What is Roof Decking: Types, Thickness, and Building Codes

Most people assume that their roof is purely made up of roofing shingles. However, while shingles play a critical role in protecting your house from outside elements, other parts of your roof play an even more prominent role. Without implementing roof decking, most roofs wouldn’t be as sturdy as they are today. So what exactly is roof decking?

Here’s what I know from inspecting thousands of roofs as a Master Inspector:

Roof decking is the supporting wood, steel, concrete, or other material that tiles or roofing shingles are placed on. Roof decking is made from either concrete, steel, or wood. They also act as the protective layer that weather seals your home or building. Roof decking is otherwise referred to as roof sheathing and forms the foundation of the roof.

It is therefore essential that you place priority on the condition of your roof decking. Here, we’ll explore roof decking and the role they play in roofing. 

What is a Roof Decking?

Roof decking is the supporting wood, steel, concrete, or other material that tiles or shingles are placed on.

A roof deck plays a critical role by forming a foundation for your roof. The roof deck is usually connected directly to your home’s rafters. After you have installed the shingles, the roof decking will support the whole weight of your roof.

Apart from being a critical component in the structure of your roof, it also guarantees your home’s integrity. Roof decking secures the rafters’ location, thus ensuring that they sustain a proper spacing with time. 

Even though roof decking hasn’t always been part of home construction, many professionals now consider it a critical structural element of any home. Due to this, it would be best if you paid attention to the integrity and condition of your roof decking.

Sheathing vs. Roof Decking

If you keep learning more about roofing, you’ll come across the terms roof sheathing and roof decking most of the time. Roof sheathing and roof decking are two terms often interchanged for the same process. 

There is no technical difference between these two terms, although some roofing contractors prefer one over the other. Here, we’ll therefore use the roof sheathing and roof decking interchangeably to mean the same thing.

Type of Roof Deck Materials

Steel roof decking is the best decking option but the most expensive too.

There are 3 materials used to make decking roofs for both commercial and residential roofing. Even within the categories of materials, there are still different varieties.  

Wood

Wood is among the most common types of roof decking material used in the industry. The material is cost-effective and durable, thus offering great value for investment. 

Even though wood is very flammable, it’s treated with retardants to add a unique protection level against disaster.  

There are four different types of wood decking and include:

  • Zip Wood: It is reputable for its high durability and strength. Zip wood has a water-resistant barrier meaning that you don’t need roofing felt.
  • FRT Plywood: They are also called Fire Resistant Plywood and help restrict fire spread from home to home. Although the roof was popular at some point, it might start to disintegrate prematurely, leading to roof replacement. 
  • CDX Plywood: It is the most recommended wood material for roof sheathing. The material is reputable for increased moisture resistance and durability. It is also the most cost-effective option. 
  • Oriented Strand Board (OSB): It is sometimes called chipboard because it is made from wood chips that are molded together in 4 ft by 8 ft sheets like plywood. OSB is commonly used in place of plywood because of its lower cost.
  • Radiant Barrier OSB: Radiant barrier OSB has a foil barrier on one side that installs facing the attic. According to the SolarBoard website, OSB radiant barrier decking can reflect up to 97% of solar radiation, lowering attic temperatures up to 30°F in the summer months.

Concrete

Concrete roof decking is gaining popularity despite not being used a lot in the past like other options. The concrete decking option is used to handle heavy loads. 

However, it would help if you understood that all concrete roof sheathing materials aren’t equal. Let’s look at some of the types of concrete roofing:

  • Structural Concrete: Structural concrete is the concrete decking type that comes to your mind when you hear the word. It is designed and developed to handle heavy loads. The roof decking is also insulated with insulation panels that are installed above the concrete sheathing. 
  • Structural Concrete Composite: It is employed in cases with an additional level over the current roof. The roof decking is a unique steel panel system that has both lightweight and structural concrete. 
  • Lightweight Insulating Concrete: It’s a low-density concrete used directly on a corrugated steel deck system. 

Steel

The use of steel roof decking materials comes with several benefits, including a chance to discount your home insurance rates. 

Steel makes up the most lightweight material for making roof decking. It’s better than both concrete and wood. 

The most important thing about steel is that it has high durability. If your home is an area that experiences severe weather elements such as hail storms, steel roof materials can extend the roof’s longevity by many years. 

Experts rate these materials to have the best level of impact resistance when compared with other roofing materials. Due to this, insurance companies will discount the insurance for your home if you have steel roof sheathing. 

Another advantage of steel roof decking is that it contributes to low energy costs. For instance, during summer, steel will reflect heat more efficiently compared to wood and concrete. The result is your home remaining cool, thus reducing energy bills. 

The only drawback is that steel roof sheathing is the most expensive option of all three. However, its benefits offset the cost making it a reputable choice for homeowners in the United States.

Do You Replace the Sheathing While Replacing Your Roof?

What do you think will happen anytime you replace shingles? Do you have to change your roof sheathing as well? It depends on the age of your roof and the condition of your house. 

