Flexural Modulus and Deflection

If flexural strength is directly related to a break load of a board (in this context) placed on supports, flexural modulus is directly related to a deflection of a board, placed on supports, under a certain load. Unlike the flexural strength of composite boards, typically significantly exceeding building code requirements at commonly accepted spans (such as 16 in. on center), flexural modulus of plastic-based composite boards often puts certain restrictions for their installation. 

There are two main situations concerning deflection of boards that may not pass the building code requirements deck boards at a certain span (distance between neighboring joists) and stair tread at a certain span. Let us consider these situations using the same examples Trex composite deck boards and GeoDeck composite deck boards. These examples would illustrate general shortcomings of plastic-based composite deck boards in terms of their flexibility and deflection. 

The building code requires that the maximum load at certain deflection of the test span shall be recorded (ASTM D 7042, Section 5). A common load requirement for measuring deflection of deck boards is uniformly distributed live load of 100 Ib/fC. 

If one is to choose common requirements for flooring, a deflection shall not exceed 1/360 of the span (The BOCA National Building Code/1999, Section 1604.5.4). 

Deflection under uniformly distributed load is determined by the following formula  

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