Roof Sheathing (AKA Decking)

What's In This Article

Are you planning to build a new roof or renovate an old one? Perhaps over a deck or a new porch? Roof sheathing, often referred to as decking, is a critical component of any roofing project. Roof sheathing provides structural support for your roof and protects it from weather and other elements.

Knowing what type of roof sheathing to use and how to install it properly will ensure your roof performs well for years.

This guide discusses everything you need to know about roof sheathing, including the types of materials available, installation best practices, and tips for success. Let's get started.

Roof sheathing, also known as decking, is a material that provides structural support for your roof. It is typically made from either plywood or OSB (oriented strand board) and can come in varying thicknesses.

Roof sheathing protects your home's interior from water damage and other elements. For instance, roof sheathing can protect your home from wind-driven rain, snow, and ice, as well as dust and debris. Roof sheathing also helps keep the attic cooler in the summer by providing an additional layer of insulation.

When selecting roof sheathing for your project, you have a range of options, but the two main options are OSB and plywood. Each has its pros and cons, as follows.

Plywood is the most common type of roof sheathing. It's made from thin layers of wood glued together to form a stronger, more rigid material.

Plywood is sturdier and stiffer than OSB and can handle higher loads in areas with heavy snowfall. It also provides better insulation for your roof due to its air pockets. The downside is that plywood is more expensive than OSB and can be harder to install.

The other lovable feature of plywood roof sheathing is that it's environmentally friendly since the manufacturing process uses fewer trees than OSB.

OSB (oriented strand board) is another type of roof sheathing. It's made from wood strands and flakes that are pressed together with glue to create a stronger material. OSB is less expensive than plywood and easier to install, but it's not as strong.

OSB roof sheathing is also more prone to water damage and can swell if wet. For these reasons, OSB roof sheathing is usually used in areas with moderate weather conditions or where there isn't a lot of snowfall.

Another option for roof sheathing is wood boards. This type of sheathing consists of individual boards that are nailed to the roof deck (the underlying layer of your roof).

Wood boards provide excellent insulation and are strong enough to withstand most weather conditions. The downside is that they can be more difficult to install than plywood or OSB roof sheathing, and they’re also more prone to warping over time.

Read More: Complete Guide To The Types Of Wood For Construction

The best choice between plywood and OSB roof sheathing depends largely on your roof's size, shape, and the weather conditions it will be exposed to.

Some features that make plywood a better sheathing material include:

If you live in a region with high corrosion levels or extremely wet weather, plywood roof sheathing may be a better choice due to its chemical resistance. With this, you don't have to worry about the sheathing warping or becoming damaged due to water.

Plywood features a high strength-to-weight ratio, making it an excellent option for roof sheathing. It's also more resistant to cracking and splitting than OSB roof sheathing. This benefit makes it a fantastic choice for roofs exposed to harsh weather conditions or heavy snowfall.

Plywood roof sheathing is usually lighter than OSB sheathing, making installation easier. This feature can save you time and money during the roof installation process.

Plywood can resist harsh impacts better than OSB roof sheathing. For example, it can withstand higher loads in areas with heavy snowfall.

Despite the numerous perks, plywood is prone to weather damage, especially during humid or wet weather. Also, the sheathing material is high maintenance, so you must regularly check for signs of damage or rot.

Below are some features that make OSB roof sheathing a popular option:

OSB roof sheathing is usually less expensive than plywood roof sheathing, making it an attractive option for roofers on a budget. It's also easy to install, saving you time and money during the roof installation process.

Unlike plywood roof sheathing, OSB is more resistant to changes in temperature and humidity levels. This benefit means it's less likely to warp or swell over time due to extreme weather conditions.

One major downside of OSB roof sheathing is its vulnerability to water damage. The roof sheathing may swell and damage when exposed to prolonged moisture or wet weather.

This issue can lead to costly roof repairs down the line. Additionally, OSB is noisy, which may be an issue for some roofers.

Generally, plywood wins in strength and durability, while OSB wins in cost and maintenance.

The size of roofing nails used will depend on the roof material. For example, roofers must use a larger roofing nail with plywood roof sheathing than they would with OSB roof sheathing.

It's crucial to ensure that roofing nails are long enough to penetrate through all roof layers, including:

It would help if you used 1-3/4 inch or longer nails for plywood roof sheathing. However, you can go as short as 1-1/2 inches for OSB roof sheathing.

