10. The importance of having a good roof deck

Roof decking is the plywood sheathing that sits below your roof’s shingles. Modern decking is made from half-inch-thick plywood or OSB panels. Before the roofing contractor lays down the underlayment and installs the asphalt shingles (or any other roofing material) they need to make sure that the roof decking is in good shape. There are times when it is clear if a roof replacement will need plywood, for example, cedar shake roofs installed over skip sheathing or old roofs with 3 or more layers of roofing material. In other cases, deck problems might not be found until after the tear off. These are three common issues found on roof decks after roof tear off:

• Rotten decking. The number one issue roofers uncover during a tear-off is rotten or soft roof decking. Decking doesn’t always get replaced in every roofing project. In fact, if the decking seems to be in good shape, an honest roofer will usually leave it alone. However, if the roofer finds rotting or soft plywood when they tear off your home’s old shingles, they will need to install new sheathing first, before adding the new shingles. The chances of finding rotten plywood increases with the number of layers of roofing being removed.
• Mold on plywood. Roof leaks and poor attic ventilation can cause mold to grow on roof deck. Sheets of plywood with mold should always be replaced before installing the new roofing material. You should regularly look inside your attic with a flashlight and look for black marks on the plywood and address the issue right away.
• Inadequate decking. The industry standard thickness of roof decking is ½”. But at one time, some local building departments didn’t require anything thicker than 3/8-inch. This thin plywood is now considered inadequate, so a roofer will usually replace it when they find it. If your home was built prior to 1970, there is a good possibility it will have 1”x6”, 1”x8” or 1”x10” board decking. Plywood wasn’t common in construction until after this time. However, this board lumber was not usually kiln-dried before it was installed. So, when the roof was originally nailed on it was likely solid, but as the wood dried over time it would typically shrink between 5%-10%, creating gaps of up to ½” or more.

Do I need to install plywood over ship lap / tongue & groove decking?

Over time, shiplap fir wood resin dissipates, causing the boards to shrink, crack, form knot holes, and become extremely brittle. Open spaces form between the once tightly fitted shiplap boards. If the roofing underlayment is applied directly on top of the shiplap, it will sag into the spaces and eventually leak.

Older homes commonly have one or more layers of roofing over the original layer. Shiplap can only take so many nail penetrations before it splinters and develops pattern inconsistency. This, and the space left by diminished overall size over the years, creates more opportunities for the new nails to miss the wood completely, causing eventual leaks. Therefore, we always recommend installing new ½” plywood over the ship lap decking.