Sheathing

The Uniform Building Code allows different shear values for walls based on the selected sheathing. Rather than view selected grade stamps, its easier to ses the practical difference by looking at the table below.

Plywood and Siding Panel Grade	Min. Nom. Panel Thickness (inches)	Min. Nail Penetration in Framing (inches)	Panels Applied Directly to Framing					Panels Applied over 1/2-inch or 5/8-inch Gypsum Sheathing
			Nail Size (Common or Galv. Box)	Nail Spacing at Panel Edges (inches)				Nail Size (Common or Galv. Box)	Nail Spacing at Panel Edges (inches)
				6	4	3	2		6	4	3	2
Structural I	5/16	1 1/4	6d	200	300	390	510	8d	200	300	390	510
	3/8	1 1/2	8d	230	360	460	610	10d	280	430	550	730
	7/16			255	395	505	670
	15/32			280	430	550	730
	15/32	1 5/8	10d	340	510	665	870	n/a	n/a	n/a	n/a	n/a
C-D, C-C Shtg. plywood panel siding and other grades covered in UBC Std. 23-2 or 23-3	5/16	1 1/4	6d	180	270	350	450	8d	180	270	350	450
	3/8			200	300	390	510		200	300	390	510
	3/8	1 1/2	8d	220	320	410	530	10d	260	380	490	640
	7/16			240	350	450	585
	15/32			260	380	490	640
	15/32	1 5/8	10d	310	460	600	770	n/a	n/a	n/a	n/a	n/a
	19/32			340	510	665	870
Plywood panel siding in grades covered in UBC Std. 23-2	5/16	1 1/4	6d	140	210	275	360	8d	140	210	275	360
	3/8	1 1/2	8d	160	240	310	410	10d	160	240	310	410

Structural I panels have a higher allowable load because of the quality of materials used in their construction.  The nails are bearing against better wood in a panel where fewer manufacturing defects are permitted.  This is not a distinction applicable solely to plywood panels.  Performance rated panels are manufactured using a variety of species with varying wood properties.  Each such panel will bear an identification stamp telling you whether it qualifies for the higher Structural I stresses.

Also, the current panel designations have added to the confusion regarding selection of materials. When a panel is marked "Exposure 1" this does not relate to its strength characteristics. Rather it tells you that the panel can be exposed briefly (as during construction) to elemental moisture. Only "Structural I" panels are permitted the higher shear values specified in the Code.

As was mentioned previously, the allowable shear in a panel also depends on the width of member to which it is being nailed.  For a number of past building codes, 3x members were required at panel edges where nailing was closely spaced or where thicker nails were installed more frequently.  The blue figures in the table above highlight those past requirements.  Beginning with the 1997 Uniform Building Code, the requirement for 3x's will be expanded to include design shears in excess of 350 pounds per foot.  I've highlighted these conditions in red. Use this table to avoid getting caught with the wrong stud width.