Selecting the best material for your roof is important to the longevity and durability of both the roof and the building. A good roof protects everything underneath it. If you’re wondering what the best plywood for roofing is, we’re here to help!
APA-rated 5/8” OSB is less expensive than comparable plywood and offers both moisture resistance and durability. CDX-graded 5/8” plywood is up to 30% more expensive and isn’t as highly rated for moisture resistance or durability. As a result, OSB currently accounts for about 70% of new roofs.
In this guide, we identify the common types of plywood used for roofing, including their pros and cons. We explain the factors to consider when selecting roofing plywood, including the importance of plywood thickness. Plus, we explore what the best plywood is for a flat roof. Our aim is to provide you with information to select the best material for your project.
- Best Plywood for Roofing
- Factors to Consider When Choosing Plywood for Roofing
- Roof Plywood Thickness
- What Is the Best Plywood for a Flat Roof
Best Plywood for Roofing
Plywood is composed of thin layers of wood glued together with the grain perpendicular to adjacent layers to form a rigid building panel. Although the Egyptians first used the technique more than 4600 years ago, the first patent for modern plywood was submitted in 1865.
Since then, plywood has undergone numerous improvements or adjustments to become what we know today. In the Table below, we identify the types of plywood most commonly used in roofing today.
|Type of Plywood||Durability||Moisture Resistance||Price|
|Structural Plywood||Best||Good to Best||Low to High|
|Oriented Strand Board (OSB)||Better||Better||Lowest|
Plywood is often double-letter graded by surface defects and strength with A being the best and D not as strong or good-looking. The initial letter is for the exposed or top face and the second for the back or hidden face. AA grade means both surfaces are high quality and free of major defects, ideal for furniture, cabinetry, and other visual locations. DD is more of a utility plywood where both surfaces are typically hidden.
Plywood is used for a variety of purposes, and modifications have been made that have resulted in products designed for specific purposes or uses. However, each product can also be used for other applications like walls, floors, and roofs.
Depending on the manufacturer, there are structural and non-structural plywoods. Structural plywoods are also stronger than wood or non-structural plywood of similar thickness. Plywoods left exposed to the elements require regular maintenance and the application of waterproof or water-resistant sealers, stains, or paints.
Below, we explain the different types of plywood commonly used for roofing and identify their uses and some of their pros and cons.
CDX is a construction-graded plywood of 5 to 7 layers and may have open knot holes and/or tight knots visible, as well as some discolorations or small splits. On the C face, the defects and knots may be filled with a football-shaped plug or putty.
On the D side, the defects are often slightly larger and not repaired. CDX has some internal voids and while not structurally rated, it is still structurally strong.
The X grade means the plywood is manufactured with waterproof adhesives for e-X-terior use and can withstand a limited amount of moisture. CDX is commonly used for exterior wall sheathing and roofing where its strength and moisture resistance are important, but it will be hidden by other construction materials as it’s not nice looking.
- ideal for wall and roof sheathing with good holding strength for screws and nails
- can handle humidity and some moisture without splitting, warping, or rotting
- will swell when wet but return to original thickness, shape, and size when dry
- cost-effective and easy to stain or paint
- knots and imperfections
- limited aesthetic appeal or visible uses
ACX plywood is a strong, structurally rated exterior-grade plywood. It has a smooth sanded good-looking high-quality veneer on the top surface and a lower quality on the back face. Manufactured with waterproof adhesives, it can handle exposure to humidity and some moisture.
This plywood may contain 1/4″ or smaller knots and no flaws on the A surface and plug or putty-filled knot holes and defects on the other surface. It is often used for visible interior or exterior locations such as wall panels, cabinetry, shelving, fencing, and exposed wall sheathing.
- high-quality, good looking, smooth sanded top surface is easier to stain or paint
- stronger than lower-graded plywood and holds screws and nails well
- won’t split or warp with exposure to humidity or some moisture
- will return to its original shape and thickness when it dries
- not waterproof so needs to be protected from significant moisture exposure
- more expensive than lower-grade plywood
Structural plywood is an interior-exterior rated plywood also known as construction or sheathing plywood. It is a strong, stable, load-carrying plywood manufactured with a durable Type A- or B-bond water and weather-resistant adhesive. It is commonly used to sheathe floors, walls, and roofs, as well as to make beams, furniture, cabinetry, crates, signs, and billboards.
