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

4(12L +j)/j

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.

## Things You Will Need

- calculator, or paper and pencil