Roof Sheathing | A Comprehensive Guide to Empower Homeowners

Damaged roof sheathing found on roof edge

For every roof, there is roof sheathing (also known as roof decking – we’ll use them interchangeably).

This is the very bottom layer of your roofing system.

Roofing contractors should replace every rotted or damaged board when they replace a roof.

Unfortunately, many do not.

And they won’t tell you that either.

So. We’ve created a guide to teach you all you need to know about roof sheathing.

We’ll cover:

After reading this guide, you’ll be prepped and ready to pick the best contractor for the project. One that doesn’t sweep sheathing under the rug (or under the shingles, in this case).

Let’s start with the basics.

What is roof sheathing?

Roof sheathing are the large rectangular boards that lay across the rafters of your house.

The boards secure to your home’s rafters, beams, and joists.

Sheathing creates a base for your roofing system. Just like your house has a foundation, your roof has the decking.

What are the different types of roof sheathing?

Most residential homes have wood sheathing materials. Some buildings have concrete or steel decking, but they’re usually commercial buildings. Today, we’ll focus on wood decking.

To understand the types of decking we use today, let’s start with a short history lesson.

The History of Roof Decking

In the late 1800s, before the invention of asphalt roofing, most residential homes had wood shake roofs. These structures had “skip” or spaced sheathing.

The decking consisted of long narrow boards with spacing between each board. The spacing created airflow so the wood shingles could dry out after each rain.

Contractors still use this method today for shake, tile, and certain metal roofing systems.

In the 1900s, once the asphalt shingle came on the scene, contractors started to install long 1×6 boards side-by-side with no spacing. This is called plank or board decking.

Although it’s no longer common practice, we come across this style in North Carolina for homes built in the 70s and 80s.

After World War II, new building fire codes led to a shift from these 1×6 boards to large rectangular sheets of fire-treated plywood. This method is called sheet decking and is the standard practice today.

Now most contractors opt for OSB sheathing (oriented strand board) instead of plywood due to its affordability.

For roofs with exposed decking (think barnhouse wedding venues), contractors install long pieces of hardwood tongue-and-groove (T&G) boards for a decorative look.

In addition to these different types of decking applications and materials, there are also different thicknesses of sheathing including ⅜ inch, ½ inch, and ¾ inch.

The heavier the roof material, the thicker and sturdier you want the sheathing.

⅜ inch is the minimum thickness allowed for roof decking. For asphalt roofs, we recommend ½ inch boards.

Infographic Types of Roof Decking roof Sheathing

What roof sheathing is best for my roof?

Now that we know a bit about sheathing’s past, let’s figure out what material is best for your roof. It’s simpler than you’d think.

The best material for your roof depends on the current material of your roof.

During a roof replacement, your contractor should utilize decking with the same thickness as your current decking. This guarantees all materials will lay flat across the roof.

However, the new decking can be a different material. For example, you can replace damaged plywood boards with OSB boards to save money.

So then which is better: plywood or OSB?

We get this question often. As with most materials, there are pros and cons to both types of roof decking.

We could get into the nitty-gritty details, but what matters is that OSB and plywood have the same structural performance, and both are good options for your home.

Roofing manufacturers also provide best practices for roof installations, including utilizing sheathing clips to properly space the boards. This accommodates expansion caused by swelling if the boards absorb any moisture.

These guidelines safeguard against the potential cons of each type of decking. In fact, failing plywood or OSB decking has more to do with the quality of the installation than the material itself.

So as long as your roofing contractor follows best practices, you can have peace of mind your decking will perform well with plywood or OSB.

At Artisan, we can install plywood or OSB depending on the homeowner’s preferences. However, we almost always recommend OSB due to its affordability and consistent production quality.

4 Purposes of roof sheathing

Your roof deck is the foundation of your entire roofing system. Naturally, it serves several functions.

1. Leak barrier

Your roofing system is layered with decking, underlayment, and shingles. Each layer acts as a covering and protection against water intrusion. Roof sheathing is that final barrier of protection for your home.

2. Solid roofing system

In addition to being that last line of defense, your roof decking supports other roofing components so they can perform well.

Whether it be drip edge, flashing, underlayment, or shingles, they all secure to the decking. If the decking is solid and secure, the other parts of your roof will be, too (if properly installed).

3. Secure nailing

This is one of the most important functions of the roof decking.

Every roof shingle nails into the decking.

Every. Single. Shingle.

And your roof has a lot of shingles.

It takes on average 66 shingles to cover 100 square feet of roof. Residential roofs are on average 2,000 square feet.

That makes at least 1,000 individual shingles.

Take it one step further. They each need 4-6 nails. That makes a minimum of 5,000 roofing nails.

5,000 nails.

Now imagine trying to hang twinkle lights on your porch, but all the wooden beams are rotted. You know those twinkle lights won’t stay up there for long.

In the same way, if you nail shingles to rotted decking, your shingles could fly off during a strong windstorm.