The diameter of the nail should match the material it's being used on. For plywood roof sheathing, roofers should use nails with a diameter of 0.131 inches or larger. For OSB roof sheathing, roofers can go as small as 0.113 inches.

Here's an overview of some benefits roof sheathing can provide if installed correctly.

Although roofs are designed to be water resistant, your roof may need additional protection against moisture and water. For instance, shingles may age over time due to exposure to weather elements. Consequently, your roof may leak easily, especially if your roof sustains cracks or holes.

Roof sheathing comes in handy to add extra protection against leaks. The sheathing materials are water-proof and help to keep your roof dry in the rainy season.

Sheathing provides additional support for roof structures, making them stronger and more durable. This perk is especially beneficial if you live in an area with severe weather conditions, such as heavy snowfall or hail storms.

As such, while the initial cost of roof sheathing may be high, you can reap huge savings in structural repairs. In addition, the protection maximizes your roof's lifespan, so you won't have to worry about replacement any time soon.

Certain roof sheathing materials are fire-resistant. The sheathing materials feature fire treatment that helps to protect Roofs from catching fire.

Roof sheathing materials are made to last, but that doesn't make the materials indestructible. Over time, roof sheathing can suffer from damage due to extreme weather elements, water damage, and other roof-related issues.

Therefore, you must regularly check roof sheathing for signs of damage.

Wood products are prone to cracking and splintering over time. Weight from roofs may also cause the roof sheathing to buckle or warp. And if you don't replace these damaged roof sheathing pieces, your roof may suffer in the long run.

You can tell if your sheathing is broken if you notice gaps in the roof deck. You may also see splits or cracks in the roof sheathing.

Let a professional inspect the sheathing to establish the extent of the cracks. In case of minor splits, your contractor may advise you to repair the damaged roof sheathing, but a replacement is the best option if the damage is too much.

Water damage on roof sheathing is a huge red flag. Roof sheathing exposed to water for too long can easily rot, leading to major problems if left unattended.

Therefore, always inspect your roof sheathing after heavy rains or snowfall to check for signs of water damage, such as mold and discoloration. When caught early enough, the damaged roof sheathing materials may be replaced or salvaged with repairs.

Roof sheathing exposed to moisture for too long will start to rot. This issue is dangerous and costly, as rotting roof sheathing can lead to major roofing repairs or even a roof replacement in the worst-case scenario.

Some signs that your roof sheathing is rotting are a musty odor or visible discoloration. Rot may even cause the wooden material to collapse, putting you and your loved ones at risk. The issue also ruins the aesthetics of the roof and devalues your property.

Roof sheathing protects against water and moisture and provides a strong foundation for roof structures. However, if you can see the light passing through the roof sheathing, this means that the sheathing has been damaged and needs to be replaced or repaired.

In such cases, you can easily spot holes in your roof sheathing caused by weathering or animals trying to make their way into your house. If left unattended, these holes can cause major damage to roof structures and increase energy bills due to drafts of cold air seeping through them.

Roof sheathing that has been damaged may cause your roof to sag. This issue is common in older roofs, as roof sheathing materials tend to weaken with age. The main sign that your roofline is sagging is if one side of the roof is lower than the other. You may also notice the roof drainage structures slowly tilting or shifting as roof sheathing weakens.

Rectify a saggy roof to avoid roof structure collapse and water damage to your property.

Additional Reading: How To Build a Roof Over a Deck

The ideal roof sheathing thickness depends on your preferred size and roof materials.

The standard thickness should be 1/2 to 5/8, but these measurements may vary based on climate and roof type. For example, roof sheathing for roofs with asphalt shingles should be 5/8 thick.

On the other hand, roofs covered with slate or tile need roof sheathing 1/2 inch thick to provide the necessary support.

Sheathing material also plays a crucial role in roof sheathing thickness. Since plywood is slightly stronger than OSB, 1/2 is the most recommended thickness. However, when using the latter, 5/8 is ideal.

Apart from roof sheathing thickness, roofing professionals also recommend roof sheathing installation guidelines that should be followed to ensure an even and secure roof deck. The guidelines include using nails or screws of the right size and type, nailing at appropriate distances, and ensuring no gaps between sheathing materials.

Therefore, it's important to involve a qualified roofing contractor for roof sheathing inspection, maintenance, and replacement if necessary for a safe structure that will last for years.

The other critical factor is the gap between trusses. For example, roof sheathing for trusses 16 inches apart needs less thickness than those spaced 24 inches. The reason is that 2×6 trusses spaced 16 inches apart only require 1/2 inch thick roof sheathing, whereas 24-inch trusses need 5/8 roof sheathing for adequate support.

