(316) 927-2233While “decking” may suggest backyard patios and raised platforms, roof decking serves a radically different purpose.
Roof decking is the structural surface that sits beneath the roofing system and above the building’s structural members. It provides a continuous, load-bearing substrate that supports the roof system and rooftop equipment like HVAC units. A well-performing roof deck can withstand wind uplift, thermal movement, and the combined weight of all roofing components. A good roof is only as strong as the deck beneath it.
This article explores the most common types of roof decking used in low-slope commercial roofing, and how they influence the project’s cost, system compatibility, and long-term durability – as well as when decking requires replacement.
What Is Commercial Roof Decking?
Decking is the foundation of the roof. It’s the material that is laid on top of the building’s structural framing and it creates a stable platform for all other roofing materials. Just like the foundation of your building, a solid, well-installed roof decking ensures the stability for everything layered on top of it.
Why Roof Decking Matters for Low-Slope Commercial Roofs
If you own a commercial building, you are more than likely aware of the unique challenges of maintaining a low slope commercial roof. While most residential roofs have a steep pitch that allows them to shed water, low slope roofs have minimal pitch, which can result in up to 48 hours of standing water, even on healthy roof systems. This means that low slope commercial roofing systems need to be waterproof, not just water shedding. Excessive standing water causes damage not only to the surface layers of your roofing material but can eventually seep through them. Since the roof deck supports the entire roofing system, damage at this level threatens the entire structure. Knowing how different decking materials respond to moisture, load, and age is essential to understanding how this foundation affects the long-term performance of your roof.
Why You Need to Know Your Roof Deck Type Before Repairs
As a building owner, the last thing you want is an unpleasant surprise when repairs begin. We should not assume or guess what your decking material might be, because getting it wrong can drastically affect the overall cost of your project and cause unnecessary delays. Concrete decking in bad condition, for example, can result in a much more expensive project. This is why it is critical to conduct a thorough investigation during the initial site visit. The most important part of this initial investigation is the core sample.
Core samples reveal every material used throughout the field of the roof. In this example, two cover boards and waterproofing layers are present. This tells us that the roof system has two layers.
An example of a core sample that illustrates the depth-to-deck for a roof system.
What Is a Roof Core Sample and Why Is It Taken?
A core sample is a small, 2-inch diameter cylindrical section of a roof system extracted using a coring tool. It’s twisted down through the membrane, insulation, and cover board to the roof deck. Core samples serve four key purposes.
- Roof Layer Count: Core sampling tells you how many layers are on the roof. Most code dictates that no more than two roofing layers may be present, so this tells roofing contractors whether laying over the existing system is possible.
- Assess Moisture in the Substrate: The symptoms of moisture in the insulation (e.g., “give” under your feet, discolored substrate, and interior leaks) might be visible, but that doesn’t confirm moisture in the substrate. Coughing is a symptom of both inhaling black pepper and pneumonia – the cause is more actionable than the symptom. Core samples help roofing experts determine the source of your roof’s symptoms.
- Identify the Type of Roof Deck: The core sample reveals what kind of deck supports the roof system. As you’ll learn later in this article, this is crucial for many reasons.
- Identify Slope Type: A single core sample reveals any slope created using tapered insulation, but identical depth-to-deck at multiple locations tells you the structural slope. This directly affects the roof’s drainage, which is an important consideration for every low slope roof system.
Common Types of Roof Decking for Commercial Roofs
The three most common types of roof decking material on flat and low slope commercial roofs are wood, metal, and concrete. Each has its own strengths and weaknesses and can affect the cost of construction, maintenance, as well as repair and replacement costs. Below, we will cover the common types of roof decking material and the cost and performance implications for each.
Plywood decking on a low slope roof. In-between some of the boards, buckling – or the deforming of panels due to compressive loads – is visible. Buckling precludes a consistent, solid substrate, so this deck required replacement before installing the new membrane.
Pros and Cons of Wood Decking: OSB and Plywood
Wood-based products are commonly used as roof decking on both residential and commercial buildings. Here, we have two options: OSB (Oriented Strand Board) and plywood. Each has its own advantages and disadvantages. A qualified roofing professional can help you choose which material will work best in your unique situation.
Wood – OSB (Oriented Strand Board)
OSB (Oriented Strand Board) is an engineered wood panel made of strands of wood that are bonded with resin and compressed under high heat and pressure. The result is a dense, fibrous sheet with no natural grain – less a piece of wood than a structural composite. The multitude of wood fibers laid in various directions creates a board that resists deflection, delamination, and warping over long periods of time.
