What is roof decking?

Beneath a great roof lies roof decking. Decking is the structural support that lives just beneath your roof's shingles, held up by your home's framing. These days, residential roof decking is usually made of plywood sheathing or oriented strand board (OSB), a type of manufactured wood. That doesn't mean all decking follows one formula, though.

Historically, roof decking came in many forms, from spaced sheathing with boards to asbestos blocks. If you own an older home, your home's roof is a unique part of its construction. If you're getting a new shingle roof and your roofer determines that your home also needs new roof decking, they should replace the deck according to the construction and design of your home.

Here's what you need to know about roof decking.

Types of roof decking

Just as there are many types of roofs, there are many types of roof decking. Three common types used today are wood and wood composite decking, metal decking and concrete decking. Since metal and concrete decking are mostly used in commercial buildings, the majority of homes will use wood or wood composite decking.

Wood and OSB

Plywood and OSB decking are the most common types of decking used today. Plywood comes in wide, thin sheets and gives shingles, slate and shakes solid support while offering insulation and resistance to wind incursion. OSB decking is made similarly to plywood and also provides good structural integrity. Many wood decks used to use planks instead of plywood. These wooden decks are sometimes still used.

Metal decking

Metal decking is used in many industrial settings, both to support metal roofs and also as support for the flat roofs you'll find on lots of shops and commercial buildings. Metal decking is often made of aluminum or steel and can be pitched or flat.

Concrete decking

Many commercial and industrial buildings use reinforced concrete to support their roofs. Concrete is extremely strong, fire- and weather-resistant. 

While wood, metal and concrete are three common decking options, New England Metal Roof , an organization dedicated to roofing information and education, lists a few other options that are also used today, including fiberglass reinforced panels and structurally insulated panels.

Historical roof decking

Because roofs need periodic replacements, most Americans will encounter roof decking systems that were used from around the turn of the 20th century and on. According to Richard S. Koziol and Christopher W. Giffin of the National Council of Architectural Registration Boards (NCARB) , structural clay tile, lightweight concrete, gypsum and masonry were all commonly used in the U.S. around 1900. Many of these supported flat roofs or architectural flourishes like stone parapets, while others, like clay book tiles, supported steep-slope roofs. 

However, the plethora of historical roof decking materials defies easy categorization, and there have always been outliers. Lyle Hogan of the International Institute of Building Enclosure Consultants (IIBEC) and owner of Fincastle Engineering, Inc., Greensboro, N.C., writes about Kaylo, an asbestos-containing block that was produced by Owens Corning, starting in the 1950s. Kaylo's fire resistance and insulation made it an attractive pick before the dangers of asbestos were widely known. It may still be found in ceilings and roof decks, and should be replaced by a trained specialist if it is.

Most commonly, older residential roofs are built on wood decking. That said, board sheathing was often used where plywood decking would be used today. These plank decks feature softwood boards laid side by side across rafters, sometimes with gaps between them. The Roof Repair Guys , a Wilkes-Barre, Pennsylvania-based roofing company, writes that plank decking is still sometimes used for roof restorations on older homes or buildings with historical significance. 

Similarly, spaced sheathing or skip sheathing is sometimes found under cedar shakes, tile or slate roofing, according to Hogan . However, the Cedar Bureau , a cedar industry authority and educational body, notes that solid plywood sheathing is generally recommended for shakes now, especially in areas with heavy snowfall and wind. 

What type of roof decking is best?

These days, plywood decking is the standard for most residential homes. It is often combined with modern barriers and contemporary shingles to build long-lasting roofs. 

While plank decks may still be used in some circumstances, they generally underperform plywood. Plank decking shrinks as it dries, potentially creating gaps and loosening fasteners. Custom Exteriors, a roofer based in Cheyenne, Wyo. , also notes that it's less effective as an insulator. Still, it has some benefits: it can be a cost saver and supply ventilation.

Homeowners won't always get a choice as to the type of decking available. Often, municipalities have specific codes for what is required. The International Building Code (IBC) lays out specific roofing requirements. Your roofer or architect should be aware of these codes and advise you on the available options that are compliant. Regardless, it's a good idea to be aware of your city's codes and permit requirements yourself, before a roofer begins work on your home.

When to replace your roof decking

If a roofer begins work on your home and finds structural damage to your roof's deck, it's no small matter to fix. A new deck can bump up the price of a reroof significantly, but decking isn't something to skimp on. 

If your roof deck has issues, new shingles may only patch up the problem. Instead, it's better to go ahead and fix the structural issues in the deck during the reroof, even if that means a few thousand dollars extra in lumber fees and labor.

How to tell if your roof decking needs replacement

As with a new roof, many homeowners will need to rely on a roof inspection to determine whether they need a new roof deck. However, there are some signs that even the untrained can spot. If you see signs of water encroachment, it's fair to assume that your roof deck may have suffered.

Look for sags in your roof. Older decks can lose their structural integrity over the decades. Plank deck roofs with large gaps between boards can be especially susceptible to sagging, allowing water to pool, and encouraging encroachment.

Catastrophic accidents, like a fallen tree or a hurricane, can also damage a roof's decking. When events like these occur, it's a good idea to check your homeowner's insurance policy to see if you have recourse.

Roof decking and insurance

Deterioration in your roof decking not only impairs the health of your home's roof, but it can also impact your finances. In many parts of the country, homeowner insurance companies are taking every excuse to purge policies . Ensuring your roof and roof decking are up to date can keep your home protected. 

Moss, sagging shingles and other signs of roofing damage can trigger insurance companies to cancel policies, but sometimes obsolete decking types will also cause problems. Custom Exteriors writes that many insurance companies will require older spaced roof decking to be upgraded to plywood decking before a home's roof is insured. 

In some cases, where an older home has obsolete decking but would otherwise qualify for insurance assistance with a roof replacement, an insurance company may mandate that the homeowner pay out of pocket to update and replace the decking, while covering the shingle reroof.

How much does a new roof deck cost?

Roof decking is an expensive part of a reroof. Not every reroof will require new decking, and some reroofs may only require small repairs, so the cost of installing new decking can vary significantly. 

Angi claims that the average cost for roof decking is $6,400, with the low end of decking jobs costing about $1,900 and the high end reaching $17,000 or more. In practice, the material you choose can greatly increase the price. Rapid Roofing, a roofer based in Michigan , writes that redecking a 1,000 square foot roof will generally cost between $2,000 and $5,000.

New England Metal Roof writes that plywood and OSB are generally the most affordable options, while metal decking's excellent longevity makes it one of the costliest.

Who should I call to replace my roof decking?

Replacing roof decking is a regular part of roof replacements. While not every reroof will require a new deck, good roofers will be able to do both. Unless your home has a niche requirement that requires a true specialist, like legacy asbestos decking, you'll generally be able to hire one contractor for both parts of the job. 

If you do have a historic home that requires something niche, like a unique cedar shake replacement , it's a good call to search for a roofer with these specific needs in mind.

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