Structural plywood is available in AA, AB, AC, BB, BC, BD, CC, CD, and DD grade configurations, with pricing consistent with grading. So, while A-grade plywood makes excellent cabinets or exposed finishes, a C or D is almost as strong but is typically used for subfloors and sheathing walls and roofs. Marine plywood is a specialized structural plywood that can resist full emersion and all climate temperatures.
- readily available
- versatile and easy to work with
- withstand moisture and weather damage
- wide range of grade finishes, strength, durability, and price point depending on grade
- not waterproof
- price is drastically affected by grade and thickness
Oriented Strand Board (OSB)
OSB is an engineered plywood manufactured by compressing up to 50 layers of wax-resin adhesive-coated leaf-like thin strands of hard and softwood into a desired thickness. The inner layers are oriented one way and the outer layers the opposing direction. The mass is heated and pressed to form large panels with consistent density and thickness.
OSB is less expensive than plywood of comparable thickness and is more commonly used today for roofing than plywood. Its resistance to moisture is better due to each strand being coated in adhesive, but it also depends on its grade.
The panels have a smooth sanded side and an unsanded side. OSB grade stamped Exposure 1 is rated for limited exposure during the construction phase, while Exterior grading is waterproofed to withstand extended exposure.
- stronger shear strength and more moisture resistance than plywood
- available in different thicknesses, types, grades, widths, and lengths
- less expensive than comparable plywood sizes
- consistent density and thickness throughout
- more flexible or pliable so thicker material is required for horizontal applications
- heavier and less visually appealing
Factors to Consider When Choosing Plywood for Roofing
Choosing the correct plywood for a roof depends on several factors. The thickness is often determined by the type of plywood, rafter spacing, roof load, durability, moisture resistance, cost, and several other factors.
Most building codes also address the thickness and type of plywood, as well as the required fasteners and nailing pattern, so ensure you consult your local one to be code-compliant.
Plywood, including OSB, is available in both square edge and tongue and groove (T&G) formats. The tongue is typically oriented toward the ridge line and the groove slides over it, closing and sealing the seam. This also prevents moisture that may make it under the roofing from dripping into the attic.
The connection provides support to the seam between the joists, helping to prevent sag. The alternative is square-edge panels and the use of H-clips on the edges between joists for support. Unfortunately, the clips also leave a drip gap between the joined edges.
Rafters or trusses are commonly spaced at 12”, 16”, or 24” on center. So, the typical eight-foot length of plywood will span and be supported by 9 joists at 12 OC, 7 at 16” OC, and 5 at 24” OC across its entire length.
The greater the spacing, the thicker the plywood needs to be, and also the more H-clips to prevent sagging if using square-edge plywood. However, roof loads and slopes also need to be considered.
Roof slope or pitch affects load, which in turn impacts plywood selection. The greater the slope, the lighter the load, and the lower the slope, the heavier the load per square foot. However, while steeper roofs shed precipitation and debris more quickly, they also tend to experience greater wind force loads.
So, roof loads and rafter spacing, coupled with pitch, may necessitate a thicker plywood selection. Always check the local codes.
Roof load is comprised of the dead load, live load, and wind load. The dead load is the weight of all permanent building and structural materials that form the roof, including sheathing materials, waterproof finishes, and HVAC. Live load refers to temporary loads or weights like people, seasonal decorations, snow, and even rain.
Snow load is a live load but is often addressed separately in areas prone to heavy snow accumulations. Wind load varies from region to region and requires design variables to accommodate the pressure or load the wind exerts to prevent damage.
Moisture resistance refers to plywood manufactured using waterproof adhesives that give it moisture-resistant properties. The moisture resistance of plywood is more important during the construction phase especially if delays are anticipated.
Roof sheathing typically isn’t left exposed once in place as it protects and keeps everything under it dry. It is commonly covered with a waterproof or resistant membrane until shingles, steel, or other finishing materials are installed.
Moisture-resistant plywood, though, will handle humidity, moisture, and leaks better than non-resistant materials.
