Roof sheathing is a covering provided over roofs to protect the inner wooden roof members and to provide support for hanging shakes and shingles. There are many types of roof sheathings, such as Wooden Planks, Plywood, skip sheathing, Oriented Strand Board (OSB), and Concrete.
This article discusses the importance of roof sheathing and explores different roof sheathing materials.
Types of Roof Sheathing: 5 Options
Roof sheathing differs depending on the type of material used for making it, and the spacing that is provided between the sheathing panels.
Roof sheathing can be made out of plywood, OSB, wood panels, and even concrete.
|Type of Roof Sheathing||Pros||Cons|
|Oriented Strand Board (OSB)||Economical, less prone to warping or swelling over time, thick panels||Vulnerable to water damage, presence of formaldehyde|
|Plywood||Lightweight, available in a range of sizes and finishes, does not swell when exposed to water||Expensive, high maintenance, smaller panels|
|Concrete||High strength, fire resistant, long-lasting, can be painted, protection from rots and insects||Costly, high maintenance, more manpower for installation and delivery|
|Skip Sheathing||Resistant to moisture, durable, less material cost||Ineffective in areas with significant rainfall and windy rains, expensive|
|Wooden Boards||Robust, excellent insulation||Difficult to install, susceptible to warping with time|
The different types of roof sheathings are discussed in detail below.
Oriented Strand Board (OSB) Roof Sheathing
It is a synthetically manufactured material made out of different fragments of wood and is a popular choice now.
The fragments are held together by special adhesives which make strong bonds between the layers of an OSB board and are less prone to warping or swelling over time.
OSB is available in thick panels that are quite strong and has a uniform finish.
Compared to its alternatives, it is less expensive.
Plywood Roof Sheathing
Plywood is another common and popular sheathing material.
Similar to OSB, plywood is made out of numerous wooden layers that are bonded using special adhesives.
The layering in plywood is quite dense and forms a thick board that is beneficial for nailing purposes.
The layers also help prevent the shrinking and warping of the plywood boards.
The high density of plywood prevents moisture from reaching the core layers and thus provides moisture resistance to it.
Due to this, plywood retains its strength and durability for a long time.
Concrete Roof Sheathing
The concrete sheathing is done where there is a high strength requirement and sufficient support is available for carrying the sheathing itself.
Concrete is being integrated more and more as part of green projects.
It replaces wood and thus saves a lot of trees, but it also has its own carbon footprint.
Concrete roof sheathing is done by pouring concrete into prefabricated boards in several sets.
Metal reinforcement is then placed within the concrete mix.
It is more uniform and continuous than wooden sheathing and does not let water enter a house.
It is categorized as low maintenance and long-lasting.
It also resists lateral wind loads effectively.
Skip Roof Sheathing
Skip sheathing is a type of roof sheathing in which the wooden planks are placed with certain spacing in between them.
The aim of doing this is to allow air to pass through the sheathing and aid the drying of roof structures after they have been exposed to moisture.
1x4 boards of plywood or OSB are used in skipped sheathing with a spacing that ranges from 5” to 10” when incorporating shakes and shingles.
Although effective, skip sheathing degrades over time and has to be re-sheeted.
In contrast, there are new materials that have built-in small pores allowing the passage of air which are largely replacing skip sheathing.
Wooden Boards Roof Sheathing
Roof sheathing started in the 20th century with the use of wooden boards made from the bark of local woods.
Wooden planks were cut into the required shape and then installed over the rafters in houses. Later on, shakes and shingles were added to the sheathing.
The shakes and shingles would eventually fall off and were later replaced with asphalt ones.
But asphalt shakes and shingles required a more solid support base for anchorage which plain wooden boards could not provide.
It was due to this problem that new and improved fabricated materials such as plywood and OSB were developed.
Plywood and OSB boards extensively replaced wooden board sheathing.
What is Roof Sheathing?
The roofs in residential construction are made using rafters or trusses. These members are placed with spacings of 16” to 24” in between.
Due to these large gaps, the roofs remain open and thus need to be closed off using a protective covering. This covering is known as roof sheathing.
In roof sheathing, boards of wood and similar materials are nailed directly onto rafters with minimal to no spacing.
These rectangle or square blocks cover entire roofs and make a support surface onto which shakes, and shingles can then be attached.
What is the Purpose of Roof Sheathing?
Sheathing helps in solidifying a roof by forming a solid covering over it that also acts as a support for shakes and shingles.
Sheathing boards are waterproof and thus protect against leakages, which helps extend the lifespan of a roof.
Roof sheathing distributes the load evenly in case of snow and heavy rainfall and thus prevents roofs from undergoing sagging.
When to Replace Roof Sheathing?
Like all other structures, roof sheathing degrades over time and there may come a time when you will need to either re-sheet it or replace it entirely.
Fortunately, roof sheathing degrades slowly, making it easier to point out when it needs replacement.
If any of the following warning signs develop to an extreme, then roof sheathing needs to be replaced.
