2696 



i 



Figure 22-64. — (Left) Grade D back of a typical plywood sheathing panel made from 

 factory grade 3 Appalachian oak logs. (Right) Grade D back of a typical southern 

 pine sheathing plywood panel. (Photo from Craft 1970.) 



hardwood blockboard, if 0.9 inch thick, could meet or exceed strength proper- 

 ties of y4-inch thick Douglas-fir plywood; a hardwood blockboard panel of 0.9- 

 inch thickness would weigh about 13 percent more than a y4-inch panel of 

 Douglas-fir plywood. 



The hardwood, face-glued, three-ply blockboard panels made by Bowyer 

 (1979a) had linear expansion, when conditioned first at 50 percent relative 

 humidity and than at 90 percent, as follows: 



Direction of measurement Linear expansion 



and width of core veneer strip (50 to 90 percent RH) 



Inches Percent 



Parallel to grain of face veneer 



l-Vi 0.087 



3 .076 



Perpendicular to grain of face veneer 



1-^ .076 



3 .094 



Three-quarters-inch-thick, five-ply Douglas-fir plywood subjected to the same 

 cycle, had linear expansion of 0.071 percent parallel to the face grain and 0.031 

 perpendicular to the face grain; face plies were 1/16-inch thick, crossbands 7/32, 



