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    Home»Home Improvement»Roof Decking: What It Is, Types, and When to Replace It

    Roof Decking: What It Is, Types, and When to Replace It

    Laura WalkerBy Laura WalkerOctober 6, 202507 Mins Read
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    OSB roof decking installation on residential home rafters during new roofing construction project
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    Your roof’s not just those shingles catching everyone’s eye from the street. Underneath that outer layer sits something way more critical—roof decking. Think of it as your roof’s skeleton, the foundation holding everything together when storms roll through or snow piles up.

    Most homeowners don’t think about decking until something goes wrong. But here’s the thing: understanding what’s under your shingles can save you thousands down the road. Whether you’re planning a roof replacement or just trying to figure out why there’s a weird sag happening, knowing the basics of roof decking is your cheat code to smarter decisions.

    Let’s break down what roof decking actually is, the types you’ll run into, and when it’s time to replace it before things get messy.

    What Is Roof Decking?

    Roof decking—sometimes called roof sheathing—is the wooden base layer installed over your roof’s framing. It’s what contractors nail your shingles, underlayment, and other roofing materials onto. Without solid decking, your roof’s just a fancy hat with no head to sit on.

    This layer does more than hold nails. It distributes weight evenly across your roof, keeps everything stable during high winds, and acts as a barrier against moisture sneaking into your attic. Most decking is made from plywood or OSB (oriented strand board), both engineered to handle serious loads without bending or breaking.

    Think of decking as the unsung hero. You don’t see it, but it’s doing all the heavy lifting while your shingles get the credit. When decking fails, everything above it starts to go south fast—leaks, sagging, even structural damage that’ll make your wallet cry.

    Types of Roof Decking

    Not all decking is created equal. Depending on your home’s age, location, and budget, you’ll run into a few different types. Here’s the breakdown on what’s out there and what actually matters.

    Plywood Roof Decking

    Plywood’s the OG choice. It’s made from thin layers of wood veneer glued together in alternating directions, creating a strong, stable sheet. Most contractors use CDX plywood for roofing—it’s moisture-resistant and comes in thicknesses ranging from 3/8″ to 3/4″, depending on your rafter spacing.

    Plywood is durable, reliable, and handles weight like a champ. It’s also easier to cut and shape for complex roof designs. The downside? It costs more than OSB, and prices fluctuate with lumber markets. Still, if you want something proven and long-lasting, plywood’s a solid bet.

    OSB (Oriented Strand Board)

    OSB’s the new kid that took over. It’s made from compressed wood chips and strips bonded together, forming a uniform 7/16″ sheet. Contractors love it because it’s cheaper than plywood and performs just as well in most situations.

    OSB’s uniform composition means fewer weak spots. It’s also more consistent in quality, so you’re less likely to deal with knots or imperfections. The catch? It absorbs moisture faster than plywood, which can be a problem in humid climates. But when installed correctly with proper underlayment, OSB holds up just fine for decades.

    Plank Decking

    Plank decking’s old-school—think 1×6 or 1×8 rectangular boards laid side by side. You’ll mostly find this in homes built before plywood became standard. It looks like a backyard deck or hardwood floor, except it’s on your roof.

    Here’s the issue: plank decking leaves gaps between boards, which reduces stability and creates potential weak points. Building codes now require gaps no larger than 1/8″ between planks. If your home has plank decking with bigger gaps, you’ll need to replace it or cover it with sheet decking during a roof replacement.

    Metal Decking

    Metal decking is rare in residential roofing but common in commercial buildings. It’s made from steel or aluminum and offers superior durability, fire resistance, and strength. If you’ve got a flat or low-slope roof, metal decking might be your go-to option.

    The benefits are obvious—metal doesn’t rot, warp, or attract pests. But it’s pricier and requires specialized installation. For most homeowners, wood-based decking makes more sense unless you’re dealing with strict fire codes or commercial construction.

    When Should Roof Decking Be Replaced?

    Here’s the reality: you won’t know the full condition of your decking until your old roof comes off. Contractors can’t see rotting boards or structural damage through shingles. That’s why most roofing estimates include a range for potential decking replacement—they’re covering their bases.

    If your decking’s compromised, it has to be replaced before new materials go on. Building code R905.2.1 from the International Residential Code states that asphalt shingles must be fastened to solidly sheathed decks. Translation: if your decking’s rotten or weak, it’s getting replaced whether you planned for it or not.

    Usually, you’re looking at a few boards or sheets that need swapping out. But if there’s been long-term water damage or poor ventilation, you might be replacing more than expected. That’s where having a solid roofing contractor matters—they’ll catch issues early and keep you in the loop.

    Signs Your Roof Decking Needs Attention

    You don’t need to climb on your roof to spot decking problems. A few key warning signs can clue you in before things spiral. If your roof’s sagging in certain spots, that’s decking starting to fail under load. Even a slight dip means moisture’s gotten in and weakened the wood.

    Dark spots or discoloration on your ceiling inside? That’s water damage working its way through. Leaks don’t always show up immediately—they’ll rot decking from the inside out before you notice stains. By then, you’re already behind the eight ball.

    Soft spots when walking on the roof are another dead giveaway. If the surface feels spongy or unstable, the decking underneath has absorbed moisture and lost its structural integrity. Mold or mildew growth in your attic also points to decking issues, especially if ventilation’s been ignored for years.

    How Much Does Roof Decking Cost?

    Let’s be real: roof decking costs fluctuate like crazy. Wood prices swing wildly depending on market conditions, supply chain issues, and regional demand. In 2025, prices are still elevated compared to a few years ago, so budgeting for decking replacement can feel like guesswork.

    Sheet decking (plywood or OSB) is priced per sheet, while plank decking is priced per linear foot. On average, plank decking costs about three times more than sheet decking after factoring in labor and materials. If you’ve got old plank decking that needs replacing, going over it with OSB or plywood is often cheaper and faster.

    Your contractor won’t give you an exact number until they tear off your old roof and inspect what’s underneath. Most estimates include a buffer for decking replacement based on your home’s age and roof condition. Ask upfront what their per-sheet or per-board pricing looks like so there’s no sticker shock later.

    Final Thoughts

    Roof decking’s one of those things you don’t think about until it’s too late. But now you know—it’s the foundation holding your entire roof together, and when it fails, everything else follows. Whether you’re dealing with plywood, OSB, or old plank decking, keeping it in good shape means fewer headaches and a roof that actually lasts.

    If you’re planning a replacement or noticing warning signs like sagging or leaks, don’t ignore the decking. Get a contractor to check it out before things escalate. A solid roof deck isn’t just about passing inspection—it’s about protecting your home for the long haul.

    Got questions about your roof’s decking or need someone to take a look? That’s what professionals are for—don’t wait until a small problem becomes a big one.

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    Laura Walker

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