Another advantage of OSB is cost. Typically, OSB is significantly cheaper than plywood and can help to lower the overall cost of your project. Because OSB is made from small pieces of wood, it can be produced quickly and cheaply from farmed trees, avoiding the need to cut down larger, more mature trees.
The disadvantage that precludes the use of OSB in many of our low slope roofing systems is that it is more susceptible to damage in wet or humid conditions. Although OSB takes longer to become saturated with water, once it’s saturated it takes significantly longer to dry out. It can hold on to water, especially at the edges, swelling up and making it more vulnerable to mold and decay. Unlike plywood, which will return to its original shape when dry, OSB will remain swollen at the edges. This isn’t suitable for roofs that may support standing water for up to 48 hours.
Additionally, OSB can be quite heavy. Because the wood fibers are mixed with heavy resins and then compressed, the resulting board is denser than plywood and natural hardwoods. A single sheet of 4’x8′ OSB at 5/8” thick, weighs approximately 66 lbs., compared to 50 lbs. per board for plywood. The additional weight that OSB adds to a building’s structure needs to be factored in to ensure that the new roofing material does not exceed the load-bearing capacities of the building.
Generally, we advise against using OSB as decking on low slope roofs with higher loads and lower slopes. It performs well on steep slope (>2:12 pitch) roofs, but low slope roofs demand stronger fastener retention and better moisture resistance than OSB can provide in most cases. OSB has a niche use as cover board on low slope systems, laid rough side up, where its rigidity and abrasive texture allow it to support fully adhered systems well.
Plywood
Plywood is an engineered, composite material like OSB, but it is much older, having been in use since the 1930s. Plywood is made from large, thin sheets of wood veneer, which are then laminated together using a hot press. Much of the strength of plywood comes from the cross-lamination process. Each piece of wood veneer is laid at a 90º angle to the piece below it so that the grain patterns of the wood run perpendicular to one another. Plywood is constructed from an odd number of layers so that the center is balanced and the exterior faces of the wood grain both run in the same direction. The cross-lamination produces a relatively light board is 10 percent more rigid than OSB, holds fasteners in its perimeter well, and dries out faster than OSB due to its layered, ventilated structure that permits moisture to escape through the grain.
Plywood dries out like a bowl of spaghetti, with air moving freely between its long, separate strands. OSB, however, holds moisture like a scoop of mashed potatoes: dense and compact, with nowhere for water to escape.
The primary disadvantage of plywood is cost. Plywood is more expensive than OSB in most markets, as it is made from larger, more mature trees. Although plywood retains its shape after limited exposure to moisture, even plywood can warp, rot, and delaminate over time, especially in wet and humid conditions, or when attic spaces are not properly ventilated.
Metal decking on a newly constructed building. Corrugated metal decking has become the most common type of roof decking on low slope commercial buildings.
A building’s decking can usually be identified from the interior of the building. If a building has drop ceilings, the decking is visible with a panel moved, as in this example.
Pros and Cons of Metal Decking
Corrugated metal decking is quickly becoming the decking of choice in commercial low slope applications. Metal decking provides several advantages over other decking materials which can save the building owner time and money.
First, metal decking is relatively thin and lightweight compared to other materials, making it easy to attach the rest of the roofing system to the decking surface. When workers attach underlayment and insulation boards to the metal deck using mechanical fasteners (typically long screws with a fastening plate) they can quickly and easily fasten everything to the metal decking at regularly spaced intervals. This ease of installation saves the property owner time and money, assuming the deck passes a pull test.
Unlike plywood and OSB, metal decking is less susceptible to water damage. Metal does not hold onto moisture, swell, delaminate, or attract mold like wood-based products can. However, because it is made from steel, metal decking can corrode if not properly protected from the elements. Regular, proactive maintenance can spot leaks early and ensure that your metal decking is not exposed to water for an excessive period.
To prevent or slow this type of corrosion, metal decking is prime-painted or galvanized. Galvanization coats the metal with a layer of zinc to protect it from oxidizing when exposed to air and moisture. The most common treatment is G60 galvanization, which cites the amount of zinc coating applied (i.e., 0.60 oz/ft2).
Metal decking also comes in different profiles and thicknesses. The corrugation or wavy pattern seen in this type of decking gives the metal sheets their stiffness and rigidity and helps transfer the load of the roof onto the structural members below. The ribs of the corrugated metal are placed at a 90º angle to the structural members underneath to provide maximum stability. Metal decking for roofing is typically available in thicknesses from 16 to 22 gauge, with the lower numbers representing thicker sheets of metal. Heavier gauge sheeting will be able to span longer distances and can support heavier roofing systems such as inverted roofs.
