Plank Roof Decking: Is It Time To Replace Yours?

Often, when we go out to someone’s home to give them an estimate to replace their roof, we inspect the roof decking.  We check for rotten wood, signs of leaks, dry rot, ventilation, and the type of roof decking.  The type of roof decking is an often overlooked component of the roofing system.  It is the foundation on which the roof is built.  Without a proper foundation, the money invested in replacing the roof can be wasted.  In this article, we will be taking a closer look at plank roof decking.

Plank Roof Decking

There are several types of roof decking:  CDX plywood, OSB (oriented strand board ) used in most houses built today due to its cost, and board planks (which come in various sizes).  Roof planks can be 4 inches wide to 12 or 13 inches wide.  Shingle manufacturers, like GAF, CertainTeed, TAMKO, and Owens Corning, have specific guidelines about roofing on roof planks determined by years of research.

Before plywood was available, board planks were used to put on the house for the roof decking.  If they are still on top of the house, they could be 50 to 100 years old.  Not everything lasts forever including wood.  When these planks have been on top of the house for that long, subjected to extreme heat in the attic, freezing temperatures in the winter, and usually no ventilation, they have become very dry.  Then when the roofing materials (shingles, underlayment, etc.) are removed, the roof deck is exposed to the outside.  The effects of osmosis will create a future problem.  The humidity in the air will absorb into the roof planks like water into a sponge.  If you have ever left a board laid out overnight, you have seen the effects of this situation.  The roof plank will expand in the areas where the humidity has absorbed causing the plank to twist, bend, and cup.  It may not be noticeable right away but it will be after the roof is completely installed.  It will create ridges and bumps that can be seen from the ground.  Not only cosmetically is it unappealing, but it will also cause leaks in the future.  This typically only happens when the planks are wider than 6 inches.  If they are 6 inches or less in width, the shingle manufacturers will still warrant their shingles since the more narrow planks are much less prone to twisting and cupping.

These areas that are cupped and raised will lift up on the shingles creating gaps for snow melt and heavy rain to slip under the shingles, allowing the water to find openings in the decking to leak into the house.  

There are a few ways to deal with this problem.  One possibility is to re-deck the house and make the roof decking solid for a long time to come.  Another option would be to layover plywood on top of the planks to create a solid and smooth foundation to install the roof.  With this option the roofers need to be sure to use long enough nails to penetrate both layers of decking.  If the nails do not penetrate, the planks, through expansion and contraction during the daily temperature changes, will push the nails up and cut the shingles above them causing more leaks known as “nail pops”.  A third option, assuming the planks are close enough to each other with less than 1/8-inch gap (required by the shingle manufacturers like GAF, CertainTeed, TAMKO and Owens Corning) is to cut relief cuts lengthwise in the middle of the planks.  The saw cut allows the plank to swell and expand evenly without twisting or cupping.  Then there is still a solid foundation to install the roofing system.

If there are large gaps between the planks, (larger than 1/8-inch) this option is not a viable one.  Just because it may not have been an issue in the past does not mean it will not be an issue in the future.  Shingles in the past had a 5 inch exposure, meaning the area of the shingle that you see on the roof.  The other part of the shingle is covered by the shingle above it which covers the nails that hold the shingle to the roof.  Current shingles produced today have a 5 5/8-inch exposure.  That extra 5/8 inch may not seem like much, but as you install more rows of shingles, the nail line gets higher and higher above the previous roof’s nail line.  At some point that row of nails will hit the gap between the planks and not be into any wood.  The nails will just float in place allowing them to tear shingles but also not hold the shingles tight.  This will make them more prone to wind damage and leaking.  The two options for correcting too large of a gap between the planks is to either lay plywood over the planks, which like discussed previously, can have its own problems if not done correctly, or to re-deck the roof with plywood.

So whenever the roof is inspected for an estimate to replace it, there should always be an attic inspection to evaluate the roof decking.  That is the foundation on which the roofing system is built.  If that is not adequate, you may have to reinvest in your roof again.  It is always best to do it right the first time, so that you don’t have to do it again a second time.

About Roof Right 

Replacing your roof can be an incredibly stressful and complicated process, but it doesn’t have to be! Roof Right, located in Carroll County, Maryland, works directly with clients to ensure that every step of the process is as straightforward and stress-free as possible. Since 1994, we have been providing excellent service to Anne Arundel County, Baltimore City, Carroll County, Harford County, Howard County and Montgomery County. Schedule a virtual estimate today!