Roof Decks and Roof Performance

Dec. 13, 2006

When considering roof systems, building owners and managers will tend to think from the top down, since the surfacing, flashing, and roof membrane are what you see (and repair when needed).

On the other hand, roofing material manufacturers and design professionals will start with the roof deck and supporting structure. With the ‘deck-up approach,’ slope, weight-bearing capacity, fire resistance, and other code issues must first be addressed.

Generally, the choices are poured-in-place structural concrete, precast concrete, poured or cast gypsum, steel, wood products (e.g. T&G, plywood, oriented strand board (OSB), structural wood fiber), lightweight structural concrete, and lightweight insulating concrete. Other specialty decks are sometimes needed, such as for extreme chemical resistance.

Roofing manuals will group these decks into:

  • Non-nailable, noncombustible.
  • Nailable, combustible.
  • Nailable, noncombustible.
  • Steel decks.

Non-Nailable, Noncombustible Roof Deck
For the non-nailable, noncombustible decks, hot asphalt is generally used to attach the roof insulation or roof membrane to the deck, since hot asphalt is both an excellent adhesive and very cost effective. The surface of the concrete deck is primed with an asphalt primer to promote adhesion, penetrating dust and surface pores, and the hot asphalt follows. In older roof systems, the insulation might have been wood fiber, perlite, or glass fiber boards. To meet today's needs for higher insulating value, polyisocyanurate insulation (Isoboards) are substituted.

The advent of the single-ply industry and ballasted membranes has provided an alternative roofing technology. Insulation does not need to be attached to the deck at all. Either polystyrene or polyisocyanurate foam insulations are laid loosely, followed by a single-ply membrane with ballast to hold everything in place. The ballasted roof systems are far heavier than bituminous roofing, perhaps as much as 20 to 22 psf for high-wind areas, whereas the BUR would be only 4 to 6 psf. This means a structural engineer needs to verify the load capacity prior to substituting a ballasted roof where previously an attached system had been used. (Single-ply manufacturers will not take responsibility for these assurances. 

Still another recent alternative for non-nailable decks is the use of slow-rising polyurethane foam adhesives. Providing they are applied to dry substrates in warm weather and that the insulation boards follow promptly (before the adhesive surface skins over), excellent attachment can be attained.

While this category of roof decking is called “non-nailable,” the fastener industry has devised fasteners that also can be used. In this case, generally the deck must be predrilled, and then various “spikes” or screws are used to attach either the insulation or the entire single-ply system.

Nailable, Combustible Roof Decks
Wood and wood derivative roof decks are regional in their popularity. Plywood and OSB are very common on the West Coast. They are light in weight and can provide a strong diaphragm against seismic loading. Since they are combustible, fire ratings are generally met through the use of interior sprinkler systems or by overlaying the deck with fire-resistant underlays such as gypsum boards, foil-polyester laminates, or glass-fiber mats. Since wood purlins support the deck and condensation is not a major concern on the mild Pacific Coast, thermal insulation may be installed under the deck in the form of flexible fiber glass batts.

Bituminous roof systems for this application generally call for a base sheet, nailed or stapled to the decking. Asphalt is not used because of the combustibility, as well as the tendency to result in full adhesion to the decking. Since wood is moisture sensitive, the decks will tend to expand and contract with seasonal changes. Full adhesion results in concentrated stresses at deck joints. Both sprinkle-mopping and strip-mopping of base sheets have been attempted to reduce joint stress, but these are difficult to control under field conditions. 

Single-ply membranes may be fully adhered to wood product decks since the membranes are capable of accommodating the deck movement. However, they may require a fire-resistant underlayment, which is first mechanically fastened to the deck.

Ballasted roofs are rarely used, since the deck systems are designed for lightweight roofs and the inertia effect of ballast under seismic conditions might destroy the building's integrity.

Structural Wood Fiber Decking
A grey area between combustible and noncombustible decking is the structural wood fiber deck system. These fibrous decks generally have a gypsum-like or cementitious binder and combine the functions of a roof deck, thermal insulation, and finished ceiling. They also have acoustical value and are oftentimes used in schools and restaurants. Combustibility is generally defined by specific UL or FM listings.