You need solid decking so it can act as a secure foothold for the roofing nails.

4. Weight distribution

Finally, decking evenly distributes the weight of your roofing system across the rafters. The decking will also distribute the weight of any foot traffic, fallen tree branches, and snowfall.

Infographic Types of Roof Decking roof Sheathing

Do I need to replace my roof sheathing?

If you want your roof to last until its “best by” date, you’ll want to replace all damaged sheathing during your roof replacement.

The 2018 International Residential Code states that asphalt shingles must have “solidly sheathed decks.” Your contractor should replace any rotted or damaged boards they find during a roof repair or re-roofing project.

However, if the existing sheathing is in good shape, it can stay right where it is.

7 Signs of sheathing damage

Sheathing rot doesn’t happen all at once. It slowly builds and develops over time at insecure water entry points.

Water entry points can include roof boots, flashing, and roof eaves.

A poorly installed roof will naturally expedite sheathing rot. Moreover, a poorly maintained roof can expedite sheathing rot. That’s why we recommend a roof tune-up every 8-12 years for maintenance and leak prevention.

So how do you know if you have damaged sheathing? Here are the top 7 factors that affect sheathing damage.

1. Age of your home & roof

The older your roof, the longer it has had to accumulate water damage.

Additionally, your sheathing will accumulate wear and tear over time. As we talked about earlier, roofs use a lot of roof nails, but your sheathing boards can only take so many nail holes before the boards start to deteriorate.

If your home has had several roof replacements since its construction, chances are it has some old, deteriorating decking.

2. Quality of your last roof installation

If your last roof was installed poorly, there may be multiple areas with water damage down to the roof decking. Additionally, there is a chance the last contractor didn’t replace all the damaged sheathing.

3. Number of roof leaks

The more roof leaks you’ve had, the more opportunities your sheathing has had to absorb water and rot.

4. Roof slope/pitch

Did you know that steeper roofs tend to have less sheathing damage? The steeper the roof, the quicker water will flow off the roof.

In contrast, if you have a low-sloped roof, the water will drain slowly and have more time to seep under the shingles.

5. Roof ventilation

Roof leaks aren’t the only cause of sheathing rot. Attic condensation is a big contributor to wood rot. The key to keeping a low moisture attic space is adequate attic ventilation.

Unfortunately, many new construction builders and roofers install the wrong type of ventilation. Rather than correct it, we see many roofers install like-for-like when they replace roofs, even if it isn’t what’s best for the home.

6. Wood rot in the attic

If you can access your attic space and the sheathing is visible, take some time to visually inspect it and check for any damaged or rotted areas. You may even notice a roof rafter that’s developed wood rot.

Remember to tell your contractor about any wood rot you find in the attic space.

7. The condition of your fascia, soffits, and siding

If other parts of your home exterior have wood rot, chances are your roof has some, too. Since wood rot forms slowly over time, if the exterior has developed wood rot, chances are your roof decking has, too.

How many new sheathing boards will my roof need?

In the end, it is impossible to know just how much sheathing needs replacement until you replace your roof, but these key indicators can help you prepare and budget for the project.

But how many boards will it be?

Infographic how to budget for sheathing board replacement

This is a very rough guideline based solely on the age of your home. We based these numbers on our years of experience completing over 1,000 asphalt shingle roof projects.

Results can vary greatly based on a variety of factors that affect decking, but we hope this guide will help you budget for potential sheathing costs.

Why do some of my roof bids not mention roof sheathing?

Many contractors will opt not to replace key components of your roofing system like flashing and damaged sheathing.

While it could save you some money in the short term, it means that you won’t really have a new roofing system, only a partial one.

Not only that, but it also means your roof will not last as long and could develop leaks within just a few years of the project.

How do we know this?

Because homeowners call us weekly to fix other contractors’ mistakes. We find ourselves repairing or replacing many roofs due to preventable storm damage or early roof failure.

And we often find way more rotted sheathing than is normal for the age of the roof – meaning the last roofer didn’t replace it.

Some roofers like to avoid talking about sheathing. Unexpected costs or issues mean a higher bid and less likely to sign the deal.

But we believe that in the end, a quality roofing system is what matters. Don’t hesitate to ask your roofer about sheathing.

You wouldn’t pay to build a house on a cracked foundation, so don’t empty your wallet to install a roof on rotted decking.

Ask how they determine what needs replacing, how they notify you, and what is included in the estimate. If you have an older home and they say there won’t be any damaged sheathing, they could be skirting the issue.

At Artisan, we replace every rotted or damaged sheathing board to ensure the home has a truly secure roofing system.

Congratulations, you are now a roof sheathing expert! Plenty of contractors skip out on sheathing. Lower prices = more sales.

So when you compare quotes, always make sure you’re comparing apples to apples, not just the bottom line. In the end, regardless of the price, a new roof is not really a new roof without some new sheathing. Am I right?

Do your research, vet your contractors, and have peace of mind for your home.

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