Even though it isn’t necessary to replace everything you replace your roof, several homeowners find it best practice to replace the roof deck during roof replacement. However, replacing your roof decking isn’t easy and will increase the cost of roof replacement.

The advantages of high-quality sheathing materials are many and hard to argue. 

Before you remove shingles from your roof, it would be best to approach a professional roofing company to assess your roof’s condition. It can offer you some insight into the state of the decking materials on your roof. If you feel that you need a new roof, it would be time to call your favorite roofing company and schedule a proper inspection. 

Warning Signs of a Roof Damage

A roof that is in a bad state has missing shingles, shingles starting to warp, and faded shingles. However, it won’t be easy for you to determine the condition of the sheathing by yourself. It would therefore help to involve a qualified professional. 

You might notice some common signs of a damaged roof deck in your home. The warning signs include: 

  • Leaks inside the house, whether small or large, can be a sign of damage with the roof decking material. 
  • Sagging ceilings also present a warning sign of a damaged roof decking. It is more common in wood decking, where water can develop staffing that may affect the ceiling in your home. 
  • Water stains also point to roof deck damage, whether the water damage results from inferior roof decking or faulty shingles, it can cause mildew, mold, and other major problems. 

These warning signs mean that it’s time to call a professional roofing company to replace your roof. Remember that your roof’s structure determines the integrity of your home. Having a great roof and a beautifully painted home can improve your home’s value and the resale price. 

It will therefore help if you keep your home in the best conditions possible.

What Causes Roof Decking to Warp?

A roof becomes warped if there is visibly sagging on the structure. You may notice parts that curl up, unevenness in the shingles, and blistering sections of the roof. Common causes of roof deck warping include: 

  • Uneven rafter levels
  • Underlayment issues
  • Water damage
  • Poor ventilation
  • Multiple shingle layers

Advantages of Investing in a New Roof Decking

There are several benefits of investing in a brand new roof decking, even when it isn’t critical to replace. A new roof decking improves the structural integrity of any house. It is true more so in areas that experience various weather elements such as rain and hail storms. 

When you want to invest in roof decking, it would be best to choose the type of sheathing material that is perfect for your home. We recommend that you replace your old wood roof decking with a more energy-efficient metal option. Although it will require a high initial investment, everything will be worth it.

How Thick Should Roof Decking be?

Roof decking is available in 4 by 8-foot grid-marked sheets. It would be best to install them perpendicular to the frame and never attach roof decking with nails or screws. The decking should be at least 19/32-inch thick. A normal roof decking has a thickness of between ⅜ inches to ¾ inches. Roof deck thickness codes do vary based on wind and snow loads.

However, there are several factors that you must consider while determining the perfect decking thickness for your roof. Some of the factors include the area for the snow load and the span of roof joists. Let’s look at more information about sheathing thickness:

Types of Roof Decking in Residential Homes

The most common types of wood applied in residential home roof sheathing are plywood and oriented strand board. The most popular option is the oriented strand board. For cases where the snow load is at least 30 pounds per square foot, you can use 7/16-inch sheets with no edge support. 

With the same snow load and edge support, you can use a ⅜-inch oriented strand board. Panel H clips or tongue-and-groove decking panels normally offer edge support. 

Determine Quantity

A single four by8-foot sheet of plywood or oriented strand board is 32 square feet. You should take the total square footage of your roof and then divide it by 32. Multiply the figure by 1.15 to factor in about 15% of waste. You will get the number of sheets to purchase.

Roof Decking Building Codes

The roof decking building codes usually focus on roofing materials, installation, structural performance, and weather protection. 

Construction Type

The building code assigns the construction type depending on the occupancy and size. It also establishes the least fire resistance of a roof assembly. It means that some buildings should have a minimum fire resistance level depending on their use and occupancy. 

Determining the Code to be Adopted

Even though no all jurisdictions will require you to have a permit to roof your home, replacements or repairs should comply with the available building code. You should therefore be aware of the codes that are in effect at the time of repair. 

If you aren’t of the local building codes, contact a professional to help you out. 

Live Load/Dead Load

Changing lightweight roofing or adding extra decking leads to an increase in a dead load of your home. You should evaluate the roof and determine whether it can withstand additional dead loads and live loads. 

Hail

There is no specific building codes resistance for hail resistance. Manufacturers have their assemblies and roofing products tested using FM 4473 and UL 2218. However, many products aren’t rated for impact resistance relating to hail performance. 

Wind

Your building’s roof decking should withstand the required wind loads of the area where your home is located. The design wind speeds in the coastal are higher due to these areas being hurricane-prone. However, the design speed is lower throughout the rest of the country. 

A roof decking shouldn’t only comply with the applicable wind speeds but also with the uplifting pressures. Here it would help if you focused on:

  • Design wind speed: It depends on the risk category and location of the building. Risk category involves the building or structure categorization depending on its use or occupancy. 
  • Wind exposure category: It is determined by the size, concentration, and type of the surrounding vegetation, the terrain, structures, and buildings that are windward of your home. 
  • Design wind loads: The structure should be designed as per the wind pressures and the accompanied uplift forces. 
  • Roof height: The wind speeds vary with an increase in elevation and height above the ground. Use the wind speeds at eave height if your structure has a low-slope roof decking of below 2:12. 
  • Variations in local requirements: Most jurisdictions that implement the residential and building codes use IRC and IBC. The old building has been developed into different building codes or no codes. It would help to understand the building code that was implemented or needs to be implemented in your home. `For instance, the nail patterns in high-wind locations require 8d nails with a maximum spacing of 6-inch. 

