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Community and Q&A

Is adding an extra sheathing layer to a 15/32″ roof deck a good or bad idea?

mark_sullivan | Posted in General Questions on


I need to re-roof my house after a hail storm damaged the asphalt shingles. The current roof deck sheathing is only 15/32″ OSB, attached 24″ OC to trusses, and lacks H-clips or other edge support.  Once the old shingles are removed, I’m vacillating between two options for the deck sheathing and would appreciate any thoughts from forum members:

Option 1. leave the 15/32″ decking as-is (exception for replacing rotted areas). At 27 years old, the underside of the decking looks fine from the attic.
Option 2. directly overlay the current OSB with another 1/2″ layer of sheathing (OSB or Advantec?), staggering the seams between the two layers.


Code in the area (Spokane WA) now requires new construction to use 5/8″ decking for the area’s 40psf ground snow load.  I also plan to install solar panels, which will probably shed some snow load, but will put another set of workers on the roof during install.  I also like the idea of Fortified Homes and wonder if another layer of sheathing would improve or reduce resiliency.


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  1. Expert Member


    Do you have access to the underside of the roof?

    1. mark_sullivan | | #2


      Yes, I can generally access the underside of the roof. I installed a catwalk along the center-line of the trusses so that region is easy to reach. The truss ends are harder to reach, especially since half the roof uses scissor trusses. NOTE: roof is 5:12 pitch.


  2. Expert Member


    if the existing sheathing is in good shape, another option to beef up the structure would be to block the panel edges with 2"x4"s.

    1. mark_sullivan | | #4

      I mulled over blocking too. Just wasn’t relishing the idea of working inside the attic. Especially since the scissor trusses make it really difficult to get anything beyond the ridge line and the next seam down.

      But I do suppose the top section of roof is where most walking will occur, and it’s the differential movement between two adjacent sheets of sheathing from footfalls that would be most likely to damage the roofing.

      1. Expert Member
        MALCOLM TAYLOR | | #5


        If I was asked to do both jobs, I'd much rather add a layer of sheathing too.

        If you confirm the trusses can take the extra loading, the only downside is cost.

      2. Expert Member
        BILL WICHERS | | #6

        The panels should already have the joints between panels staggered. A roof should be sheathed in a sort of brick lay-like pattern, to minimize the movement of panels with respect to each other. Putting another layer over the top should definetely lock things together better (assuming all the joints are staggered), and will beef things up to be able to handle additional snow load.

        My preference with solar panels is to put in "hard points", which are strategically located points with solid blocking from the structure up through the roof to support a frame for the solar panels. This is good for two reasons: one is that you don't put any additional point loads on the sheathing itself, since all load is transfered directly to the rafters/trusses, and second is that those hard points are usually able to be better weather sealed compared to trying to fasten over the top of the finished (shingled) roof. If you're pulling off all the shingles, it shouldn't be that bad to add some hard points, since they can be fastened through the sheathing to the structure below.


        1. Expert Member
          MALCOLM TAYLOR | | #7


          I think Mark means staggered so the horizontal joints aren't lined up, to compensate for the absence of H clips.

          1. Expert Member
            BILL WICHERS | | #8

            I was thinking staggering ALL the joints on the second layer with respect to the first, to lock everything together. Same basic idea.


        2. mark_sullivan | | #9

          Thanks Bill,
          We'll definitely anchor the solar framing through to the trusses, so the sheathing won't carry the load of the solar array. The only concern with the solar array is that, during install, workers could unknowingly step on one side of an unsupported roof decking edge and weaken/tear the tar paper or asphalt shingles above that edge (assuming the edge support wasn't improved).

          And I was mainly thinking of staggering the horizontal seams when adding another sheathing layer, but you're right that it would also help to stagger the vertical seams.

          1. Expert Member
            BILL WICHERS | | #11

            The usual way to warn workers where not to step is either those strings of flags you see marking edges, or orange "trail tape" and/or paint. Mark the part of the roof that isn't supported sufficiently to support workers, then tell the crews to avoid the marked areas.

            Note that I don't think any area of the roof should be so weak after construction that it couldn't support the weight of a worker though. Snow load applies to the entire area of the roof. Any unsupported areas should only be temporary during construction.


  3. mark_sullivan | | #10

    Malcom and Bill,
    Thank you for your thoughts! I'm leaning towards doing the overlay since it sounds like there shouldn't be any major problems with adding a second layer of sheathing (besides cost).

    But I will first research what truss load capacities were required in 1994 when my house was built, compare that to now, to make sure adding 1/2" of sheathing and a solar array remain within limits.

    Thanks again!

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