Roof Sheathing Thickness: Essential Guide

There’s a new house being built on my street and they’ve just finished up the framing and sheathing. One thing I noticed was that the entire house frame was covered in OSB – even the roof deck. This got me thinking about how thick my roof sheathing thickness should be on the shed I built.

The recommended thickness for roof sheathing is ⅝” thick. Sheathing with a thickness of less than that is much more likely to result in bowing between framing which could potentially compromise your exterior roof material.

The choice of thicknesses actually varies widely depending on what store you shop at and where you are in North America. When it comes to sheathing your roof, however, it never hurts to go thicker. A thicker panel means less sagging and a stronger roof deck.

In this article, we’ll go over some tips for choosing the right sheathing thickness for your shed or house, including the types of substrate, techniques for installation, and what works best in different geographic areas.

What is Roof Sheathing?

Roof sheathing is the solid surface below your exterior material, whether it’s shingles, metal, or another type of covering. It is most often OSB – a type of particle board – or plywood. In the past, roof substrate was simply dimensional lumber but today’s options are cheaper and easier to install.

The function of sheathing is to provide a smooth, flat, and solid surface for the roofing material to attach to with nails or screws. It also serves as a secondary barrier between your house and the roofing material in keeping water out and heat in, to some extent.

Sheathing is not meant to be a moisture barrier or vapor barrier primarily. You put underlayment on top of the roof deck to act as a moisture barrier, and your primary vapor barrier will bein your attic, not beneath your shingles or other outer layer.

The thickness of your roof sheathing is important. Thicker panels will result in a more solid roof deck and resist bowing between the rafters or trusses. Less bowing means a more secure surface for roofing material, improving the longevity of your roof.

What is the Standard Roof Sheathing Thickness?

Standard roof sheathing thickness is ½” to ⅝” thick. If you are using OSB for your roof deck, then ⅝” is the recommended thickness. For plywood, which is slightly stronger than strand board, ½” thick is appropriate.

Another factor is how far apart your trusses or rafters are spaced. 24” and 16” are the most common spacing, and you’ll often find 16” standard for rafters and 24” standard for trusses. A thicker panel may be required for 24” compared to 16”, so if you have larger gaps between the framing, consider a thicker substrate.

It is important to note that there is no “standard” sheathing thickness, at least officially. When you purchase roof decking, it will have an “APA” stamp with two numbers beneath, divided by a slash. The first number tells you the maximum distance that the panel can bridge between gaps.

Therefore if you pick up some ⅜” sheathing that is 12/0, that means the max gap it can bridge between rafters or trusses is 12”. If you get ⅜” panels that say 24/0, it can span up to 24” between framing members. Therefore, there are many options available.

What is the Minimum Thickness of Roof Sheathing?

Minimum sheathing requirements dictate that ⅜” thick is the lowest you can go in terms of thickness. That minimum number refers to either OSB or plywood.

Even you have a ⅜” thick piece of strand board and the same for plywood, they may likely be rated for different spans. Thus not all sheathing was created equal. There are even different types of plywood of the same thickness that will be rated for different spans, so it is critical to pay attention to the labels on the actual panel.

And if you are looking for a deal, it may pay to hesitate before you go cheap and purchase ⅜” panels. Remember that ⅜ is very thin, and any load placed atop ⅜” panels will likely, over time, make the strand board or ply panels sag even if placed at the rafter/truss spans as designated by the panel label.

Even in your neighborhood you’ll notice homes with roofing that sags. Often you can see the outline of the rafters or trusses beneath the shingles. Will thicker sheathing help mitigate that problem? Probably, but the key is not to sheath using the minimum thickness.

Roof Sheathing Code Requirements

The minimum requirement for roof sheathing is ⅜” thick. There is no upper limit on how thick your roof substrate is. Thickness allowed depends on how far apart rafters or trusses are spaced and the rating of your roof decking. For instance, there are ⅜” panels rated for 24” rafter spacing.

Another consideration is if your rafter or truss spacing allows you to place your sheathing – usually 4’x8’ panels – so that each edge of the panel is supported. If that is the case, you’ll find that your framing can support thinner panels than if an edge does not sit directly above a framing member.

Of course, nearly all frames are designed to fit roof sheathing edges, so when you install your panels you’ll be able to screw or nail your panel edges into the framing. However, if you are retrofitting an old house or shed, you may encounter unusual spacings which will indicate your roof decking may need to be thicker than the spacing and label indicate.

Finally, code requirements do not differentiate between types of roof decks. Why? Because the APA – the Engineered Wood Association – creates labels for every piece of lumber they certify, OSB or plywood. The label clearly states maximum spans and loads allowed for that piece of roof substrate.

This makes it simpler for the consumer to pick out the right piece of product under their finished roof without having to worry about telling the difference between types of wood products.

Types of Roof Sheathing

There are many different types of roof sheathing, the main ones are:

• OSB (oriented strand board) – uses adhesives compressed with layers of wood chips that alternate direction between each layer
• Plywood – compresses large, ultra-thin sheets of wood into a sandwich using adhesives and heat to form one board
• Dimensional Lumber – an older way to sheath a house, in which tongue and groove lumber was frequently used or 1” lumber butting up against one another

Today the most common types of sheathing are plywood or strand board. Plywood is more expensive but slightly stronger. Dimensional lumber is not used because it would be much more expensive and provides a lesser surface for nails or screws due to gaps between all the boards.

