Roof replacement
Roof decking is the wood foundation your whole roof sits on. Learn what it is, the different types, the warning signs of rotting, and what causes it.
What is roof decking?
Roof decking, also called sheathing, is the layer of wood that forms the foundation of your roof. Installed over your roof's framing, decking supports the shingles and every other roofing material above. Because the whole roof rests on this layer, healthy decking is essential to a sound, long-lasting roof.
Key takeaways
- Roof decking, or sheathing, is the wooden foundation that your roof is nailed to.
- OSB, plywood, and plank sheathing are the common decking materials.
- Because decking is wood, water and moisture can cause damage to it.
- Roofing over an old roof neglects damaged decking, leading it to worsen.
- Ceiling stains, a sagging roofline, and musty smells can signal rotted decking.
A roof is made up of several components working together as one complete system, all supported by the roof decking beneath them. While decking often goes unnoticed, it plays a critical role in the strength, stability, and overall performance of your roof. Since 1991, On Tops Roofing has been committed to educating Raleigh homeowners about how their roofing systems are built, helping them understand what goes into their roof and exactly what they’re paying for.
- What roof decking is
- Roof decking materials
- Why you should not roof over old decking
- What causes decking to rot
- Signs of rotted decking
- FAQ
What Roof Decking Is
Roof decking, also called sheathing, is the foundation of your roof that ties everything to your home's frame. Laid over the structural framing, decking supports the weight of the shingles and every other roofing material on top. Since the entire roof sits on this layer, its condition sets the tone for the whole system, and when decking is failing, the rest of the roof is not expected to hold up either.
Roof Decking Materials
Decking is usually made from one of three materials, and some suit certain roofs better than others.
- Oriented strand board (OSB) is the go-to decking today, since it is durable, easy to find, and affordable. OSB is made by compressing layers of wood strands with adhesive, with the strands overlapped for added strength.
- Plywood is similar to OSB, but is a bit pricier and considered to be stronger due to its design of glued layers of wooden veneer sheets.
- Plank sheathing is less common and is wood planks that are spaced about an inch and a half apart.
Why You Should Not Roof Over Old Decking
Hiding under the shingles, decking is hard to judge until the old roof comes off, which is a big reason to avoid a roof-over, meaning new shingles laid over the old ones. If the decking underneath is already damaged or starting to rot, covering it up only lets the problem grow and weakens the whole roof over time. When decking does need attention, a roofer usually replaces only the damaged sections rather than the entire deck. For a full look at when replacement makes sense, see our post on whether you need to replace your roof decking.
What Causes Roof Decking to Rot?
Since decking is made from wood and wood composites, water is the threat to watch. Moisture can reach the decking in plenty of ways, including worn or old shingles, clogged gutters, poorly installed flashing, ice dams, and a buildup of heat and humidity from poor attic ventilation. Any of these can let water into the system and lead to rot over time.
Signs of Rotted Decking
A few clues point to decking trouble, and they usually trace back to a roof leak.
- Wet spots or stains on ceilings and interior walls
- Water stains on the rafters or attic insulation
- A musty, moldy smell inside the home
- Daylight showing through the roof when you are in the attic
- A sagging or wavy roofline, which can mean the decking has warped from moisture
If you notice any of these, especially after a storm, the next step is a professional inspection so a roofer can pinpoint the problem and the right fix.
A Solid Roof Starts Underneath
Roof decking may sit out of view, but a lasting roof starts with a solid deck. Keeping water out and catching problems early protects the foundation everything else depends on. On Tops Roofing has helped Raleigh-area homeowners understand and care for their roofs since 1991.
FAQ
What is roof decking?
Roof decking, or sheathing, is the layer of wood that sits on your roof's framing and supports the shingles and other materials. It is the foundation the entire roof is built on.
What is roof decking made of?
Usually oriented strand board (OSB), plywood, or plank sheathing. OSB is the common choice today for its strength and price, plywood is a close second, and plank sheathing appears mostly under wood shakes.
What causes roof decking to rot?
Moisture is almost always the cause, since water from worn shingles, clogged gutters, bad flashing, ice dams, or poor attic ventilation can reach the wood decking and cause it to rot over time.
How do I know if my roof decking is rotted?
Watch for ceiling and wall stains, water marks on rafters, a musty smell, daylight in the attic, or a sagging roofline. These often point to a leak that has reached the decking, so an inspection is the right next step.
Can you put a new roof over old decking?
Roofing over is not a good idea, since a roof-over hides the decking's true condition, and any existing damage will keep spreading underneath. Replacing damaged sections is the better path, which we cover in our post on replacing roof decking.
