Plywood For Roofing Everything You Need To Know

Plywood For Roofing: Everything You Need To Know

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James
March 1, 2022
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If you want a strong, durable roof, you have to ensure you have installed the correct roof decking.

This is made up of sheet materials that produce a flat surface so that shingles can be nailed onto it securely, and decking is usually made out of plywood.

What is plywood, exactly?

Plywood is a wood-based material that’s constructed from thin layers of wood. These are glued crosswise along the grain to prevent shrinkage and swelling from occurring.

With this in mind, let’s explore what plywood is used for in roofing, and why it’s a good choice. We’ll also explore what you need to know about the different types of plywood. 

How Is Plywood Used In Roofing? 

Plywood For Roofing Everything You Need To Know

Plywood is usually used as a form of roof decking, the material that is placed on top of the roofing structure.

While we often think of the main roof structure as being what needs to be strong and handle large amounts of weight, the roof decking is a component that has to be strong enough to support the entire system from collapse!

Since decking is made from layers of wood that are glued together, the grains of the wood run in alternate directions throughout each layer. This gives an even distribution of the wood’s strength so that it’s resistant to contracting and expanding. This also prevents it from being weak that it splits when a nail goes through it.

In case you’re wondering, roof decking is one of the most important structures of your roofing system. This is made up of wooden boards that produce the roof frame.

These boards are basically what your roof shingles and other roofing elements are installed on, so you have to ensure that the decking is strong enough to handle all their weight. 

There are good reasons why plywood is ideal for decking construction. These include the following: 

  • Plywood handles moisture well. When it gets wet, it tends to swell evenly all across the plywood sheet surface. Then, when it dries, it regains its normal size. Since the drying out period is short, this doesn’t affect your roofing system.
  • It can be cut to size. What further makes plywood ideal for roofing purposes is that it can be cut to size with a handheld saw. Generally, the standard size for plywood sheets is four feet by eight feet, so it’s easy to work with while giving you a good surface coverage on your roof. You can find plywood in different thicknesses that can handle different loads. 

What Are The Different Types Of Plywood?

Plywood For Roofing Everything You Need To Know

There are two main types of plywood that are used in the roofing business. Let’s explore them in greater detail. 

Structural plywood

This comes with strength performance proof from the supplier to show that it’s passed the building regulations so that it can be used structurally in buildings. Since it’s water-resistant it can be used effectively for roofing purposes.

To glue the layers of wood together in structural plywood, phenol formaldehyde resin or melamine-urea-formaldehyde is used. Phenol formaldehyde is a type of bond that’s said to perform well as it’s resistant to temperature fluctuations and prevents moisture from penetrating into the wood, so it gives the plywood the highest grade of quality. 

General plywood

This type of plywood doesn’t come with proof of strength performance, so it’s not used for structural purposes. It’s reserved for interior use. Since its layers are glued together with urea formaldehyde resin, this can start to deteriorate when exposed to wet conditions or temperature changes. 

What Size Of Plywood Does Your Roof Need? 

Plywood For Roofing Everything You Need To Know

The plywood thickness your roof requires depends on the load that the roof will need to be able to handle. If you have a flat roof, it will have a heavy load so you will need a thick plywood. By comparison, a pitched roof handles a lighter load as the weight gets evenly spread across the roof, so the plywood can be thinner.

Some older homes have ⅜-inch thickness but this is not the norm because it’s a bit too fragile. You therefore will have to settle for plywood thickness that’s between ½ and ⅝ inch thickness, both of which are suitable for standard loads with roof rafters being placed 24 inches apart.

Should You Choose ½-inch or ⅝-inch Roof Decking?

These are the most common thicknesses for roof decking, so you might wonder which one you should use and if it even matters. Basically, plywood that’s ⅝-inch decking is stronger and more durable than ½-inch decking because of its extra thickness. It also produces a smoother finish as it hides any valleys between joists and trusses.

But, it takes more time and money to install. It’s a good choice if your home is susceptible to hurricanes, a lot of snowfall, or other extreme weather conditions, because it’s thicker and therefore more protective. 

By comparison, ½-inch decking is almost as strong as ⅝-inch decking, and it’s much more cost-effective for your roofing needs. It can tolerate harsh weather conditions, but it’s most suitable for you if you live in mild climates.

Extra Things To Consider:  

The thickness of plywood that you need will vary depending on the slope of your roof, the spacing of the rafters, and the weight of other material that will be placed over the decking.

You will also need to consider the design load, which is the quantity of pressure that’s exerted by the weather, such as when snow accumulates on the roof.

Basically, the wider the spacing of your rafters, the heavier your shingles, the lower your roof slope, and the heavier the weather-related load, the thicker you’ll have to make your plywood so that it has the correct supportive features. 

