Skip sheathing refers to roof sheathing with gaps. 1x4 boards of plywood or oriented strand board (OSB) are nailed onto the rafters with spacing between the boards. This spacing is useful in allowing air to flow through the roof and drying out the moisture.
Sheathing can be done in two ways, either as continuous sheathing or as skip sheathing.
Commonly, continuous roof sheathing is done with uniform planks made of oriented strand board (OSB) or plywood.
Sheathing done in this way does not have any gaps between the panels of wood. The entire roof deck is continuous and can be stepped on.
It acts as a surface for other roofing materials such as shakes and shingles.
This form of sheathing is still quite popular, but it is prone to degradation caused by moisture which decreases the life of the boards.
In contrast, skip sheathing or spaced sheathing is a more efficient form of roof sheathing as it enables airflow, consequently reducing the effect of moisture.
This article provides a detailed guide on skip sheathing and how it's installed.
What is Skip Sheathing?
Skip Sheathing is a form of roof sheathing in which the wood boards are placed with certain spacing between them.
Normally 1x4 or 1x6 wooden boards are used in skip sheathing, with a maximum spacing of 3.5” in between.
The sheathing is then nailed directly onto the roof rafters.
Sheathing boards must be nailed correctly onto the wood rafters to make a firm connection.
Shakes and shingles are then attached to the sheathing.
The boards installed this way have openings between them, allowing air to flow under the wooden shakes and shingles.
This airflow moves laterally allowing for faster drying of these members from underneath.
The hot air then rises through the openings drying the shakes and shingles more evenly.
Is Spaced Sheathing same as Skip Sheathing?
Spaced sheathing is an alternate term used for skip sheathing.
Both these terms refer to the same process of installing sheathing with gaps in between the wooden boards or shakes over the rafters in roofs.
Another term used for skip sheathing is gapped sheathing.
Why is Skip Sheathing Important?
All sloping roofs shed water, but the roofs made of wood are not waterproof.
When wooden roofs get wet, their boards end up absorbing moisture.
Additionally, a wooden roof is also exposed to extreme heating by the sun.
Hot air also rises and gets stuck under the enclosed attic or roof.
Over time the wooden panels of the roof dry out from the outside-in.
But the extreme heating together with the moisture gives rise to steam on one side.
This one-sided (non-uniform) steaming is all too common and is known to cause bending in the wooden shakes of roofs due to uneven drying.
This bending or curving is undesirable as it leads to cracking and ultimately splitting of the shakes from the roofs.
After the wooden shakes crack and split, the sheathing is likely to absorb even more moisture which causes the underlying wooden boards to degrade.
If the damage is left unchecked, continuous freezing and thawing of water comes into effect and causes further degradation of the wooden members.
These issues can be greatly reduced by letting the airflow through the roof.
Skip sheathing allows the passage of air that facilitates faster drying of the roof members.
If the sheathing dries out quickly and evenly, wooden shakes can last longer.
Advantages of Skip Sheathing
Skip sheathing aids in reducing moisture effects on wooden roof structures by enabling airflow.
It also reduces sheathing material cost as the spacing provided means less material will be required to form the sheathing layer.
The shakes and shingles remain attached to skip sheathing for long periods of time and do not have to be replaced often.
Limitations of Skip Sheathing
Skip sheathing is very effective when provided on wooden roofs supporting wooden shakes and shingles.
However, the spacing cannot be supported by asphalt composite shingles as it does not form a solid deck.
Skip sheathing is also not effective in regions with heavy snowfall and windy rains.
Furthermore, re-sheeting over degraded skip sheathing using small pieces of lumber is not enough.
Nailing small new pieces between old skip sheathing leads to cracking.
Thus, full-length solid sheets must be applied on top of the old skip sheathing which can be very expensive.
Spacing in Skip Sheathing
For wooden shakes and shingles, 1x4 boards are used in skip sheathing with 10” and 5” spacing respectively.
For concrete tiles, 1x4 boards are not enough to carry the weight so 1x6, 1x8, or 2x4 boards are used with 12” to 14” spacing.
Metal plates are lighter and usually, 1x6 boards with 24” spacing provide sufficient strength to support them.
Why Should Skip Sheathing be Re-Sheeted?
As the skip sheathing gets older, it must be re-sheeted.
Every time a new sheet of wood is applied to the previous skip sheathing, more holes are introduced in it because of nailing.
Thus, after some time, the old skip-sheathing will not be able to act as a good nailing surface for new sheets.
It has been observed that using new wooden blocks on top of existing skip sheathing leads to several problems.
When new wood shakes and shingles are applied on top of older wood, cracks are likely to develop after some time.
Consequently, the shingles applied on such frames have weak wind resistance and can fall off.
Therefore, new solid wooden sheets should be applied on top of the skip sheathing after some time to reinforce it.
How to Install New Sheathing Over Old Spaced Sheathing?
Installing a new sheet over skip sheathing is quite common.
It is done when a solid surface is required over the roof, or as a design choice when shingles and shakes have to be installed.
