Roofing Plywood: Sizes, Material Options, Thickness, Calculations, OSB
While it’s not the main material for the roof itself, roofing plywood is an important part in a roofing structure. It’s so important that you have to arm yourself with knowledge and know everything there is to know so you’ll know the best way to utilize it for your roof.
Read on to know more about this material. You’ll know how it’s used for decking and sheating. You’ll also know the different size and thickness requirements. You’ll also learn how to calculate to find out how many sheets you need for your roof.
On this page:
- What is Roof Decking/Sheating?
- Different Material Options & Types of Roof Plywood
- Roof Sheathing Thickness
- What Size Plywood should You Use for your Roof?
- How Many Square Feet in a 4×8 Sheet of Plywood – How Many Sheets of Plywood Do I Need for My Roof?
- Plywood vs OSB
- DIY or Hire a Professional?
- Get Quotes from Competing Contractors
You’ll also know about the different material options. OSB is another popular choice. We set aside a whole section in this article for you to compare the two and determine which one is better.
What is Roof Decking/Sheating?
Also known as sheating, decking is basically where the roofer attaches the shingles or whatever your chosen roofing material is. Obviously, it should be graded for exterior use meaning it should be able to withstand rain especially during the construction of the roof. The waterproofed coat of plywood makes it a top choice along with OSB, planks and wood boards.
What is plywood? Plywood is made out of laminated sheets of veneer. It’s a top choice because of its characteristic wherein it expands evenly when exposed to water unlike OSB wherein only the edges expand.
Speaking of OSB, it’s a top choice of homeowners looking for something more affordable than plywood. As a warning, it’s considered by some to be inferior in quality as compared to plywood. We’ll discuss plywood vs OSB more later.
Different Material Options & Types of Roof Plywood
Yes, there are several different material options and types of roof plywood. An experienced roofer can help you pick out the right one, but it’s good for you to know what these options are.
Before we start talking about the different options, it’s important to note that plywood has different grades:
This is the highest grade there is. You won’t see imperfections. Understandably, it’s the most expensive. Thankfully, it’s not really a requirement to use this grade for all roof applications.
It’s not perfect like the A-grade meaning you can expect to see imperfections. However, it’s good enough to be used underneath roofing felt and shingles.
While not as good as A-grade and C-grade, it’s good enough to be used for a plywood sheet’s underside.
Here are your options:
This option is basically a specialized plywood grade. Its front has a C-grade while its back has a D-grade. X signifies the glue that bonds the veneers.
This is a popular option because it presents homeowners with functional but affordable roof sheathing. A half inch CDX plywood is often used with rafter with 24-inch spacing. If your roof carries a heavier load, like if there’s less pitch and/or it snows in your area, a roofer may recommend the use of the ¾ inch variety.
- Zip wood
Strong and durable, this engineered wood product has a barrier that resists water but still allows it to breathe. This means that there’s no need to use roofing felt. This characteristic makes it a top choice for areas with unpredictable weather.
Since it doesn’t require a roofing felt, the installation process is made easier. The roofer simply puts the panels in place and then seals the seams using tape. Hence, the cost to install it is lower. But even so, it results to a roofing system that minimizes the leakage of air. This makes it more energy efficient.
- FRT plywood
If you live in a townhome with other townhomes around you, building codes may dictate that you use FRT or fire-retardant plywood. It works by a process called acid hydrolysis that’s activated when it’s exposed to fire.
Roof Sheathing Thickness
Unlike with plywood siding, the plywood sheet to be used for sheathing should be at least 3/8 inch thick. That’s considered to be the minimum. This minimum will only work if the rafters are set 16 inches apart at most and if the roof doesn’t have too much load. If your roof is more on the flatter side, then it’s expected to have a heavier load. This minimum wouldn’t work if you have a flat roof.
Most roofs are spaced at least 20 inches apart. In this case, you can’t use a 3/8 inch thick sheet. For this, you can either go for a half inch or 5/8 inch thick sheet. Once the spacing hits 24 inches, a 5/8 inch thick sheet is more ideal.
In addition to the thickness, you also need to make sure that you choose plywood that’s graded for outdoor use. What plywood should you use for a roof? Look for something that’s graded as Exposure 1 or Exterior.
What Size Plywood should You Use for your Roof?
The industry offers a thickness of anywhere from 5/16 inch to ¾ inch. So, what size plywood should you use for your roof?
There are several factors to consider:
- How the roof is designed.
- The spacing of the rafter.
- The expected load.
A half inch thickness is considered to be standard in a lot of areas and it should work just fine considering that it’s deemed okay by the roofing contractor after considering the things above. It’s the standard for spacing of 24 inches in the rafter, although as mentioned, a 5/8 inch thick is more ideal. You might be tempted to choose a ¾ inch thickness although it’s unnecessary in most cases.
You can get plywood sheets with a length of up to 10 feet. The standard width is 4 feet.
Talk to a professional so that the factors will be properly considered so you can make the right choice on the best wood.
How Many Square Feet in a 4×8 Sheet of Plywood – How Many Sheets of Plywood Do I Need for My Roof?
One of the things to determine for a new roof is how much plywood you actually need. Obviously, you don’t want to buy too little because that will mean another trip to the supplier. You don’t want to buy too much because that will be an expensive waste.
A standard sheet of plywood is 4×8. How many square feet are there in a 4×8 sheet of plywood? You just need to multiply the length by the width. In this case, a 4×8 sheet of plywood is 32 square feet. Knowing this number can help you determine how many sheets of plywood you need for your roof.
You need to measure your roof using a tape measure. Get the width and length of each slope of the roof. You’re in luck if you have a conventional gable roof because it should have slopes on each side that are identical in dimensions.
Multiply the width and length of each slope and you should get the square footage of that slope. Multiply that by 2 if you have a gable roof and that’s the total square footage of your roof.
Let’s say that your roof has a total of 1,500 square feet. Divide that by 32 (the square footage of each 4×8 plywood sheet). The number you’ll get is the number of sheets that you need. In this case, you need 47 sheets (rounded off from 46.88). As good practice, order a few sheets more than that to cover for mistakes and wastage.
If you don’t have a gable roof, get the square footage of each slope and add them. What if you have a complicated roof wherein you can’t measure this way? There’s a formula to be applied. Bring out the calculator
Measure the floor area of everything covered by the roof. Let’s say that the main floor area is 10 by 20 feet and the wing floor area is 10 by 15. The main floor area is 600 square feet and the wing floor area is 150 square feet. You now have a total of 750 square feet.
Make sure that you add for the slope of the roof. Adding 20% is industry practice. In this case, add 150 to get a total of 900 square feet. Divide that by 32 to get the number of sheets needed. In this case, you need 29 sheets (rounded off from 28.13). As always, add a few sheets to be sure.
Plywood vs OSB
Plywood vs OSB board has been an ongoing debate. Is there a better choice between the two or will either one work just fine?
Just to get one thing straight, either one will work just fine. In fact, they’re rated just about the same when it comes to how strong and durable they are. Even the Engineered Wood Association agrees to this. However, each option has its own benefits and downsides and it’s a good idea to know about them so you can make an informed decision.
It’s important to note that industry experts, including the National Roofing Contractors Association and Asphalt Roofing Manufacturers Association, recommend both. However, they have a preference for plywood. This sentiment is shared by representatives from TAMKO and Cellotex – 2 known manufacturers of roofs. These manufacturers have shown that they feel more comfortable working with plywood.
Plywood has shown better resilience against water. It doesn’t swell when it gets wet unlike OSB. However, it’s important to note that OSB will only swell when it gets really wet to the point that it’s soaked for a long time. If it does, it can stay that way even if you dry it out.
OSB offers several benefits. If you choose the textured varieties, you’ll have a less slippery option. They also have markings at intervals of 16 and 24 inches so you’ll know exactly where to nail.
OSB is also more affordable. The savings you’ll get per sheet will really add up. Speaking of sheets, it’s available in 4×9 sheets.
While plywood remains a good and viable option, the fact remains that OSB is now the preferred choice. It has been estimated that OSB has a stronghold of ¾ of the market share, mainly because it’s the more affordable option.
But still, no one will blame you if you choose plywood over OSB. A lot of people still think that plywood is higher in quality and in some cases, it is. It has its own strengths. This is especially true among experienced DIYers who mastered DIY tasks working with plywood.
DIY or Hire a Professional?
Speaking of DIYers, should you do the sheathing yourself or should you hire a professional? It might be tempting to do it yourself especially if you’re looking to save money. To know the answer to this question, answer these simple yes or no questions:
- Do you want the best results?
- Do you want to stay away from potential accidents?
- Do you want to save money in the long run?
If you answered yes to these questions, then you need to hire a professional. It doesn’t matter how experienced of a DIYer you are. A professional will always do a better job provided that you hire the best one for the project. After all if you’re better than them, you should be getting paid for it, right? Besides if you’re not satisfied with their job, a professional will make it right to ensure the best results.
More importantly, hiring a professional will keep you away from potential accidents. Working on the roof is dangerous. You can easily slip and/or fall. You can create more damages. Simply put, it’s not safe.
In addition, hiring a professional can help you save money in the long run. Since they can ensure the best results, you will get your money’s worth. The job will also be done right which will minimize mistakes and wastage.
Get Quotes from Competing Contractors
We recommend hiring a professional contractor for this task. What we don’t recommend is for you to hire the first contractor with a good offer. You need to shop around in order to get the best quotes to choose from. If they know that you’re shopping around for the best deal, you can expect to get the best quotes. Be sure to also read our article about getting free roof repair estimates.
We made this process easier. All you need to do is to use the form that you see on this page. Complete the form with the basic details asked and send it. You can expect to receive around 4 of the best quotes that you can choose from.
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