Roofing Companies: Sheathing or Roof Deck - Part 1

 As a roofing contractor, I field a lot of questions about different components of a roof.  Enough so that I think a series of blog topics to educate on all the parts would be useful for my prospective clients (and honestly for any homeowner, even if you don’t call me).  Let me know if you find these useful.

 Asphalt shingles need to be installed over continuous wood decking; this wood decking is commonly referred to as sheathing or roof deck.  In the ancient past, this may have been wood planks.  However plywood and oriented strand board (OSB) have long since replaced wood planks, and as engineered wood products these are both superior options to wood planks.  When we say, “They don’t build them like they used to,” in this situation I must admit the buildings are superior with engineered wood as the roof deck.

 Asphalt shingles are flexible and will telegraph any imperfections in the roof sheathing itself.  The most common areas of imperfections in roof sheathing take place at the joints.  Plywood or OSB come in four (4) foot by eight (8) foot sheets; these will have much fewer joints than wood planking, greatly reducing potential for imperfections at the roof joints.

 Wood is what is known as a “hygroscopic” material => this means that variations in relative humidity can impact its dimensions.  As moisture content in the air increases (high humidity), the wood sheathing can absorb some of this water from the air, and this causes the wood to swell and the dimensions increase.  As moisture content in the air decreases (low humidity), the wood sheathing will dry out and this causes the wood to shrink and the dimensions to decrease.  The wood sheathing is exposed in the attic space; a properly vented roof will have the wood sheathing exposed to whatever seasonal weather fluctuations occur.  In my service area of Jackson County, Missouri, our seasonal fluctuations are significant and range from very cold dry air in the winter to very hot humid air in the summer.  And if you have a roof leak, the sheathing is exposed directly to water and not simply humidity in the air, making this problem even worse.  While any individual component of the roof sheathing dimension may change only marginally; over a space as large as your roof the changes are cumulatively significant.  With our local climate, a 4x8 sheet of plywood or OSB may expand and contract as much as 1/8 inch.  This expansion and contraction itself leads to imperfections in the roof sheathing that will telegraph through the asphalt shingles.

 To compensate for this, when roof decking is installed it includes gaps to account for the seasonal fluctuations.  This is usually measured and consistent by using the shank of a nail used to secure the roof sheathing as a guide.  I already identified that the most common areas of roof imperfections occurs at the joints, and here we are required to make imperfect joints to account for the wood to expand and contract, so minimizing joints is important. 

In addition, both plywood and OSB are much more dimensionally stable than wood planks.  Wood planks, when they were used, had to have larger gaps.  Again, imperfections at the joints are where asphalt shingles will telegraph imperfections in the roof sheathing; wood planks have many more joints and the joints are larger.  They don’t build them like they used to, and as a roofing contractor I am certainly glad they don’t!!!

 

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