Because these decks are porous, roofing cannot be adhered directly to the deck. Generally, a base sheet is mechanically fastened, using a specially designed divergent fastener for wind resistance. While these decks provide some thermal resistance, laminations with styrene or urethane foam and felt facers are used for the high R-values needed today. Both single-ply and bituminous roofs can be directly attached to the faced products.

Nailable, Combustible
Prior to and during World War II, poured gypsum and gypsum plank decking were quite popular. The poured decking was poured over a breathable form, such as glass or wood fiber or asbestos cement boards. Because the gypsum was prone to developing cracks, it was treated much the same as wood fiber by using mechanically fastened base sheets. While gypsum is rarely installed in new construction today, there are still structures around with these old decks. One concern is deck integrity, and prolonged roof leakage could corrode the wire reinforcing and leach binder to the point that an unaware person could fall through the deck. The Westford, MA-based National Roof Deck Contractors Association has published guidelines for rehabilitation of old gypsum decking. 

Lightweight insulating concrete is also considered non-combustible and nailable. The light weight is achieved through use of expanded aggregates such as perlite or vermiculite, air entrainment, or both. Because of the relatively high aggregate content, excess free water is used in pouring the deck. A permeable form (such as slotted steel subdecking) is needed to permit the fill to dry out toward the building interior. Coated base sheets are nailed to the fill using special diverging fasteners designed for this type of decking. With today’s energy demands, additional thermal resistance is achieved by embedding water-resistant polystyrene foam in the poured fill. This system is widely used to achieve positive drainage when the structure or subdeck is not adequately sloped to drain.

While lightweight insulating fills are very durable, you should be aware that at the time of re-roofing, the dust generated may contain tremolite, a form of asbestos. The tremolite is associated with expanded vermiculite from only one source: a mine in Libby, MT (this mine was closed in 1992). It is recommended that building owners have tests conducted on decks poured prior to 1992 to determine if vermiculite aggregate with tremolite is present. This might have a bearing on whether the deck should be encapsulated rather than having it disturbed by a roof tear-off

Steel Decking: Non-Nailable and Noncombustible
Steel decking is widely used. While it is technically non-combustible, rapid heat conduction can cause roof system ignition in the case of an under-deck roof fire. FM Global Class I and UL Insulated Metal Deck Tests evaluate the potential for fire propagation by testing complete roof assemblies. Polyisocyanurate insulations are compounded to meet these requirements when mechanically fastened.

While steel decks are termed “non-nailable,” they are generally roofed by using screws or special penetrating fasteners to anchor a layer of gypsum board or board insulation. The rigid boards are needed in steel deck applications to span the flutes in the deck.

Some slow-rise foam adhesives are marketed for application to steel, but these do not yet have FM or UL approval. FM global has recently updated information on fastening patterns for adequate wind resistance. 

Welds hold the steel roof deck to the purlins. When re-roofing, failed welds should be repaired or the loose decking should be screwed back onto the purlins.

Steel decks are produced with galvanizing for corrosion resistance, or a thin layer of coating called “shop-primer.” The primed deck is not very corrosion resistant, so the Rosemont, IL-based National Roofing Contractors Association (NRCA) recommends that owners always specify galvanized decking. If the deck is badly corroded, these sections should be replaced at the time of re-cover, as the corroded deck may not provide enough fastener withdrawal resistance.

Steel decks are provided with different levels of minimum yield strength. While the roofing industry generally has used grade C with a minimum strength of 33 ksi, some recent EPDM specifications require a minimum of 80 ksi decking to meet their mechanically fastened membrane wind requirements. Since steel is not normally marked with the yield strength or grade, it may be necessary to have your steel deck tested before permitting one of these unique EPDM specifications to be installed.

Hopefully, this column will encourage you to look at your roof system from the bottom up to understand how the roof deck is the foundation of your roof system and how it may impact roof decisions.


American Concrete Institute
American Institute of Steel Construction
American Iron and Steel Institute
Asphalt Roofing Manufacturers Association
APA - The Engineered Wood Association (APA)
FM Global
Gypsum Association
National Roof Deck Contractors Association
Precast/Prestressed Concrete Institute
SDI - Steel Deck Institute
Single Ply Roofing Industry (SPRI)
Steel Roofing
Underwriters Laboratories Inc.

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