Cracks in Roof Sheathing
Cracks may develop in the sheathing due to heavy loads not being distributed correctly.
An example would be a tree branch falling onto the roof and causing a direct impact on the sheathing.
Other reasons for the development of cracks include continuous freezing and thawing of moisture within the sheathing.
Cracks then cause water to seep into a house and affect other structures.
Keep an eye out for cracks because if they are left unchecked, they can propagate and give rise to larger cracks.
Moisture can cause damage in a lot of ways to roof sheathing.
Long-term exposure to moisture degrades the quality of the sheathing making it lose its strength.
This can ultimately lead to warping or twisting of sheathing boards.
Consequently, the shakes and shingles start to fall off.
It is thus important to check that the roof sheathing does not retain moisture for long periods of time.
If you see any twisting in the sheathing, followed by falling off of shakes and shingles then your sheathing is in need of a replacement.
Rotting in Roof Sheathing
If you start to notice blackened spots on your roof, then that means that the wood panels have started to rot.
Press against these spots and check if the material gives in easily. If so, then the integrity of your roof has been compromised.
Hence, always keep an eye out for black spots along the roof sheathing.
Holes in Roof Sheathing
Holes in roofs are never a good indication. They allow water to leak through the roof and enter a house.
Roof holes can develop due to a number of reasons which include moisture damage, load impacts, and pest action.
Check your roof in broad daylight to spot any holes in it and call in a roofing contractor for them to be repaired.
Sagging in Roof Sheathing
Sagging is one of the major signs that indicate if your roof sheathing is in need of a replacement.
Roof sagging is both dangerous and aesthetically unpleasant.
You can check the alignment of your roof from the outside to see if it has formed any waves.
Roof sagging can also be checked from the inside, by going up to the attic and seeing if any part of the roof seems closer to you than the other parts.
Roof Sheathing Dimensions
Roof thickness is selected based on the steepness of a roof, the amount of snow load it has to bear, and the spacing of rafters.
Roof sheathing panels come in 4’x8’ rectangular blocks.
7/16” is the common thickness used for roof sheathing.
In general, roof sheathing thickness lies between 1/2” and 5/8”.
3/8” is the minimum thickness recommended for plywood sheathing.
The thickness of roof sheathings is determined by rafter spacing and snow load.
8d nails are used to install the sheathing over the rafters.
Cost of Roof Sheathing
The cost of sheathing is dependent upon the thickness of the sheathing boards.
The average roof covers an area of about 3000 square feet and the average 4x8 plywood sheet covers about 32 square-feet roof area.
A single standard plywood sheet having 4’x8’ dimensions costs anywhere between $70 to $100.
The range of thickness is usually between 1/4" to 5/8”. The thicker it gets, the more it costs.
In comparison, OSB boards are cheaper than plywood.
The average cost of a 4’x8’ OSB board lies between $32 and $77, again depending upon its thickness.
The total cost of a sheathing project varies depending on labor costs as well.
How Long Does Roof Sheathing Last?
With proper care, roof sheathings can last well over 100 years.
It is generally designed to last the life of a house itself, but its lifespan can be significantly shortened by moisture action.
Water causes roof sheathings to degrade, and rot and can lead to the development of cracks.
Sheathing surfaces lose strength as they are exposed to moisture for longer periods of time.
OSB and Plywood are more susceptible to moisture effects than traditional lumber.
Roof sheathing is a common way of securing roofs and provides a base to fix shakes and shingles.
It has been in practice for more than a century, however, the type of material used for roof sheathing has changed over time.
Previously, sheathing was done using local wooden planks, but nowadays it is being done by utilizing boards of newly engineered materials.
These materials include Plywood and OSB, which offer a variety of advantages over normal wooden sheathing.
Out of the two materials, OSB is cheaper and is widely used in residential construction.
Concrete is another material used for roof sheathing processes, where heavy loads need to be carried.
The sheathing is done using boards of 4’x8’ dimensions, where the thickness can range between 1/2" to 5/8” depending on the rafter spacing and snow load etc.
Roof sheathing tends to degrade over time and should be replaced or repaired to avoid any risk of collapse.
Frequently Asked Questions (FAQ)
What Type of Sheathing is Best for a Roof?
Oriented strand board (OSB) is the material that is extensively used in sheathing roofs.
It has sufficient thickness; making it strong, it resists water, it is lightweight, and is also very cost-effective.
It is also employed in the production of sheets for re-sheeting skipped sheathing.
Do You Need to Glue Down the Roof Sheathing?
Roof sheathing shouldn't be glued like flooring. Nail the panel.
When using a power tool, be careful not to overdrive the nails through the panel surface.
Can I use Screws for Roof Sheathing?
Screws provide greater holding strength than nails and can be used for securing roof sheathing.
But they should be sized properly by a designer or an engineer.
It is recommended to use galvanized screws and nails to help protect against weather action from the exterior side of the roof.