The toughness of timber is calculated by power grading, which is dependent upon the species and grade of timber. The higher the amount, the more robust the timber. Hardwoods vary from D18 to D70, and C16, C18 and C24 are classified as the mostly obtainable energy classes for softwood. e. http://felixjkgig.blogprodesign.com/9379892/myths-about-composite-decking-can-be-fun-for-anyone