How Many Plywood Sheets Will Your Roof Require? 

Plywood For Roofing Everything You Need To Know

You will have to work out the amount of plywood sheets you’ll require, bearing in mind that a sheet of plywood is 4×8 feet in size, with a surface area of 32 square feet.

So, with the average roof size being 1,700 square feet, this means that you’ll need approximately 53 sheets of plywood.

To calculate how many sheets of plywood your roof requires more accurately, you should follow these steps. 

  1. Use a tape measure to measure the width and length (in feet) of each roof slope. 
  2. Multiply the length by the width to get the square foot area of one slope. 
  3. Then, multiply that number by 2 to get the total roof area. 
  4. Now you want to divide the total roof area by 32. This gives you the number of plywood sheets you’ll require to adequately cover your roof. 
  5. Add 10 percent to that number so that you accommodate for any waste.

If you can’t directly measure your roof, you can use a calculation that converts the floor area covered by the roof so you can get an estimate of how much plywood you will need. So, if your house has a main area of 20 by 40 feet and a wing that’s 15 by 20 feet, multiply 20 by 40 to get 800 square feet.

Then multiply 15 by 20 to get 300 square feet for the extra wing. Add 800 and 300 to give you the total floor area, which will be 1,100 square feet. Then add about 20 percent to the number to cater for the roof slope, which gives you 1,320 square feet of roof area. Finally, divide 1,320 by 32 so you get 41 plywood sheets. 

How Much Does Plywood Cost? 

A regular piece of plywood sheathing will measure 4×8 feet and can be from ¼-inch to ⅝-inch thick.

The thicker your plywood, the more you’ll have to pay for it and its installation. The price you pay for the project will depend on a variety of factors, such as the labor costs. Generally, a sheet of 4×8 plywood will cost about $10.

Note that a plywood sheet that’s 4×8 feet will be large enough for a roof area that’s 32 square feet. Since the average roof is approximately 3,000 square feet, you can see how installing decking in your home can be very expensive. 

Plywood Vs OSB: Which One Is Better? 

Plywood For Roofing Everything You Need To Know

When strengthening your roof, you will have to choose between using plywood or OSB, or oriented strand board. Both of these are made out of wood and are joined with resins or glue.

They also come in a variety of sizes, but they have some important differences you should know about. 

Durability 

OSB absorbs water at a slower rate, which is useful if you have areas of low dampness. After absorbing water, it slowly dries. This is problematic because it swells and won’t return to its original shape. By comparison, plywood absorbs water faster but dries faster, too. It doesn’t warp and easily regains its original shape. 

Eco-friendliness

If you’re looking for “green” materials for your roof, you’ll do well to choose OSB instead of plywood. This is because OSB is made out of small pieces of wood so it can be produced with the use of smaller trees that grow quickly and can be farmed.

By comparison, plywood is derived from larger trees that are rotary cut in order to produce the layers that are required. These larger-diameter trees take longer to grow. 

Installation

When it comes to their installation process, both OSB and plywood undergo a similar installation. The big difference, however, is that OSB is a bit more flexible than plywood. Both OSB and plywood will have to be installed against the joists and then nailed into place to keep them secure.

Cost

When it comes to their costs, which one is more budget-friendly when you’re installing a new roof? Plywood is actually more expensive than OSB. This is because it has to be thicker and has to be built to more specific standards.

A 4×8-foot sheet of plywood costs approximately $10, whereas an OSB sheet of the same size will cost you a few dollars less. However, when it comes to installing them, they both cost about $70 per hour. 

Related Questions 

Can you use plywood around the home? 

You can use plywood for a variety of applications, such as walls and furniture, but this type of plywood is thin and flexible. 

Why is rotted plywood a problem?

When your plywood rots, it battles to support the roof weight. Moisture causes the wood to rot. If you try to install shingles on rotten wood, this will cause the roof to move and shift, which can lead to water getting into the construction. 

How does your attic affect your roof decking?

Too much moisture in the attic can cause the decking to become moist. This is as a result of hot temperatures that rise from the attic space and make the decking a site for moisture, mold, and mildew. 

Conclusion 

Decking is important to seal your roof as it ensures greater stability for your roof by being secured to the joists and trusses. When shingles get attached to the planks of plywood, this makes your roof watertight to prevent leaks and water damage.

In this article, we’ve featured some important things to know about plywood, and how it compares with OSB, which is another popular decking material.

Sources:

James

James Weldon is a professional roofing contractor with many years of experience on the job. He does not only handle large projects and provide excellent services for his company’s many clients; James Weldon also dedicates his spare time to teaching others useful tricks of the trade. Using BuildCampus as an avenue to reach many roofers and aspiring roofing contractors, James Weldon continues to provide high-quality educational posts and buying recommendations for anyone who visits the website.