Structural wood panels fastened to a stick-framed wall add stiffness and strength. That's the principle behind braced-wall requirements in the International Residential Code (IRC), and the principle underlying engineered shear wall design in the International Building Code (IBC) and in engineering standards such as the American Society of Civil Engineers standard ASCE-7. So if one layer of plywood or OSB on a framed wall is good, would two layers be better? That's the question considered recently by subscribers to the Structural Engineers Association mail list at www.seaint.org. Although the case in point related to seismic issues, not wind resistance, the discussion offers lessons for builders in high-wind country too. The problem was raised by Long Beach, California, engineer Charles Laines. In a posting, Laines wrote, "Contractor 'forgot' the interior layer of plywood in a shear wall spec'd for sheathing on both sides. Wants to put double layer on the outside since the interior has been finished. I cannot find any restrictions in the codes." Several engineers responded skeptically to the idea of doubling up "shear panel" on one side of the wall, instead of on the inside and outside faces. One engineer's gut reaction: "I can’t imagine that two layers of sheathing of the same side of the wall would accomplish anything other than tearing the studs up." But Tom Skaggs, Manager of Product Evaluation at APA - The Engineered Wood Association, chimed in to note that there is some precedent for the proposed idea. "We did testing on double layered singled sided diaphragms," wrote Skaggs. The concept was a retrofit of an existing roof diaphragm with a second layer of panels, fastened with 14 gauge staples. Panel edges were staggered so that edge nailing would miss the framing, Skaggs noted, and "results were favorable." Download "Research Report 138" from the APA Web site. Coastal Connection contacted Tom Skaggs to ask if the idea of adding panels to the outside of a structure might be valid in high-wind situations — for example, as a wall retrofit in Florida or elsewhere on the Atlantic or Gulf coasts, when existing siding was being replaced anyway. After consulting with several other APA engineers, Skaggs got back to us with some comments. "Tests have shown that it can work," he noted, and the building code states that "shear walls are permitted to be calculated by principles of mechanics without limitations." But field installation details are significant, Skaggs warned: "The biggest concern is splitting of the shear wall boundaries. And it’s extremely difficult to assess splitting in a non destructive fashion." Skaggs also passed along a more detailed response from APA Help Desk engineers, who noted two building code references: "2006 International Building Code, Section 2305.3.9, Summing shear capacities. The shear values for shear panels of different capacities applied to the same side of the wall are not cumulative except as allowed in Table 2306.4.1." That table makes no mention of two layers of sheathing on a single side of the wall, the Help Desk said. But when it comes to two-faced shear walls, the code reads, "The shear values for material of the same type and capacity applied to both faces of the same wall are cumulative. Where the material capacities are not equal, the allowable shear shall be either two times the smaller shear capacity or the capacity of the stronger side, whichever is greater." Can these principles be extended to shear walls with double sheathing layers? The Help Desk offered a cautious "yes":

"Even though two layer shear walls aren't directly addressed in the code, there is no technical reason to prohibit their use provided that the calculated design capacities can take the calculated loads and a load path is provided. The calculated design capacities could be developed using the principles established in the high shear load diaphragm noted above. The capacity of the wall hold-downs should be given careful consideration; their ability to resist overturning may be the limiting factor".
To our knowledge, this particular system has never been tested either monotonically (static) or cyclically. APA Engineers see no reason why this system could not be used in ways similar to use of a double-sided shear wall or a double-sheathed diaphragm (as discussed in APA Report 138) if:

1.) minimum fastener penetration into the framing is met

2.) panel vertical edges are staggered to minimize lumber splitting

3.) consideration is given to top and bottom plate fastening

4.) overturning, including hold down and bottom plate crushing, is studied

One engineer, formerly with APA, has suggested using 50% of the shear capacity of the top layer as additional capacity. It would appear, that this approach would be a conservative alternative. However, if care is taken in fastening the top layer, this approach might be too conservative. Is it a good idea? Maybe. But as Tom Skaggs remarks, "The shear wall capacity of the panels are limited by nail bending, and not panel shearing through the thickness. So, perhaps a more cost effective solution is to just add fasteners into the existing panels."