by 8-foot high-density wood fiberboard was set in full-coverage spray polyurethane foam adhesive with a 60-mil EPDM membrane fully adhered to the high-density wood fiberboard.

Additionally, the architectural drawings called for rooftop relief vents to be removed and capped over.

Around June 2008, a Nor’easter (an intense rainstorm), coming in from the east off the ocean, swept into the city. This resulted in the new roof system being lifted off the roof deck. Mode of failure was the fiberboard detaching from the precast concrete roof deck.

Investigation revealed several acts and conditions that contributed to the wind damage.

PHOTOS: Hutchinson Design Group Ltd.

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Roof Decks: Don’t Underestimate the Backbone of the Roof System

NOTE: This article is intended to provide general information while conveying the importance of the roof deck as an integral part of a roof system. Additional information about specific effects and concerns in regard to roofing can be found in The NRCA Roofing and Waterproofing Manual and various roof-cover manufacturers’ design guides.

Wood plank decks can provide a dramatic exposed roof deck.

Wood plank decks can provide a dramatic exposed roof deck.

The roof deck is the backbone and an integral component of all roofing systems. Its main function is to provide structural support for the roof system and, therefore, is a building element that needs to be designed by a licensed design professional because proper support of the roofing above is critical to the roof system’s success.

Roof decks also add thermal performance and fire resistance and ratings, provide slope for drainage and enhance wind-uplift performance. They must accommodate building movement and often determine the attachment method of the vapor retarder, insulation and membrane.

Roof Deck Types

There are many types of roof decks being installed today:

  • Steel
  • Precast concrete panel
  • Structural concrete
  • Cementitious wood fiber
  • Wood planking
  • Plywood/OSB
  • Poured gypsum

Some decks are covered with topping fills that become the base for the roof system and may also be an integral structural component:

  • Concrete
  • Lightweight insulation concrete topping
  • Lightweight aggregate concrete topping

Other deck toppings are available, such as poured gypsum and lightweight concrete with integral insulation, but these are considered substrate covers and not roof decks.

The most prevalent roof deck in the U.S. for commercial buildings is steel. On the West Coast, plywood/OSB is very popular. In addition to the roof decks already mentioned, in the course of roof-replacement work the designer may come in contact with the following:

While the “plate” test is not a preferred method, it can quickly and inexpensively give an indication of retained moisture in lightweight aggregate concrete roof deck covers.

While the “plate” test is not a preferred method, it can quickly and inexpensively
give an indication of retained moisture in lightweight aggregate
concrete roof deck covers.

  • Book tile
  • Lightweight precast concrete planks
  • Precast gypsum planks
  • Transite

Collaboration with the Structural Engineer

Because a roof deck is the foundation for the roof system, the designer needs to coordinate the roof system design requirements for the roof deck with the structural engineer to ensure the performance of the roof system. For example, the roof deck may need to extend to the roof edge. In this example, the roof deck may not need to extend to the roof edge for structural concerns but is needed to support the roof system; the roof designer must address this. If the roof deck is structurally sloped, the designer and engineer must determine whether the low point is a potential drain location. Are there steel beams in the way of the drain location? The roof deck must be attached to the structure to prevent uplift. And the designer and engineer must determine what the deflection of the roof-deck span may be between structural supports. For example, steel deck is sometimes installed with spans of 7 feet between joists and flexes (deflects) under foot traffic. This typically is not a good condition onto which a ridged roof system, such as a bituminous one, should be installed. It cannot be expected to accommodate such deflection. PHOTOS: Hutchinson Design Group Ltd. [Read more…]