Roof decking and sheathing: what it does and when to replace it

What is roof decking (or sheathing)?

While some roofers prefer one term over another, roof decking and sheathing are the same thing. Both terms refer to the flat wood boards that form the base of your entire roof. Roof decking (or sheathing) is secured across the trusses and joists of your home. When a new roof is installed, roofers will nail or staple your underlayment and shingles directly to the decking (or sheathing).

Material options

Originally, before the mass availability of plywood, roof decking was made of planks (namely 1×6 or 1×8 boards, laid across the full length of the roof to cover it completely). Today, plank decking isn’t used as much anymore. However some older homes may still have original plank decking and some homeowners may install it as a deliberate choice. That said, plank decking is very expensive and can be quite susceptible to leaks between boards.

Because of this, homeowners choose either OSB (oriented strand board) or plywood when building a new home or replacing a roof.

Here’s a look at each type of material:

• OSB (oriented strand board) – While OSB is made from heat-resistant and waterproof materials, it often does not last as long as CDX plywood. That said, OSB is (typically) cheaper than plywood even though it appears very similar. If you decide to build or replace your roof decking with OSB, you will likely need to replace it again in a few decades, regardless of roof installation quality.
• Plywood – Plywood is made up of thin layers of composite wood glued together at right angles to prevent warping. Because this design has a lower risk of wood split and warping from nailing, heat, and moisture, CDX plywood is a superior choice for roof decking or sheathing. If a roof is installed properly, CDX plywood can often last up to 100 years before needing to be replaced.

How to tell if your roof decking (or sheathing) needs replacement

Several things can damage your roof decking, including water, hail, heat damage, fungus, termites, and even falling trees. Unfortunately, not every type of damage can be spotted from the ground. You may require the help of a professional roofer to diagnose the true problem. That said, here are some ways you can tell if your roof decking or sheathing needs to be replaced.

Get a roof inspection

The quickest way to tell whether or not your roof decking needs to be replaced is to inspect your roof. As a homeowner, you may not be able to safely climb onto your roof or properly inspect it for damage without a professional eye. But you will likely be able to inspect your ceiling and attic.

Look out for leaks or signs of deterioration. If your attic is unfinished, you should be able to look at the underside of the roof decking directly for signs of damage or rot. If you are able to climb a ladder safely, you can also survey your roof from the edge to look for sagging or other concerning signs of damage.

In any case, it’s always best to have your roof inspected by a trusted roofing expert. These professionals have an eye for roof damage and can recommend the best course of action to bring your roof back up to code.

Signs of roof decking/sheathing damage

Roof damage is usually evident. However, some forms of damage are harder to spot than others. In some cases, you may not notice them until your roofing materials are completely removed during the replacement process.

Here are some signs to check for:

• Musty smell – A musty smell inside your attic may indicate that the roof decking/sheathing is rotting.
• Dark spots – Dark spots on the ceiling may indicate active leaks or fungus growth. Dark spots on the roof decking itself may indicate rot or water damage.
• Fungus – Over time, water intrusion can cause mold to grow into your decking and deteriorate it from the inside. It may spread into the walls if the leak or moisture problem is not quickly repaired.
• Leaks – If your roof or ceiling is leaking water, this is something to take care of right away. This likely means there is a hole in your roof or part of your roof decking is compromised and needs to be replaced.
• Holes – If there are any holes in your plywood decking where water or light can get through, the entire section will need to be replaced (if not the entire roof decking).
• Roof sagging – If you view your roof from the top of a ladder, you may notice a large dip in the middle of your shingles. This is a sign that your roof decking is compromised and may indicate structural damage in other areas. While this isn’t a sure sign that your roof is about to collapse, that may not be far off in future.
• Spongy texture – A well-installed roof should feel sturdy. But when a roofer walks or presses on your roof, they may notice it flexing slightly under that pressure. This may be a sign of deterioration, wood rot, or waterlogged decking.
• Thin wood – After the shingles are removed, your roofer may notice that the wood material is extremely thin. This means it has deteriorated beyond use and needs to be replaced with new wood.

Don’t delay roof replacement

If your roof decking or sheathing isn’t deteriorated or rotted in any way, you may not technically need to replace it just yet. If your roof is installed properly in the first place, the roof decking should last nearly 100 years. However, if your roof wasn’t installed or maintained correctly, rain, wind, heat, snow, and other types of weather may shorten its lifespan.

