2998 Chapter 24 



In roof and wall sheathing applications, a 1/2-inch flakeboard panel of mixed 

 southern woods (42 to 46 pound/cu ft density) will increase in thickness about 10 

 to 12 percent as relative humidity increases from 50 to 90 percent. This will, of 

 course, lift shingles or siding by this amount — but should cause no particular 

 problems. Floor decking will similarly increase in thickness, uniformly lifting 

 floor tile, rugs, or other floor covering. 



If thoroughly wetted, as by a long-term plumbing leak, such a flakeboard 

 originally pressed to 1/2- to 5/8-inch thickness may swell 25 percent. After such 

 swelling, panel stiffness and load carrying capacity may be slightly diminish- 

 ed — but not excessively, as indicated by the following data on southern hard- 

 wood flakeboard from Price (1978, p. 114). 



Maximum load Deflection under a 



Panel thickness supportable on a 2,000-pound uniform 



and type 3-inch-diameter disk load 



Dry Wet Dry Wet 



Pounds-— — - Inch 



24-INCH SPAN: 2- BY 4-FOOT PANEL 



1/2-inch 



Oriented 573 388 0. 189 .245 



Random 672 575 .249 .334 



5/8-inch 



Oriented 769 514 .115 .150 



Random 938 740 .146 .188 



16-INCH SPAN: 2- BY 2.67-FOOT PANEL 



1/2-inch 



Oriented 748 494 .039 .043 



Random 850 612 .040 .062 



5/8-inch 



Oriented 1,015 600 .032 .026 



Random 1,173 861 .027 .032 



Limits on thickness swell are not prescribed by the National Particleboard 

 Association for 2-MW waferboard and 2-MF flakeboard. Neither has the Ameri- 

 can Plywood Association set a limit on thickness swell of APA RATED 

 STURDI-FLOOR and APA RATED SHEATHING. 



FACTORS AFFECTING THICKNESS SWELL 



Flakeboards swell in thickness when their moisture contents increase because 

 dry wood swells when wetted (fig. 8-28) and because individual flakes that have 

 been compressed during mat compaction tend to revert to their original dimen- 

 sions. Most of the thickness swell occurs at relative humidities above 70 percent. 

 Since the heat and pressure of the pressing cycle cause some compression set, 

 complete thickness regain is unlikely even after numerous cycles. Once water- 

 swollen, most flakeboards do not return to their original pressed thickness. If 

 flakeboard could be made with no compression of flakes and with no voids, 

 maximum thickness change would be approximately equal to values for radial 

 and tangential shrinkage of wood given in tables 8-9 and 8-10. 



