For most builders & crews the biggest question when it comes to sheathing a wall is “should we do it while the wall is being framed on the ground, or after we have stood it up?” For others that have to add foam on the exterior, some start to wonder, “if I need to add this insulated foam board, do I really need to use wood sheathing everywhere or can the foam replace some of it?” With all the changing codes many are starting to also wonder, is what we did in the past still the best method, or are there easier & better means to do it now?
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The What & the Why?
If I had to sum up sheathing I would refer to it as; A material used to enclose & protect the structure (along with its contents) which also helps provides the needed structural stiffness & backing required by that assembly. For most residential structures the most typical materials used now are OSB or plywood while in some commercial wall assemblies a specialized gypsum type product is used.
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Horizontal or Vertical?
For years, many of us simply installed sheathing horizontally & used a 16 penny nail slid between the sheets to help ensure there was room for expansion & to help reduce the swelling caused by an moisture that got between the two sheets. With changes to the codes (caused mainly by a better understanding of seismic shear forces & what was required to help prevent a wall from racking in the wind…) it became a requirement in certain areas to start sheathing the corners & a few select other areas vertically.
As you can imagine this started leading many to start asking, if sheathing these areas vertically is good, how about the rest of the house, should we go back to horizontal in those areas or simply stick with doing it vertically? Well with the exception of maybe a few large windows, going with 100% vertical easily wins out especially when you consider a few more of the unexpected benefits. The dreaded wavy panel seam ceases to be an issue & low & behold the amount of air leakage took a major nose dive. Why, well you remember that 1/8” expansion gap that is required for expansion & contraction – well it now falls on a joist eliminating that path.
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Installing sheathing vertically:
This is a picture I took when doing a presentation at the Local 372 Training Center in Tuscaloosa which primarily offers programs related to welding & plumbing. They created this 2 story mockup to help assist in their training & it just so happens to makes a great example. For a one story house, the sheathing should extend from the bottom of the lowest framing member to the top of the highest framing member (as shown in blue).
For multi-story buildings the sheathing on the bottom should extend down to the bottom of the lowest framing member & extend to the halfway point on rim-joist (as shown in red). The next piece above it should be allowed a 1/8” expansion joint ending at the top plate or at the midway point of the next rim joist. For more on these connections & other best practices you may wish to check out the APAwood.org site.
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Stepping it up a notch:
While raised heel trusses are still considered the domain of “green building” at this moment, with the stated goal of the ICC, AIA, & the others striving to reach net-zero by 2030, you can expect that they will soon become mandatory. Not only are they great from a thermal efficiency standpoint, one can extend the sheathing up eliminating some bracing (allowing for more insulation), the need for baffles, but also help create a more sturdy structure.
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Sheathing Grade Marks:
One of the most confusing aspects for many people is understanding what the grade marks mean on lumber.
- This is commonly called the “panel grade” which can refer the grade of the veneer or its intended use.
- Span Rating — in some cases you will see two numbers separated by a slash while in other cases you may just see one. The left-hand number is maximum spacing for when used for roof sheathing with long dimensions across supports while the right is for flooring.
- Occasionally you may see one of two terms here:
- Sized for Spacing refers to a panel that is trimmed a fraction shorter so that when installed with the proper expansion gap it will equal its nominal size.
- Tongue and groove
- Bond Classification refers to the exposure rating based on the severity and duration of the weather and moisture exposure the panels are designed to withstand.
- Manufacturing mill’s thickness declaration
- Manufacturing mill’s identification number
- Product meets Voluntary Product Standard (typically PS 1-09 for Construction and Industrial Plywood & PS 2-10 for Wood-Based Structural Use Panels)
- Product meets APA’s Performance Rated Panel Standard
- Product meets HUD Standards
- The performance category is linked to the nominal panel thickness designations used in the IRC & IBC
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