# applying osb over plank decking



## joe h (Jun 8, 2011)

is it ok to apply osb over plank decking to add smoothness and strength and what type deck nail would be rwcommended


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## gotogregg (Sep 16, 2010)

Hey Joe H, 

Are you talking about adding OSB over your exterior deck boards? Like on your deck so you will walk on OSB instead of the decking boards?

If that is the case the answer is no. Definitely not ok. OSB, even when painted or sealed, doesn't last very long when exposed to nature and foot traffic. 

Why do you want to do this? What are your main goals? Let me know so I can figure out a better solution for your problem. -Gregg


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## Grumpy (Dec 12, 2003)

Gregg, I do beleive when Joe H says "deck" he is referring to the roof substrate which is commonly referred to as the roof deck by most roofers. I try not to use the words roof deck anymore when referring to the substrate since the lay person thinks about a patio type deck on a roof when the words "roof deck" are used. 

It is ok to install OSB over spaced decking. I personally prefer CDX, but regardless of my personal prefrence, we do it several times a year on various projects.

The only reason I would consider appying the osb over the plank decking is if the planks were spaced, otherwise I don't see the point. If there is uneven-ness and wavy-ness in the roof substrate it could likely be due to a framing issue, not a sheathing issue. If it is a framing issue the application of the plywood will be a waste of time and money. What ever shape the existing roof has with wavy-ness or deflection the new plywood will follow. 

If you choose to install the plywood or OSB over the decking, you want to ensure that your nails are long enough to penetrate through the decking and into the rafters. The plywood may not always line up rafter to rafter, which is in theory OK, as long as you have enough nails at the edge of the plywood sheet and into the planking. Assuming the planking is 3/4" actual (1" nominal) there should be sufficent holding power. 

I would fasten my plywood with minimum 3" nails spaced 6" OC at all edges and rafters. I would leave a 1/8" gap at all sides of the plywood for expansion and contraction. We commonly use 1/2" 3 ply cdx for applications over existing roof boards.


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## oh'mike (Sep 18, 2009)

Are you talking about roof sheeting? Just guessing here---If that's the case then yes you can--I like plywood better,for its nail holding abilities.

However OSB is rated for that use and is acceptable.----Mike----


Grumpy is faster at typing---and much more experienced with roofing--listen to him.-M-


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