Fire resistance refers to how long a material or assembly can withstand the heat and flames of a fire. Both OSB and plywood are available with fire-resistant treatments, but it isn’t commonly used.
Most codes address the resistance of the whole roof construction, so unless fire-resistant plywood is required, it isn’t commonly used. However, the thicker the plywood used, the greater its resistance to heat and fire spread.
The durability of a material not only affects its lifespan, but also that of the roof. Selecting higher quality materials, though more expensive, will usually result in lower repair and maintenance costs in the future.
Thicker materials will last longer, as will moisture-resistant ones, however, it’s not necessary to break the bank. The durability and lifespan of OSB and plywood used for roof sheathing are almost the same.
OSB has better moisture resistance but once water is inside it, it can swell, while plywood will usually return to its original thickness. Unfortunately, the hidden voids in plywood can trap moisture and lead to rot.
Additionally, the high heat of the roof deck can cause the adhesives in plywood to break down allowing the plies to separate. Provided the roof is properly maintained, the sheathing shouldn’t get wet, so its lifespan and durability should endure.
Properly constructed roofs will require less maintenance than poorly crafted ones, but they still need maintenance. Much, though, depends on the roof slope, type of shingle, steel, or other finish. Dirt and debris need to be removed seasonally as they can trap moisture and allow it to work through protective layers around fasteners.
Damaged or missing shingles and caulking need to be addressed before moisture can cause rot or mold development in the wood sheathing. Many roofers apply a waterproof sealant to all cut edges and holes to minimize moisture absorption.
Additionally, ventilation is also important as a damp attic can cause mold, mildew, and rot that can destroy both plywood and OSB from the underside.
The square footage of the roof surface determines the number of pieces of plywood required to cover it. Labor costs to sheathe a roof in OSB or plywood are comparable, but material costs can vary depending on thickness, grade, type of material, and ZIP code, as well as the time of year – hurricane season affects supply and demand, which also impacts nation-wide pricing.
A quick scan of 5/8” plywood and OSB prices in your area can be very illuminating and almost shouts the reason OSB accounts for about 70% of roof sheathing today.
Some say the cost difference between OSB and plywood is 10 to 15%, but experience shows it can be as high as 30% or more. In my canvass of the 3 big U.S. home improvement stores in one east-coast city, 5/8” Exposure-1 OSB ranged from $0.59 to $1.66, 5/8” CDX $0.98 to $2.09, and ACX $1.30 to $2.48 per square foot. That’s a significant difference depending on the size of the roof and what price you pay, so shop around!
Roof Plywood Thickness
The thickness of roofing plywood usually depends on rafter or truss spacing, slope, loads, and type of finishing material to be used. Plywood is about 10% stiffer than OSB of the same thickness, but in most roofing situations, that isn’t significant. Nor is OSB’s greater shear strength or plywood’s greater impact strength.
Rafters spaced at 16” OC or less in some regions only require 3/8” sheathing depending on roof loads and other factors, but thicker 7/16” or 1/2″ is commonly used. 5/8” thick sheathing is typically used for rafters spaced up to 24” OC depending on loads and finishes.
Roofs experiencing heavy loads, having lower slopes, rafters more than 24” apart, or finished in steel often require 3/4″ plywood or OSB. However, always check with the local Building Department for requirements and specifications.
What Is the Best Plywood for a Flat Roof
A flat roof has a pitch of 5 degrees or a slope of 1/12 or less. For edge support perpendicular to rafters, T&G is recommended but square-edge can be used provided edges are supported by blocking. Sheathing must be stamped APA Rated Structural 1 Sheathing and graded Exposure 1 or Exterior.
Plywood used for flat roofs must be 5/8” or thicker, 5 or more ply, and suitable for exterior use. If using OSB, it should be at least 5/8” thick OSB-3 or OSB-4.
The thickness of both types of sheathing depends upon rafter or truss spacing, the type of finish it must support, calculated loads, and environmental conditions that affect roof loads. Additionally, the type of finish going on the sheathing affects the choice too.
Some adhesives such as GRP bond better to OSB and others better to plywood. It should be noted, though, that both types of sheathing must be dry before covering.