2878 Chapter 23 



23-12 BOARD STANDARDS AND PROPERTIES 



COMMERCIAL HARDBOARDS 



Hardboard standards. — Hardboard product standards do not distinguish 

 between medium- and high-density boards, but define hardboard as fiberboard 

 compressed to a density of 31 pounds/cu ft or greater (specific gravity ^ 0.50). 

 The three voluntary product standards for hardboard are as follows: 

 Product Standard Reference 



Basic hardboard PS 58-73 U.S. Department of Commerce (1973a) 



Prefinished hardboard paneling .... PS 59-73 U.S. Department of Commerce (1973c) 



Hardboard siding PS 60-73 U.S. Department of Commerce (1973d) 



The basic hardboard standard makes no reference to end use but classifies 

 hardboards as SIS or S2S, and by tensile strength parallel and perpendicular to 

 the surface (table 23-10). 



The prefinished hardboard paneling standard classifies boards into two 

 quality classes according to resistance to heat and humidity and according to 

 resistance to abrasion, scraping, fading, and staining, and according to coating 

 adhesion, gloss, and washability. Paneling property requirements are in addition 

 to those for basic hardboard. 



Hardboard siding is intended for exterior exposure, so weatherabiHty, sta- 

 bility of finish, and dimensional stability are specified in the product standard. 

 Linear expansion of siding first equilibrated at 50 percent relative humidity and 

 then at 90 percent, cannot exceed the following percentages: 



Type of siding Maximum 



and thickness linear expansion 



Inch Percent 



Lap 



0.325-0.375 0.38 



over .376 .40 



Panel 



0.220-0.265 .36 



.325-. 375 .38 



over .376 .40 



Thick medium-density fiberboard is not covered by the hardboard standards and 

 is discussed in the next subsoction. 



