Q&A – Replacing old decking

When it came time to replace a piece of decking, this homeowner wanted to make sure they got it right the first time. 

John, a homeowner from Georgia, came to the AskARoofer Experts with a question about replacing a deck:

"Should old 11-inch wide plank decking be replaced with 4x8 OSB decking? If so, can you install new OSB on top of the existing plank decking?"

A variety of our experts chimed in with thoughts, including Jose Castellanos with Bumble Roofing, who said:

We always suggest using OSB or plywood with roof replacements to abide by manufacturer standards! With plank decking (we call it shiplap), there is a possibility that the nail driven into the shingle does not hit a board and is therefore not truly installed to the sheathing. It is possible to install OSB on top of the existing plank decking!

Roofing expert John Kenney from Cotney Consulting Group also responded with: 

Although the 11-inch plank decking may still have some structural integrity, it doesn’t always meet today’s manufacturers' standards for shingle application. Installing a layer of 4x8 OSB or plywood over the existing deck is a smart upgrade that solves those concerns and helps ensure the roof system performs as it should in the long term.

You can install OSB over the existing plank decking, assuming the planks are in good condition, meaning they’re flat, secure and rot-free. Doing so gives you a clean, consistent nailing surface and reduces the chances of shingles telegraphing board movement underneath. This is a common and accepted approach when reroofing older homes. Just make sure to use the proper fasteners that secure the OSB to the planks and penetrate into the rafters below. That’s key for maintaining structural integrity.

Now, if any of the existing plank decking is soft, warped or shows signs of decay, it needs to be replaced before proceeding. Laying OSB over bad wood doesn’t solve the problem; it just covers it up. That said, if the planks are solid, this method brings the roof up to modern standards cost-effectively. I’d also recommend checking with your local building department, but this method is acceptable in most areas as long as structural load requirements are met.

Donavan Morgan from The Roof Gallery, another one of our roofing experts, shared his thoughts: 

Yes, you can overlay old 11” plank decking with OSB instead of tearing it all out. Ideally it should have space less than 1/4 of an inch. 

• Replace planks only if they’re rotten, split or badly cupped.
• Overlaying OSB (7/16”–5/8”) gives a smooth, flat base and saves labor.
• Fasten into rafters, not just planks, so it’s structurally tied in.
• Watch weight, ventilation and fix any rotten boards first.

Rule of thumb If planks are mostly solid and less than 1/4 inch gap → overlay with OSB. If they’re shot → tear out and replace with full OSB decking.

And last but not least, our roofing expert Henry Staggs with The Arizona Roofer sent in this response! 

Continue the conversation in our Forum!