Fire

Roof decking assemblies are rated as class C, B, A, or maybe non-rated, all of which depend on effectiveness against exposure to fire. Class C, B, and A assemblies effectively against light, moderate and severe fire exposures, respectively. However, some roof decking types don’t need a fire rating. 

The IRC and IBC require the construction of roof decking where there is a termination of certain fire-resistance walls at the underside of roof sheathing. The requirements are complex for houses developed as per the IBC and depend on the user group, wall rating, type of construction, and fire separation distance. 

Cold Climate

It also creates a unique condition for the prevention of snow and ice-related damages. In areas with a history of ice accumulating along the eaves, you should include an ice barrier. If your building is in a cold climate, your should also implement proper ventilation.

Frequently Asked Questions

What is Radiant Barrier Roof Decking?

Radiant barrier roof decking is an energy-efficient way to keep your home cool in the summer and warm in the winter. It works by reflecting heat away from your home, which helps to keep your indoor temperature comfortable year-round.

Radiant barrier roof decking is made from a variety of materials, including a reflective aluminum foil applied to OSB wood sheets. Radiant barrier roof decking is an easy and cost-effective way to improve the energy efficiency of your home, and it can help you save money on your energy bills.

Read more on radiant barrier decking.

What is the Decking on a Roof?

The decking on a roof is the surface that you walk on. In residential homes, it’s usually made out of wood, although there are also metal decking options available for flat roofs. The decking is attached to the underlying structure of the roof (known as the rafters in pitched roofs or joists in flat roofs) with nails or screws.

The roof decking serves three main purposes:

  • it provides a surface for you to walk on
  • it protects the underlying roofing structure from UV rays and weather damage
  • it provides a surface to install roof coverings like asphalt shingles or metal sheeting

When to Replace Roof Decking?

Roof decking should be replaced when it is damaged or worn out. Damaged roof decking can allow water and excess moisture to enter your home, which can lead to mold and mildew growth.

Wood decking can last 20 or more years depending on the type and thickness. As a rule of thumb, wood roof decking weakens the more it is nailed into. Generally, after two shingle roof replacements, the decking weakens from numerous nailings and should be replaced. Roof decking under metal roofs can last the lifetime of the metal and may never need replacement.

If you are unsure whether or not your roof decking needs to be replaced, you should contact a professional roofing contractor for an inspection.

How Long Does Roof Decking Last?

Plywood and OSB roof decking typically lasts for 20-30 years. However, the lifespan of your roof decking will depend on the type of material it is made from, the climate you live in, and how well it is maintained. Regular inspections and maintenance can help to extend the life of your roof decking.

What is the Best Material for Roof Decking?

There are a lot of different materials that can be used for roof decking, but not all of them are created equal.

In residential houses, plywood is the best roof deck material, however, OSB decking is more commonly used and meets residential building codes. Plywood offers more strength than OSB and can handle more load, which is important if you live in a colder climate where snow accumulation on the roof is common.

How to tell if Roof Decking is bad?

There are a few ways to tell if your roof decking is bad. If you see any cracks, splits, or warping, then that’s a sure sign that it’s time to replace it. When plywood or OSB roof decking gets wet for a prolonged period of time, the wood swells and separates.

When roof decking is bad, it is often because of numerous nail holes weakening the wood or due to excess moisture exposure. The roof deck will flex when walked on and you’ll often hear a cracking sound indicating the wood is breaking underneath your feet.

If you’re not sure whether or not your roof decking is bad, then it’s always a good idea to have a professional inspect it. They’ll be able to tell you for sure whether or not it needs to be replaced.

Can You Install New Roof Decking over Old Roof Decking?

In some instances, you might be able to install new roof decking over old roof decking. However, it’s usually not a good idea to do this. The old decking might not be attached properly, which could cause the new decking to come loose.

It’s always best to remove the old decking before installing the new one. This way, you can be sure that the new decking is installed properly and that it won’t come loose.

Also, installing new decking over old decking can cause the roof to sag under the additional weight. If installing new decking over old decking, your contractor may need to reinforce the rafters with additional bracing.

Conclusion

If you follow the roof decking above, there is no question you’ll have a strong roof decking on your home. Although steel roof decking is expensive, we recommend you to use it since it offers extra strength and is very durable. 

However, if you don’t know much about roof decking, it would be best to contact a professional to help you out. We hope that you have the best roof decking structures in your building. 

Photo of author

Hubert Miles

I've been conducting home inspections for 17 years. I'm a licensed Home Inspector, Certified Master Inspector (CMI), and FHA 203k Consultant. I started HomeInspectionInsider.com to help people better understand the home inspection process and answer questions about homeownership and home maintenance.
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