Different Types of Commercial Roof Decking

What is roof decking for commercial applications?

A roof deck is designed to serve as the foundation for a roof. From the bottom up, a roof deck’s main function is to provide structural support for the entire roof system. The deck material is an important structural element that helps support the insulation and waterproofing (membrane) layers.

The roof deck must also accommodate building movement, so the roof deck type often determines the attachment method of the system components, such as the vapour retarder, insulation, and membrane.

So, what are the main types of commercial roof decking? In this article, we will examine the basic types of roof decking used in commercial flat or low-slope roofing systems — roofs found on such buildings as hotels, schools, or warehouses. To begin, ask yourself a couple of questions: 1) Is your project a membrane tear-off, complete reroof or new construction and 2) Do you have any special features to consider, such as rooftop HVAC equipment? The selection of roof deck material should be based on how much weight the roof system must carry, including potential snow buildup or vegetative green roof systems.

Membrane tear-offs and reroof projects carry the same concerns. Depending on the age of the building and roof, critical information about its history may not be available. The first step in the installation of any new roofing component is to prepare the roof deck. The roof deck performs several key functions, including structural support, dimensional stability, and fire resistance. The architect or specifier is responsible for meeting the roof deck requirements for a particular project.

The roof deck must be visually inspected before the application of the roofing system. Any deficiencies found must be reported to the project superintendent or general contractor. Do not proceed with roofing installation until all issues are addressed.

In new construction, the roof deck must be assembled based on the aforementioned factors, such as the weight of the selected membrane system, rooftop equipment, foot traffic and potential snow load. It is the responsibility of the roofing contractor to ensure that:

The roof deck is secured properly to the supporting structure.
The roof deck surface is clean, dry, smooth, and free from depressions, waves, or projections before roof system installation.
All nailing strips, equipment curbs, and penetrations are in place and secured before roof system installation.

Design Considerations

All types of roof decks on commercial buildings should be designed with a minimum slope of 1:50 (1/4 inch in 12 inches).¹ An ideal roof deck has the following characteristics:

Provides a clean, dry, and even surface for nailing, mopping, or cold process applications.
Does not release moisture into the roofing system.
Has the required strength to meet applicable building codes.
Can resist dimensional changes.
Resists fire, rot, and decay.
Can support specified structural loads.
Resists damage from handling, transit, installation, and foot traffic.

The roof deck should be inspected to ensure all deck construction is complete. Once the inspection is complete, and any unfinished or improperly installed construction details found in the deck are repaired or satisfactorily complete, the roof application can begin.

Questions roofing contractors should consider include:

Can the deck handle a heavier commercial roof type, such as replacing a built-up roof membrane (4-10 psf) with a ballasted single-ply system (10-20 psf)?
If mechanical fasteners will be used to attach insulation or a single-ply membrane to the deck, will the deck provide adequate pullout resistance? Some single-ply systems require a stronger steel deck (80 ksi instead of 33) to meet wind design requirements.
If the existing roofing membrane is removed down to the deck, and tapered insulation or a sloped fill is installed to solve ponding problems, will clearance to access doors, windows or equipment hatches be at risk?

What are the main types of commercial roof decking?

Available roof deck options include:

Wood-sawed lumber, planks, plywood, or oriented strand board (OSB).
Steel — cold-rolled, corrugated.
Structural concrete — precast or poured in place.
Lightweight insulating concrete.
Poured or precast gypsum.
Thermoset insulating fill.
Other composite deck materials

The most common roof decking types in Canada are constructed of wood, steel, and structural concrete.²

Wood Decks

Wood decks should be constructed of sound, air-dried or kiln-dried, solid-sawn lumber, securely fastened to joists, beams, or their supports. The surface should be smooth and firm, free of cracks, knotholes, depressions, or other defects. Wood roof decks are available in three basic types:

Composite board sheathings (up to 25-mm or 1-inch nominal thickness), such as OSB and wafer board.
Plywood.
Plank (up to 125-mm or 5-inch nominal thickness).

Steel Decks

Steel roof decks are constructed of cold-rolled steel sheets with ribs in each panel that provide strength and rigidity. The panels are available in a variety of profiles and gauges. Gauge (thickness), rib width, depth and configuration are critical; and because of the ribbed configuration of the deck, a leveling board may need to be installed over the roof deck.

The most widely used steel deck panel in commercial low-slope roofing is fabricated from 22-gauge steel with a minimum yield strength of 33 ksi or Grade 33 steel, with a rib depth of 38 mm (1½ inches). Steel panels are generally attached to the supporting structure by welding or mechanical fasteners.

It’s important to note that because of the iron content, steel decking is susceptible to corrosion. Typically, panels are treated with a galvanized coating or may be protected with paint, epoxy, vinyl, or acrylic polymers.

The Steel Deck Institute Standard RD1.0-2006 for Steel Roof Deck provides most of the information the contractor needs to know.

Commercial steel roof decks are available in different gauges, profiles, and sizes. They are:

Wide-rib steel deck (Type B).
Intermediate-rib steel deck (Type F).
Narrow-rib steel deck (Type A).
Deep-rib (Type N).

Flute span compatibility is important and can be found in the Directory of Roof Assemblies.

Concrete Decks

General types of structural concrete roof decks include:

Cast-in-place: Concrete is cast at the job site over form material and reinforced with deformed steel bars or wire mesh.
Post-tensioned: Reinforcement cables or tendons are tensioned after the concrete has cured or set.
Lightweight Cementitious: Typically non-structural cementitious fill material poured over supporting structural panels or reinforcement.
Precast (concrete is cast into units at the plant. Once the concrete has been set, the units are transported to the job site and erected. Most precast concrete is prestressed at the manufacturing facility).

Structural concrete is produced by mixing aggregate (crushed stone or gravel) with cement, water, and chemical additives. The density of reinforced structural concrete is typically in the order of 2400 kg/m³ (150 lbs. per cubic foot). Structural lightweight concrete decks include lightweight aggregates, such as shale or clay, which are used instead of sand and crushed stone.

Lightweight cementitious fill decks include gypsum, lightweight aggregate, and cellular concrete. Aggregates may include vermiculite or perlite.

Precast (preformed) decks include concrete, lightweight concrete, organic fibre, and cement slabs. Preformed roof panels have also been used as low-slope decking materials.

A roof deck may be classified as nailable or non-nailable. Nailable decks include wood, structural wood fibre, gypsum, and lightweight insulating concrete. Non-nailable decks include steel and structural concrete. Additionally, a roof deck is classified for fire resistance as combustible or non-combustible. Wood and wood fibre are combustible; steel, concrete, and gypsum are non-combustible.³

Several North American trade associations that focus on these types of commercial roof decking also provide a wealth of resources to help determine your options. Consult them through the links below for details:

The successful installation of a roofing system begins with a structurally sound roof deck. It is the responsibility of a structural engineer to confirm that the roof deck is suitable for roofing before an installation begins. The roofing contractor is responsible for determining if the surface of the deck is in suitable condition for the installation of the roofing system. Generally, the roofing contractor accepts the condition of the roof deck once installation begins.

¹ Roofing Contractors Association of British Columbia, RCABC Roofing Practices Manual.

² Common roof deck material information courtesy of Canadian Roofing Reference Manual.