Plywood is one of the most common construction materials often used in roofing. It’s less expensive, easy to work with, and does a remarkable task of spreading weight evenly.
Plywood has been a standard roof sheathing material since the 1980s and is the second most preferred sheathing for many roofers. Since the most common rafter spacing is 24 inches, the standard plywood recommended for most roofing situations is 5/8-inches since it supports roof loads for most situations. However, a 3/4-inch thickness can be used for roofs with heavy loads.
That’s only a short answer to the question but we dive deep into the types, sizes, and advantages and disadvantages of using plywood for roofing
Let’s get started!
What is Plywood?
Plywood is a building wood consisting of a minimum of 3 thin layers of wood -commonly more than 5- that are stuck together on top of each other using glue.
Each layer is referred to as a veneer. The grain of each veneer runs in the opposite direction of the layer before (a right angle to the previous veneer), which gives plywood incredible stiffness and durability.
When we use enough layers of plywood to create a sheet that’s at least 3/8 inches thick, it makes for excellent roofing material. Generally, plywood is a highly versatile construction material because it’s very firm, possesses a high ratio of strength to weight, and offers good shock/impact absorption.
As such, plywood isn’t only used as a roofing and sheathing material and as a subfloor.
What Makes Plywood a Good Option for Roofing?
The fundamental framework of roofs consists of rafters and trusses. On top of this frame, a flat deck should be laid down to fix a final material onto it, such as roofing felt or tar paper.
One of the best materials you can use for making the deck of a roof is plywood, and here’s why:
Plywood is Durable and Inexpensive
Since the deck of the roof won’t be visible once the finishing material is installed, it’d be a waste and a huge, unnecessary cost to use solid wood as looks don’t play a factor in the matter.
Instead, using plywood provides the required strength for supporting the load of the final material on top of it and heavy ice or snowfall while being very affordable.
Plywood Resists Moisture Well
Thanks to its impressive performance upon exposure to water for extended periods, plywood makes for a handy material in outdoor uses such as roofing.
Even if it does expand or swell due to moisture, this happens uniformly across the entire deck so that it doesn’t affect the overall structure of the roof. Additionally, plywood quickly dries out, which minimizes the risk of mold building up and other moisture issues.
Plywood is Easy to Work with
Cutting plywood to size is a simple job whether you’re using circular saws or handheld saws. The standard sheet size for plywood is 4 x 8 feet, which strikes a good balance between being a reasonable size to work with and providing coverage of a fair surface area of the roof.
Although not as common, larger sheet sizes of plywood are available. It also comes in a wide range of thicknesses to accommodate various loads.
Types of Plywood for Roofing
When considering plywood for roofing, there are 2 major types to choose from: general and structural plywood.
General Plywood
This type of plywood is a lower grade that’s best used for general and interior purposes. In some cases, however, general plywood is acceptable for roofing, particularly in areas of dry climates.
The layers of wood or veneers of general plywood are glued on top of each other using urea-formaldehyde resin, which is prone to degenerating with repeated exposure to wet conditions. Not to mention, it can lose its adhering strength with drastic temperature changes.
The advantage of using general plywood for roofing mainly lies in its significantly reduced cost compared to structural plywood, so it can be a major money-saving option if you’re looking to cover a large roof.
To help keep moisture-related problems to a minimum, you can install waterproof sheeting on top of general. This can notably prevent moisture penetration, but if you want the best protection against wet weather, you should opt for structural plywood as it’s designed for outdoor applications.
Structural Plywood
This type of plywood is the highest grade available of the material, with a performance rating that meets or exceeds structural building regulations.
It has a wide range of uses in various applications, but as you can probably tell from its name, this type of plywood is intended for structural purposes.
It’s especially suitable for roofing thanks to its high level of moisture resistance, which comes in extremely handy for use on a roof that’s likely to deal with wet conditions.
The key difference between general and structural plywood is the type of glue adhering the veneers together. In the latter, the layers of wood are held on top of each other using melamine-urea-formaldehyde resin or phenol-formaldehyde resin.
Phenol formaldehyde resin forms what is regarded as the best performing bond. As a result, it produces the highest grade of structural plywood.
Additionally, this type of resin is resistant to moisture penetration of wood and sharp temperature changes.
Plywood Grades for Roofing
Besides choosing a suitable type of plywood for roofing, you also need to select a plywood grade to work with. Here’s a breakdown of the most common plywood grades used for roofing:
A
This is the highest grade of plywood, which is structural plywood coupled with a powerful glue that creates a tough bond between the layers of wood together to grant the plywood high resistance against wet conditions and temperature changes.
Not only is this a very sturdy grade of plywood, but it’s also the most aesthetically pleasing with virtually no imperfections when it comes to appearance. This makes grade A plywood ideal for indoor applications or projects in which it’ll serve as a surface.
The absence of visual imperfections doesn’t matter in roofing. However, the lack of knots helps maximize the strength of the plywood because their presence in wooden sheets can cause weak spots down the road.
B
Grade B plywood is a category of structural plywood that’s often manufactured using moisture-resistant adhesive known for being tough. It possesses more visual imperfections than grade A plywood, so it’s considered lower quality, although it still offers excellent build quality.
Grade B plywood shows a few minor holes or knots or holes on its surface that are pretty much undetectable unless you’re actively trying to find them.
C
Compared to grade A and grade B plywood, grade C plywood is produced using layers of wood having more defects on the surface, resulting in plywood with more noticeable visual imperfections including knots.
Still, grade C plywood is suitable for various structural applications, including roofing. You can even use this type of plywood on projects where you want the surface to be seen, especially if you’re planning on painting it.
Opting for grade C plywood for this purpose can save you a nice sum of money while still giving you a good-looking surface. As far as roofing goes, grade C plywood is categorized under the structural type, so it can work just fine for installing on a roof.
D
The lowest grade of plywood, grade D, isn’t an indication of the quality but rather the number of visual imperfections in the wood layers and how obvious they appear.
Grade D plywood shows a notable amount of holes and knots, but it’s still going to be tough and water-resistant. Consequently, it can be used for roofing.
That being said, grade D plywood isn’t the best type to use in any projects where you want the surface to be seen, as it often results in an unfinished and ugly look. Instead, it’s ideal for structural applications where another more good-looking material is laid on top.
One of the biggest advantages of using grade D plywood is being very budget-friendly. So if you’re looking to reduce building costs, this is a good choice.
CDX
CDX plywood is a hybrid grade common to use on building projects, thanks to its strength, versatility, and affordability. In CDX, the ‘C’ refers to the grade of one of the plywood sides, while the ‘D’ refers to the grade of the other face.
As such, the uppermost layer of the plywood will be grade C to serve as the visible surface with some imperfections, whereas the undermost side of the plywood sheet will be grade D with more obvious imperfections.
CDX plywood is an excellent choice for projects where you need to cut down on costs by installing grade D plywood while still producing a relatively google-looking finish by using a grade C on the visible surface.
The X in CDX indicates the type of adhesive used to glue together the different layers of wood. Here, an extremely strong glue gives reliable plywood resistance against moisture and sharp temperature changes.
Consequently, CDX plywood makes for a suitable roofing material as it can handle exposure to wet conditions or sudden temperature changes.
This hybrid grade of plywood is designed for outdoor applications, and it’s the most commonly used type of plywood for roofing due to its durability and cost-effectiveness.
So,What Kind of Plywood for Roof?
CDX plywood is considered the best roof sheathing material because of its better dimensional stability. That means it’s hardly waterproof and readily absorbs water, but it also sheds the water quickly and reverts to its original shape and thickness effortlessly.
Roof Plywood Thickness
When it comes to selecting an appropriate thickness of plywood for roofing or roof sheathing thickess, a few factors come into play. The choice will mainly depend on:
- The space between rafters.
- The slope of the roof.
- The weight of any other material that you plan on installing on top of the deck.
- The load of the design is the stress that the roof is expected to endure due to weather elements and various forces, especially the weight of ice or snow build-up.
In general, if the space between the rafters is too wide, the shingles become heavy. Since the weathered load is heavier than normal, the roof slope is affected. To fix the low roof slope, you must use thicker plywood panels.
What’s the Minimum Thickness of Plywood for Roofing?
The minimum thickness of plywood for most roofs is 3/8 inches. It’s the common choice for roofs with rafters set 16-inches apart with minimal roof load. However, roof pitch will affect the load; a flat roof will have a heavy load on any section, while a steep roof will have a lighter load per square foot.
There are a few things to consider when choosing the minimum thickness of plywood for your roof sheathing.
The first is the span of the rafters or trusses. The longer the span, the thicker the plywood must provide adequate support. The second is the load that will be placed on the roof. If you expect heavy snowfall or high winds, you need to use thicker plywood.
Finally, you need to consider the climate in which you live. If you experience extreme hot or cold temperatures, you will need to use thicker plywood to prevent the sheathing from warping or cracking.
What’s the Most Common Thickness of Plywood for Roofing?
On roofs where rafters are spaced at least 20 inches apart, the recommended thickness of plywood is 1/2 or 5/8 an inch.
That said, 24 inches is the most common rafter spacing, and for that, plywood of 5/8 an inch thick is most suitable. Such thickness is enough to will most roof loads, which is why it’s the standard choice.
On roofs with particularly heavy loads, you may need to use plywood of 3/4 an inch thickness. No matter the thickness you opt for, make sure the plywood is suitable for exterior use, rated “Exposure 1” or “Exterior.”
What Size Plywood for Roofing
5/8-inch plywood is the most common choice for roofing with 24-inch rafter spacing. It’s also the standard for most roof applications because it supports roof loads in most situations. However, you may require a slightly thicker 3/4-in CDX plywood for heavier roof loads.
You can get sheets of up to 4 x 10 feet long, whereas OBS is available in lengths up to 16 feet. So the question here is, how many plywood sheets does your roof require?
You should know precisely how much plywood your roofing job needs to avoid buying extra plywood and wasting money.
The standard plywood size is 4 x 8 feet. To calculate how many square feet a 4 x 8-foot sheet of plywood can cover, multiply the given length by width. You’ll conclude that 4 x 8 feet of plywood cover an area of 32 square feet.
Now, to calculate how many sheets of plywood you need to cover your roof, first multiply the length of the roof by its width to get the area of the roof, then divide this area by the square footage of your sheet.
For example, if the area of your roof measured 1000 square feet, divide it by 32 (the square footage of a single 4 x 8 sheet). The resulting number is how many sheets of plywood your roof required.
Remember, it’s a good idea to buy a couple of extra sheets of plywood to make up for wastage and mistakes.
Plywood vs. OSB (Oriented Strand Board)
Is OSB better than plywood for roofing?
As we explained above, plywood consists of multiple layers of wood called veneers. These are arranged in alternating grain and glued on top of each other with a hot press.
Finished plywood sheets always feature an odd number of veneers, so cross-lamination techniques are balanced.
As for OBS (oriented strand board), it’s manufactured by adhering together small pieces of wood then compressing them. OSB panels are available in lengths up to 16 feet, and you can easily identify them by observing their distinct wood strands.
The following is a simple breakdown of plywood vs. OBS for roofing:
- Price — OBS is less expensive compared to plywood.
- Available sizes — OBS is sold in larger panels up to 16 feet long, whereas plywood panels come in a maximum length of 8 or 10 feet.
- Water test — most types of plywood have good resistance against moisture. OSB plywood, on the other hand, easily absorbs water when exposed to moisture. A 1/2 inch OSB panel can swell up an additional 1/4 of an inch overnight.
- Formaldehyde — OSB has a higher concentration of formaldehyde compared to plywood. Formaldehyde has been associated with occupational asthma.
- Market share — OSB accounts for about 70 to 75 percent of the market share, while plywood makes up around 25 percent.
- Wood constituents — OBS comprises 95 percent wood and 5 percent wax and resin, while plywood is 100 percent solid wood veneers.
- Grades — OBS is available in several grades, including OSB/1, OSB/2, OSB/3, and OSB/4. As for plywood, it comes in 4 primary grades of plywood veneer, which are A, B, C, and D.
- Strength — OSB is more fragile than plywood panels, which possess less sheer strength when it comes to strength.
Plywood for Roof Sheathing
Roof sheathing or roof decking involves installing a layer of plywood or OSB to cover the joists and trusses, the beams that make up the skeleton structure of a roof.
The sheathing is placed below the roof’s shingles and above its framework. It provides the roofing tiles and shingles with a support layer to rest on.
The purpose of sheathing is to:
- Provide support as it prevents the direct attachment of shingles to joists and trusses by evenly distributing their weight shingles.
- Provide extra protection by incorporating an additional layer of insulation to resist leaks and reduce the spread of fires.
Roof sheathing is typically made out of wood, either plywood or OSB. While the latter is less expensive, it’s more fragile and lightweight, so many roofers go for plywood.
Plywood for Roofing FAQs
What is the Best Plywood for Roofing?
5/8-inch plywood is the most common choice for roofing with 24-inch rafter spacing. It’s also the standard for most roof applications because it supports roof loads in most situations. However, you may require a slightly thicker 3/4-in CDX plywood for heavier roof loads.
What Thickness Plywood for Roof?
The minimum thickness of plywood for most roofs is 3/8 inches. It’s the common choice for roofs with rafters set 16-inches apart with minimal roof load. However, roof pitch will affect the load; a flat roof will have a heavy load on any section, while a steep roof will have a lighter load per square foot.
Can you Use 3/8 Plywood for Roofing?
Yes, 3/8 inch is the standard plywood size for most roofs in areas with light load requirements on rafters at 16″ centers. 1/2 inch is the minimum, while 5/8 is the most commonly used with rafters spaced at 24″ centers. However, 3/4-inch plywood is recommended for heavy load-related areas, regardless of rafter spacing.
What Size Plywood for Roof Sheathing?
The spacing of the rafters governs the thickness of plywood used for roof sheathing. Most roofers suggest that 5/8-inch plywood should be standard if the underlying rafters are spaced 24 inches apart or less, but slightly thicker 3/4-inch plywood should be used for rafters spaced further apart.
What Size OSB for Roof?
In the past, 1/2-inch thick OSB was the most commonly used thickness for OSB roof sheathing. However most roofers have switched to 7/16 panels that is durable and maintains the integrity and efficiency of the building and ensuring the performance of the finish material and aesthetics of the roof.
What is the Best Wood for Roofing?
Cedar is the most popular option among roofing shingle manufacturers because of its flexibility, durability, and weather resistance. While cedar is a softwood, it resists rot and repels insects better than most other wood – qualities that make for an effective roofing shingle.
Wrap Up
There you have it, everything you need to know about using plywood for flooring. Whether it’s a floor or a roof, it seems that you can never go wrong with this incredibly versatile material.