Floor - Joist count

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Equation / Last modified by KurtHeckman on 2018/08/07 15:42
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KurtHeckman.Floor - Joist count
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The Floor Joist Count calculator FloorJoistcount-illustration.png computes the number of joists (joists and end plates) required for a floor or deck based on the dimension of the room or deck and the on-center joist separation. 

Floor Joist Count Instructions

  1. (W) -  Enter the width of the floor across the joists
  2. (L) -  Enter the length of the floor parallel to the joists
  3. (oC) - Enter the On-center Separation distance between joists

Floor Joist Board Count: The Floor Joist Count calculator computes the total number of boards needed for the joists and end-plates combined.   

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The Math / Science

The algorithm assumes a span (Floor Length) smaller than 20 feet because it is calculating the number of joists (lumber boards) using the lumber typically available up to 20 feet at the national suppliers (Lowe's & HomeDepot).  It takes the width and calculates the number of joists needed to meet the on-center spacing requirement, and includes one for the beginning and an extra for any partial space.  The algorithm then calculates the number of end pieces (plates) needed based on the width and the joist lengths.  

The algorithm will calculate a joist length based on the required board length for the span (e.g. 20' board for an 18.5' joist/span).

For spans (floor length) greater than 20 feet, please use the Floor - I-Joist count equation.