First, determine the values of the variables below. You will need these measurements to plug into the formulas.
W = width of deck parallel to joists (ft.) L = length of deck perpendicular to joists (ft.) j = joist spacing (in.) x = width of top deck boards (in.) d = spacing between top deck boards (in.)
W and L are measured in ft, while j, x, and d are measured in inches. Your value for d may be equal to zero.
Calculate the total length of board (in ft.) needed to construct the frame of the deck using this equation,
(1 + 12L/j)W + 2L.
You can also use the simplified form,
12LW/j + W + 2L.
Next, use the following equation to calculate the total length of deck boards needed to construct the top of the deck,
L(12W - x)/(x + d) + L.
You can use the simplified form,
(12WL + dL)/(x + d).
Add up the two numbers found in Steps 3 and 4, and this is the total length of board needed to construct the deck. To account for waste when cutting the boards, factor in an extra 4% for boards that are laid at a 90 degree angle the the joists, and an extra 8% for boards that are laid at a 45 degree angle to the joists.
Compute the number of screws needed for the top of the deck with the formula
2[(12W + d)/(x + d)][(12L + j)/j],
and compute the number of screws needed for the frame of the deck with the formula
Add the two numbers that you get, and this is the approximate number of screws you will need for the entire deck. You may want to factor in an extra 3%. (Note, these formulas assume that you will insert 2 screws at each joint.)
For practice, apply the deck material formulas to the following example: a deck has a width of 12 feet parallel to the joists, length of 10 feet perpendicular to the joists, joists spaced 12 inches apart, top deck boards that are 4 inches wide and spaced 1 inch apart.
We have W=12, L=10, j=12, x=4, and d=1. Using the formulas, this means that the length of board needed is 442 feet, and the number of screws needed is 682.