Roofing materials play a huge role in protecting your roof decking. For example, synthetic shingles are rated to last between 30 and 50 years on average. If you have these installed with new decking, it will likely protect it for just as long (if not longer). On the other hand, asphalt shingles (while more affordable) are only rated to last about 20 – 30 years and may offer less protection for your roof decking in terms of longevity.

If for some reason your roof decking is only projected to last another decade, you may want to proactively replace it. In many cases, waiting too long to replace your roof may cause major issues and expensive repairs down the line. If your roof system isn’t up to code, your home may become vulnerable to leaks and water damage.

With that in mind, it may be a good idea to replace your roof decking somewhat early to make sure the rest of your home is protected from the elements. Doing this just before you decide to sell your home may also add tangible resale value to your property.

The roof decking/sheathing replacement process

If your roof decking or sheathing needs replaced due to extensive damage or rot, you will need to replace the entire roof. That means adding new shingles and underlayment along with new decking/sheathing. This may cause the project to take longer than the initial estimate.

Here’s how our replacement process at Baltic looks when roof decking is included in the project:

• Inspection and consultation – Before we tear off or nail down anything on your roof, we take time to evaluate the roof’s current condition with a detailed roof inspection. If we don’t believe the decking needs to be replaced, we will explain why. We will then discuss your goals and develop a plan to repair or replace your roof to your liking, as well as extra services like gutter or skylight installation.
• Roof removal – Once the scope of the project is approved, we will then remove the old materials for disposal (starting with the shingles). We will then tear off the underlayment and remove the decking/sheathing. If your decking doesn’t need to be replaced, we will still clean it and nail down any protruding nails before we install the new roofing materials.
• New roof installation – We then install a new roof with all new materials (based on your preferences), starting with new decking/sheathing. Following that, we install the underlayment, flashing, and shingles according to local building codes.
• Clean-up and final walkthrough– After the job is complete, we will clean up our work area and comb the property with a powerful magnet for stray nails, staples, and metal pieces. Our goal is to leave your property looking exactly the same as when we found it. Then, our Quality Control officer will do a final walkthrough to make sure you’re happy with both the workmanship of the finished product and the condition of your property.

How to protect your decking or sheathing for the future

To make sure you get the most life out of your new roof, it’s important to make sure your decking or sheathing is properly protected. While it’s true that roof decking naturally decays over time, there are ways to preserve the materials so that they last longer in general.

Here are a few best practices to help extend the life of your roof decking/sheathing:

Regular roof maintenance and inspections

Roof maintenance is an incredibly important part of taking care of your roof. That’s why some professional roofing companies offer roof maintenance programs. In exchange for a monthly or yearly fee, the roofing company will regularly inspect your roof and repair any damaged areas such as cracked singles, broken flashing, or torn sealant. It’s important to clean off any debris that could cause water drainage or clog your gutters. When possible, removing snow from your roof can also help to prevent ice dams.

Keep your gutters clean

Keeping gutters clean is essential to ensuring proper water runoff. To protect your roof decking from water damage, you want to be sure that water has a clear, controlled path away from your home. If something gets lodged in the gutters, it may cause water to run back onto your roof, leak underneath damaged shingles, or run straight off the edge of the roof to eventually erode your foundation.

Install ice and water shields

In Illinois, ice and water shield membrane is mandatory with any roof replacement project. It’s a great way to protect your home from harsh winter weather no matter its age.

Typically, these large pieces of specialized underlayment are installed in the same places you might find roof flashing. You may use ice and water shields to trim eaves and rake edges, line roof valleys, or seal gaps around skylights, chimneys, and roof vents.

Especially in the Midwest, ice and water shield is an essential part of a roofing system. It can help keep your roof decking safe from ice dams, rain, and snow that may melt through damaged shingles.

Install proper flashing

Roof flashing helps to ensure proper water runoff in multiple areas on your roof. Most of the time, metal flashing is installed near roof joints near sidewalls, in roof valleys, and around other protrusions on your roof. Without flashing, water could run between roof joints and gaps between pieces of roof decking.

Drip edge is a specific type of flashing that protects your roof edges, as well as prevents water from running back against flashing or underneath your shingles onto your roof decking. It is typically installed at the eaves or rake edges of your roof to direct water into the gutters or away from the foundation. Without proper drip edge flashing, water may quickly ruin your roof decking or other key structural elements of your home.