Roof Decks – Comprehensive Overview

The following blog consists of brief descriptions of the most common types of roof decks encountered, their particular strengths, liabilities and suitability for coating with Conklin roofing systems. To learn why Choice Roof Contractor Group and its commercial roof contractors exclusively use Conklin for all projects, visit the following page: The Conklin Difference.

Steel Decks

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There are three types of common steel decks:

• Narrow rib (maximum 1”)
• Intermediate rib (1¾”)
• Wide rib (maximum 2 ½”)

The rib is considered the width of the channel between the flat upper sections of the metal sheet. Steel decks are typically installed onto steel framing and fastened by means of welding or mechanical fasteners. Corrugating allows greater spans and amount of load per square inch (psi). Rib style and type are determined by a structural engineer and are given in specifications details. Another type of deck or roof is standing seam metal roofs. These roofs are classified as either structural panels of architectural panels. For specific details on stand seam roofs, contact the Conklin Building Products Department.

Wood and Plywood Decks

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Solid wood boards and plank were a very popular choice in construction many years ago. Today, they are still occasionally used, along with the more prevalent composite boards, but plywood is by far the more commonly used deck material. Plywood is a manufactured material comprised of a composite wood core sandwiched between solid wood veneers, which are “laminated” or glued together. The grades of plywood considered acceptable decking by the American Plywood Association (APA) are APA structural I and II rated sheathing, exterior or exposure 1.

The panels are also assigned span ratings indicating the maximum distance allowable between supporting rafters. To ensure that plywood meets Conklin specifications, an APA rated board, rather than a mill-certified board, is needed. Mill-certified boards are not tested as thoroughly as APA-rated boards and may be more subject to buckling or delamination.

Plywood Under Special Coatings

Chemical coatings for roofs have increased the range of design possibilities, particularly in larger commercial structures where contoured or steel-pitched roof surfaces are exposed to view.

Exterior plywood is recommended for use under special coatings for roofs. When the coating requires a very smooth base, use APA A-C Exterior or APA B-C Exterior plywood. When maximum smoothness is not essential, use APA C-C PLUGGED Exterior. Tongue-and-groove plywood (½” or thicker)or lumber blocking at panel edges is recommended. A 1/8” space is recommended at all edge and end joints unless otherwise indicated by panel manufacturer. If high-performance coatings are to be used for finish, check coating manufacturer’s recommendations for panel joint treatment.

Grades should also be specified when the structural wood deck is to be overlaid with a separate plywood layer to serve as substrate for special roof coatings. A 1/8” space is recommended at all edge and end joints unless otherwise indicated by panel manufacturer. Although minimum ¼” plywood may be used over structural decks, 15/32” or thicker panels should be considered for best performance over uneven surfaces or when rain or high humidity is anticipated prior to application of roof coating.

APA – The Engineered Wood Association has several publications available containing extensive information on plywood and Oriented Strand Board (OSB) and its uses.

They can be obtained at the following location:

APA – The Engineered Wood Association
P.O. Box 11700
Tacoma, WA 98411-0700
(206) 565-6608

Precast Concrete Deck Systems

Typically, precast concrete deck systems are concrete slabs, most commonly found with metal edges or tongue and groove edges. The slabs are reinforced with wire fabrics or preformed steel bars and are either slipped to the steel frame members or welded to concrete frames by means of metal plates. Depending on the design of the slab, they span anywhere from four to twelve feet. The joints between the slabs are sealed with flexible grouts or other seals.

Gypsum Concrete

Gypsum concrete is a poured system composed of a mixture of gypsum, wood chips, and shavings combined with water. After mixing at the job site, the substance is poured over a wire mesh, which is supported on form boards of either fiberglass, gypsum board, or other material. The minimum acceptable thickness of a gypsum concrete deck is 2”. One advantage of this system if the feasibility of varying the thickness to provide slope for good drainage.

One cautionary note: Because the material is applied in a wet state, it is very important to allow sufficient drying time before installing a roofing system over a new gypsum.

Further, gypsum concrete is not suitable for direct application for Conklin coatings because its chalky surface prevents proper adhesion. Before roofing over this system, some method of moisture detection must be used to ensure that the deck is dry, followed by a recommended “divorcing material” to act as a buffer between the concrete and coating.

Cement-Wood Fiber Panel Roof Deck (Tectum)

Cement-wood fiber roof deck panels are composed of treated wood fibers that are bounded together with Portland cement or other binder and compressed or molded into flat panels. The panels provide both acoustical and thermal insulation. They are furnished either as tongue-and-groove planks suitable for application directly to joists or with rooted edges, which must be secured with bulb-tees.

Reinforced Concrete Roof Decks

Reinforced concrete is produced by mixing aggregate (usually stone or crushed gravel and sand) with Portland cement, water, and in some cases, chemical additive. Steel bars, welded steel mesh or both are used to reinforce the concrete. Then, the surface should be finished to a texture equivalent to either a light power-trowel finish or a firm wood-trowel finish. The density of these decks is from 100 to 150 pounds per cubic foot. Reinforced concrete should not be confused with either lightweight structural concrete or lightweight insulating concrete.

Lightweight Insulating Concrete

Lightweight insulating concrete is usually made of perlite or vermiculite and Portland cement. Occasionally, a foaming agent is also added to create bubbles within the concrete, lessening its density. Minimum standard density is twenty-two pounds per cubic foot. This is a poured system, which is installed over wire mesh or form boards much like the gypsum deck. The material used in making these units can also be used as a fill over pre-cast and pre-stressed deck systems at a minimum thickness of 2”.

Note: Lightweight insulating concrete is not suitable for direct applications of Conklin coatings due to its high water absorption properties. Before roofing over this system, use some method of moisture detection to ensure that the deck is dry. Then, use a recommended “divorcing material” to act as a buffer between the concrete and coating. Conklin Building products Department will provide specific recommendations for this procedure.

Do not apply Benchmark or Rapid Roof III where temperatures exceed an ambient of 100o F and a surface temperature of 120o F. Do not apply at temperatures below 40o F, during inclement weather, when temperatures may drop below 40o F within forty-eight hours of when the temperature is not at least 5o above the dewpoint.

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Written by: Ian Myers
Ian Myers
Ian was the very first team member brought on by the original co-founders of the Choice Roof Contractor Group to assist with nationwide growth. Since then he has written 120+ blog posts and provided excellent support to the roofing contractors in the group. Through his multiple roles in the commercial roofing industry, he has gained tremendous knowledge in the sector.