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…]

Transite Roofing: Friend or Foe?

As Transite, or Asbestos-containing, Roofs Come to the End of their Life Cycle, Contractors Should Know When Retrofitting Is an Option

The use of asbestos dates back thousands of years. For millennia, cultures across the globe embraced asbestos’ super-strengthening properties. Asbestos’ popularity peaked in the late 19th century during the Industrial Revolution when commercial asbestos mines sprung up across the U.S. and Canada. Before its carcinogenic properties were discovered, asbestos was used in hundreds of applications, including walls, roofs, coatings, fireplaces, shingles, insulation, pipes, furniture, paper products, automobile parts, fabrics and packaging. In the construction industry, in particular, it was considered a “super-product.” Whether mixed as a binder with cement or used as a coating on steel panels, asbestos is insulating, non-combustible, corrosion-resistant, inert, humidity-tolerant and sound absorbent.

One major application of asbestos over the past 100 years has been transite roofing panels. Asbestos was essentially used as a binder in cement slurry and then formed into profiled or flat sheets. Transite roofing panels can still be found across the country; many are still in place after 50, 60 or even 70 years of life. Because transite panels acquired asbestos’ super-strengthening properties, they made (and in some cases continue to make) ideal roofing for foundries, forges, chemical plants, paper plants, wastewater treatment plants and sewage facilities. The roofing material withstands high heat, chemical emissions, humidity and other elements emitted by these facilities that other building products could not tolerate.

Transite roofing panels can still be found across the country; many are still in place after 50, 60 or even 70 years of life.

Transite roofing panels can still be found across the country; many are still in place after 50, 60 or even 70 years of life.

Despite the strength of asbestos, even transite roofs can deteriorate or require renovation. In fact, most roofing contractors have encountered or will soon encounter transite roof jobs. The job where transite is in good condition with no airborne particles may be a perfect candidate for a retrofit.

(For the purposes of simplicity, this article uniformly refers to corrugated, asbestos-containing cement roofing sheets as transite. Professionals may also encounter names like 4.2 cement asbestos or corrugated cement. It’s important to note not all corrugated cement roofing sheets contain asbestos; some manufacturers substituted wood fibers for asbestos during the height of asbestos panic in the 1980s. Few of these products successfully penetrated the market because performance did not match that of original transite.)

Leave It In Place

Contrary to popular belief, asbestos is still legally used in many commercial applications in the U.S. today, including roofing and flooring materials; in fireproofing; and in friction products, like brake shoes and clutches. With the surge of installation of transite roofs in the 1950s and 60s, the lifespan of many of these roofs’ components is just now ending. Common factors attributing to roof or structure deterioration may include longitudinal cracks along the panel highs, broken or brittle fasteners or washers, friable panel material, and/or building shifts due to expansion or contraction. Additionally, other renovations on a building may require that old roofs that are still intact be brought up to new codes.

Today, a good general rule about asbestos is “leave it on if you can,” meaning it must be in good condition with no airborne particles.

Today, a good general rule about asbestos is “leave it on if you can,” meaning it is in good condition with no airborne particles.

Professionals encountering transite roof jobs may feel confusion about how to handle asbestos-containing materials. Some may even avoid transite jobs altogether assuming they will equate to expensive asbestos-removal procedures and red tape. However, asbestos abatement (the process of removing or minimizing asbestos health hazards from a structure) can take many forms, including removal, enclosure, encapsulation or leaving the material undisturbed.

In the past, abatement through removal was the recommendation of many asbestos professionals. Traditionally, transite was replaced with fiberglass. This solution is imperfect, however, because of the expenses of asbestos removal and new fiberglass, as well as the lower heat tolerance of fiberglass-based materials. More recently, approaches have changed and several other options present themselves.

Today, a good general rule about asbestos (and in fact the position of the Washington, D.C.-based U.S. Environmental Protection Agency) is “leave it on if you can,” meaning it must be in good condition with no airborne particles. Although not always possible, when the contractor can leave the asbestos-containing material in place, asbestos should be considered friend, not foe. Regulations might prevent installation of a new asbestos transite roof, but old buildings that can keep their transite roofs in place will continue to reap the benefits of the product’s super-strengthening properties. [Read more…]