Here's how to install a new sheet over the old skip/spaced sheathing.
Select the material for your new sheet. Generally, plywood or OSB sheets are applied over old sheathing.
Check with your city's building department and building codes to see the required size of sheet panels.
Cut the plywood or OSB panels accordingly and start installing them from any corner of the roof.
The size of the panels should be such that they fit center to center between two rafters. This helps in nailing them properly.
You will notice that at some intervals there is no surface available for nailing.
In such instances, install filler wood between the skip sheathing to act as a nailing surface.
Keep repeating the process until the whole roof has been applied with a new sheet of wood.
Types of Skip Sheathing
There are a number of materials that can be used in roof skip sheathings, such as OSB, plywood, and wood boards.
The materials used in sheathing are specially certified. Each material must be exterior graded to ensure its long life.
This certification indicates that the material has been treated with chemicals and is water resistant.
This is important because sheathing is constantly exposed to harsh exterior weather conditions like precipitation and snow.
Steel and concrete are also used in roof sheathing; however, they are not preferred for skip sheathing.
The materials that can be used in skip sheathing are as follows.
Wood Board Sheathing
The original material used in roof sheathing back in the 20th century was the local wood.
Planks of wood were cut and installed to form a base over the roofs of houses. Later on, shingles and shakes were installed on top of the base.
Spaces were left between the panels to allow airflow for faster drying of the wood.
Due to progressive rotting, the shingles and shakes would fall off and they would have to be replaced with new ones.
It was due to this problem that stronger roof sheathing materials such as plywood and OSB were later introduced.
Oriented Strand Board (OSB) Sheathing
OSB is a synthetically manufactured material made from different wood components.
Special adhesives are used to make bonds between the layers of an OSB board.
OSB is a relatively new engineered material that provides strength and uniformity at a cheaper rate than its alternatives.
It has gained a lot of popularity over the last few decades due to its durability and cost-effectiveness.
Roof sheathing has been in practice since 1970.
By 2007 around 50% of residential construction had incorporated OSB as the primary roof sheathing material.
Plywood is another common sheathing material.
Like OSB it is made of numerous wooden layers held together by special adhesives.
This layering helps in making plywood a thick surface for nailing purposes and helps avoid the warping and shrinking of the boards.
The high density of plywood provides strong resistance against moisture content, keeping it away from the central layers.
In this way, plywood retains its durability for a long period of time.
Plywood is similar to OSB in providing strength and durability however it has an additional benefit in terms of moisture resistance.
Skip Sheathing vs Modern Roof Sheathing
The spacing in skip sheathing allows for wood shakes to breathe air which helps them in drying.
However, skip sheathing is not very effective for the drying process in regions where there is snow and rain accompanied by winds
In such areas, uniform roof sheathing or decking is provided in the roofs, made from synthetic materials and roof underlayment to counter this problem.
Air can pass through these materials without the need to provide spacing between their panels.
Thus, sheathing done with modern materials enables more airflow making it more efficient as compared to wooden skip sheathing.
What is the Impact of Skip Sheathing on the Lifespan of a Roof?
Years can be added to the lifespan of wooden roofs by using skip sheathing.
The ventilation provided by the spacing reduces the effects of moisture on the wooden members of the roof.
The sheathing panels, wood rafters, shake, and shingles all degrade at a slower rate due to efficient drying.
The sheathing panels retain their strength for a long time and keep on acting as a sufficient nailing surface for shakes and shingles.
As a result, the shakes and shingles do not fall off easily.
If proper maintenance is provided to a skip-sheathed roofing system then it can function properly for decades.
Skip sheathing is a process applied to roofs to enable airflow that leads to faster drying of wooden members.
It is done by fixing wooden boards made of OSB or plywood over rafters, having spaces in between.
This spacing allows for air to flow through and dry out the moisture in the roof components in an even manner.
It is an efficient way of ensuring the longevity of wooden panels, shakes, and shingles.
Note that skip sheathing is only effective in wooden roofing. It cannot be incorporated into concrete and steel roofs.
In regions with heavy snowfall and windy rains, skip sheathing is not very effective in drying out moisture.
The roof sheathing in such regions is generally made of modern synthetic materials, through which air can easily pass through and no spacing is required.
Frequently Asked Questions (FAQ)
Are Roof decking and Skip Sheathing the same?
No roof decking and skip sheathing are not the same things.
Roof decking refers to the application of continuous wood panels to make a uniform deck over the rafters of a roof.
Whereas in skip sheathing the wooden panels are not installed continuously, rather they have spaces in between them.
What Type of Sheathing Is Best for a Roof?
Oriented strand board (OSB) is the most durable material being used in sheathing roofs.
It is also used in making sheets which are used in the re-sheeting skip sheathing.
What Material Can be Used for Re-Sheeting Skip Sheathing?
Oriented strand boards and plywood are the common materials used for making solid sheathing sheets.
The sheets made from plywood should be at least 1/2” thick. While the ones made from OSB should have a minimum thickness of 7/16”.
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