The minimum thickness should be 3/8 inch, which applies to plywood and OSB. Minimum thickness means that sheathing should be at least 3/8 inch when installing roof structures such as trusses or rafters, even if you use roofing materials like asphalt shingles.

Remember, there are different types of plywood, each with different thickness requirements. For instance, when using Southern Pine plywood, roof sheathing thickness should be 1/2 inch or more.

As much as you may want to take advantage of deals and discounts, it's best to invest in quality roof sheathing materials such as plywood for a safe and secure roof structure.

The recommended roof sheathing thickness should be at least 1/2 inch using plywood or 5/8 using OSB. This thickness is the standard for most roofing materials, such as asphalt shingles, tiles, and slates.

Apart from roof sheathing thickness, it's also crucial to ensure that roof structures like trusses and rafters are nailed properly at appropriate distances when installing sheathing to provide adequate support.

When deciding on the ideal sheathing, consider economic benefits without compromising the quality and strength of your roof sheathing. For example, if you are going for roof sheathing thicker than 1/2 inch, consider using plywood rather than OSB because it's more economical.

In the same way, if your trusses are uneven or widely spaced, roof sheathing should be thicker than when the trusses are evenly spaced. Other tips to consider when choosing roof sheathing thickness include the following:

In a nutshell, don't decide on the thickness entirely based on roofing material or roof sheathing cost. Remember to include roof structures and trusses, as well as roof sheathing thickness, when deciding on the right sheathing for your home.

Ensure you involve a qualified roofing contractor to inspect, maintain, and replace roof sheathing if necessary for a safe roof structure that will last for years.

Roof Sheathing code requirements differ from one place to another. For example, in the United States, roof sheathing is usually governed by local building codes and International Residential Code (IRC).

Building codes must be followed for roof sheathing installation and repair work. Some roof sheathing code requirements include:

These are just some roof sheathing code requirements when installing roof sheathing on your house. Remember that additional regional roof sheathing requirements may be based on your area's building codes and climatic conditions.

Climate is a primary consideration when determining sheathing thickness for your shed. First, consider the snow load and how much roof sheathing thickness is needed to bear the extra weight. Thick sheathing can take off the extra weight your roofing materials would otherwise have to bear, which minimizes the chances of damage and costly repairs.

Also, pay attention to the insulation factor. Roof sheathing with thicker insulation boards or plywood can help keep your roof warm and prevent weather damage to roofing materials.

Finally, look at the roof pitch of your shed. Roofs with low-pitched sheds should have thicker roof sheathing to bear the extra weight from snow and rain accumulation.

Ideally, roof sheathing on a metal roof is not much different from sheathing on an asphalt shingles roof. The roof sheathing should meet your local building codes’ roof sheathing code requirements.

If you opt for battens, the roof sheathing can be thinner, as battens act as additional support for roofing materials. However, if you don't use roof battens, it is recommended to go for thicker roof sheathing – at least 1/2 inch or 5/8 inch plywood or OSB boards, respectively.

On the other hand, if you use metal roofing materials like corrugated or standing seam roofs, roof sheathing should be thicker and can go up to 1.5 inches in some cases.

Don't decide on the thickness based on roofing material or roof sheathing cost. Instead, remember to include roof structures, trusses, and sheathing thickness when deciding on the right roof sheathing for your home.

Remember to hire a qualified roofing contractor to inspect, maintain, and repair your roof sheathing. In addition, pay attention to local building codes regarding minimum roof sheathing thickness and other roof sheathing requirements.

Now that you know about roof sheathing, you can make the right decision when installing roof sheathing on your home. Before making a purchase, keep in mind all the roof sheathing code requirements, climate conditions, and roof pitch. Investing in quality roof sheathing will contribute to a durable roof structure that will cost less in repairs over time.

Always follow local building codes for roof sheathing installation and repair work. This way, you’ll rest assured that your roof is safe and secure for years to come. And most importantly, work with a professional to guarantee safety and quality.

Chief Building Officer at Green Building Elements, joined us after a successful career and business owner at Alexander Architecture, which served the New York City Metropolitan area. Prior to running his own firm, John was an architect working for Gensler projects in Boston which is the largest architecture firm in the United States. John holds both a bachelors and masters degree in architecture from the Rhode Island School of Design. John is the current contact for InformeDesign and The 1 Percent program of Public Architecture.