Many builders who are redoing a roof that has original dimensional lumber as a roof deck will put plywood or OSB over the top before finishing the roof with new roofing material as it is simply a more reliable substrate than many pieces of lumber on end.

What’s Better for Roofing: OSB or Plywood?

OSB is the better option for roof decking because it is cheaper and comes in a wider variety of larger sizes than plywood. Strand board does not react as well to moisture as plywood does, but it will have a more consistent thickness and fewer weak spots – like knots – than plywood.

If you are concerned about moisture getting into your sheathing, then plywood may be a better option. Plywood dries much faster than OSB because it is not as dense. There are usually only several layers of plywood per sheet while there are up to 50 layers of wood chips in a sheet of strand board, plus much more adhesive than a sheet of ply.

Since there are so many layers in OSB it is somewhat heavier than plywood. It takes water longer to penetrate it, but once it gets into the sheet then it takes a much longer time to dry. A board that stays wet longer is prone to sagging, which is why strand board tends to sag over time on many homes.

Finally, if you are going to sheath a shed that isn’t heated, then OSB would definitely be a better option, provided you install the roofing material properly. There won’t be any condensation issues, so your roof deck is liable to stay dry. In that case, strand board is best instead of plywood.

What Size of Roofing Nails for Roof Sheathing?

When installing sheathing, use 8d roofing nails. An 8d nail is 2.5” long and is more than enough to adequately fasten most thicknesses of roof substrate.

If you opt to use screws, then #10 wood screws would work. Make sure they are 1.5” long, which would work for ½” thick panels. If your deck is thicker, then you’d want #12 wood screws at 2” long.

Recommended Thickness of Shed Roof Sheathing

The recommended thickness for roof decking on a shed is ⅝”. At that thickness, you can economically sheath your shed while still getting an extremely solid roof substrate that will stand the test of time.

OSB board is preferable to plywood for sheathing a shed. Strand board that is tongue and groove makes for a very straightforward install on your shed. If your rafters or shed trusses are widely spaced, or uneven (if it’s an old shed) then the tongue and groove give the edges additional strength, particularly if they aren’t supported by framing at every edge.

The strand board, besides being cheaper, also works extremely well with tar paper. The rough surface of OSB as compared to the smoother surface of plywood allows the tar paper to hold better and makes installation much faster.

And while plywood may dry faster than strand board, it’s important to note that all strand board roof decking you buy these days has blue edges. That means the edges have been waterproofed, which makes it much less likely water will penetrate the board. The adhesives that bond the strand board are also waterproof.

You could always opt for thicker roof deck panels, but for a shed there is no need to go thicker if your framing is of standard width. Once you start buying panels that are greater than ⅝” thick, you’ll notice the cost rise significantly.

Thickness for Roofs in Cold Climates

When it comes to choosing a thickness for the solid decking to put atop your framing on your shed or home, one of the primary considerations should be the weather.

Snow load should factor into your decision in terms of how thick your strand board or plywood will be beneath your exterior shingles or metal roof.

As mentioned above, all types of roof sheathing comes with a label that indicates spacing allowed between framing members. Beneath those numbers are the maximum allowable live and dead loads that panel can support.

For areas with extreme snow, look for panels that have total loads of 50 psf – pounds per square foot. While that might not sound like much weight, just know that you’ll multiply that number over the number of square feet on your roof.

Therefore if you have a total of 2000 square feet, then the total load spread out over your roof is multiplied times 50, which is 100,000 pounds. That is more than enough strength to keep the snow from falling into your structure.

Metal Roof Sheathing Thickness

Roof decking that will have metal for its outer roof covering is no different than sheathing a roof that will have asphalt shingles. Of course, some shed owners, or homeowners, may opt to use battens in conjunction with their metal roof, but battens don’t offer nearly the amount of protection that strand board or ply does.

Thus, for sheathing a roof with metal panels, you’ll want to stick to ⅝” panels. Overall metal roofs use less nails than asphalt shingle roofs, so you have fewer fasteners holding everything together. Therefore you’ll want to ensure you have the most solid substrate possible for that metal roof to screw into.

If you are considering battens in place of a solid substrate, just know that strand board or ply acts as a noise barrier as well as a secondary moisture and temperature barrier. You lose all of that if you opt for battens.

Conclusion

One of the difficult parts about figuring out roof coverings for your shed or house are all the options. Roof sheathing is one area where it pays to spend a little more money. Sagging roofs are very common – and visible – and are easily rectified at the time of install.

Sheathing a shed roof is no different. Save money by using a strand board, then use that savings to up the thickness a little bit. You’ll find that the performance of the OSB will be more than adequate for your shed’s needs.

I wish you luck on your roof project. Remember to always use your fall protection when working on the roof. Handling large panels on a windy day while on a roof sometimes doesn’t end well.