Pros and Cons of Concrete Decking
The Cost Implications
Concrete roof decking is also found on commercial low slope roofs and adds a high level of structural support to the building. Since concrete is heavier than wood or thin-gauge metals, it is often found on buildings that were specifically designed to bear the weight of a heavy roof. Concrete provides long-lasting protection against the elements, is fire resistant, dampens the noise of loud thunder and heavy rains, and can easily bear the weight of heavy roof-mounted equipment.
However, a concrete roof deck can add significant cost to your project both in terms of materials and labor. Concrete is a dense substrate that does not easily accept fasteners. Drilling into it can cause cracking, spalling, or structurally compromise the system if done improperly. As a result, most roofing systems installed over concrete are fully adhered using specialized low-rise foam adhesives. A significant amount of glue must be used to adhere subsequent roofing materials, which increases the weight of the roof, the drying time, labor costs, and the overall project timeline.
Additionally, if you have concrete that is in bad condition, it is costly to break up and remove the existing concrete decking. This is why a qualified roofing professional will complete a thorough investigation of your roof decking during the initial site visit.
Concrete – Lightweight Concrete Decking Options
It should be mentioned that the type of concrete that is typically used in roof decking is not the same as foundation or standard concrete, which is a much heavier version of concrete that is designed to withstand the weight of the entire building. Instead, lightweight concrete is often used in roofing applications, as it provides many of the same benefits of standard concrete without the added weight.
Lightweight concrete typically used in roofing applications is mixed with alternative materials rather than dense natural stone. Porous and lightweight aggregates such as pumice, perlite, shale, or slate that has been expanded through pressure treatment, as well as manufactured products, such as fly ash and slag, can be added to lightweight concrete to open up its texture and reduce its overall weight. Lightweight concrete can also be “foamed” by introducing tiny air bubbles into the concrete slurry, thereby reducing its weight and increasing its insulating properties. These processes create a lightweight concrete that retains many of the benefits of standard concrete. However, because more water is added to lightweight mixtures they typically take longer to dry, which can lengthen your project’s schedule.
How to Choose the Right Decking for Your Commercial Roof
Each material has advantages and disadvantages that need to be carefully considered within the context of your unique project. Below are the types of questions skilled roofing professionals use to determine the most suitable decking material for your building.
- Does the deck type impact the ideal attachment method?
- Concrete functionally requires full adhesion, not mechanical attachment.
- Most metal decks and at least ½” thick plywood decks work well with mechanical fasteners.
- What’s the structural load it needs to support?
- Concrete supports heavy loads, as does thick-gauge metal.
- Plywood can support most roof systems.
- How will it deal with moisture long-term?
- Plywood dries quickly but can still delaminate and rot with chronic exposure to water.
- Treated metal resists corrosion.
- Concrete in good condition is unlikely to be directly compromised by water exposure.
- How does the upfront cost compare to long-term costs?
- Plywood costs more than OSB but is much more viable long term.
- Both metal and concrete roof decks have higher material and labor costs.
While a qualified roofing contractor can advise on roof system compatibility and installation methods, structural load calculations must always be performed by a licensed structural engineer.
When to Replace Your Roof Decking
When the integrity of your roof decking has been significantly compromised, adding a layer on top can compound the problem. To learn more about when the roof deck is salvageable versus when it needs replacement, read our article outlining layovers vs. removal and replacement.
Keep in mind that the decking supports the weight of all roofing materials placed on top of it, including insulation, the waterproofing membrane, ballast, skylights and vents, as well as human foot traffic and heavy mechanical systems. It’s the decking’s job to transfer that load onto the rafters, trusses, and structural members of the building. It also provides a solid base on which other layers of roofing material are fastened or adhered. If water intrusion has caused metal decking to become corroded, or plywood to de-laminate, it cannot properly do these jobs and we need to know about it before work begins, so the problem can be assessed and repaired appropriately.
Key Takeaways
- The three most common types of roof decking are wood, metal, and concrete. Each comes with distinct cost, compatibility, and moisture resistance considerations that will affect the project’s design.
- Your roof is only as strong as its decking. Roof decking is the foundation of your roofing system. A functional, solid roof deck is a prerequisite for a functional roof.
- Core sampling is the first step in deck evaluation. It reveals how many roof layers exist, whether slope is structural or tapered, identifies deck type, and tells you if you have moisture in the substrate.
- Decking condition influences project scope. Damaged, warped, or corroded decking requires replacement, which alters your project’s timeline and budget.
- A licensed and qualified roofing professional can help you identify and assess the health of your roof decking and help you make an informed choice that meets both your goals and budget.
