The Floor Joist Count calculator 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 Board Count (JC): The Floor Joist Count calculator computes the total number of boards needed for the joists and end-plates combined. The calculator also returns the number of joist hangers (ties) needed.
The basic math is to divide the width by the on center spacing, round up to the nearest integer and then add one for the first joist (at the zero position). The length is used for the length of the joists or I-joists. Note: the number of floor joist hangers needed is usually the number of joist minus two for the first and last, and that number times two for both ends of the hung joists.
The algorithm assumes a span (Floor Length) smaller than 20 feet for normal (solid wood) joists, 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, Home Depot & Menards). 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). It is important to note, that the strength of the joist and its width are driven by the use of the floor. A free standing outside deck for some light furniture and a few people will have a different load requirement than a deck for a hot tub. For this reason, this calculator does not tell you what boards to be used, just how many. The most common driving factor in joist selection is the Live Load which is the specification for the load (weight) that is placed on a square foot of the floor. Local building codes also dictate what joist boards can be used. The live load will most commonly drive the Maximum Joist Span.
Once the floor joists are in place, then one needs to compute the: