Calculating materials is one of the toughest jobs for beginning and do-it-yourself carpenters to master. Getting the right amount saves time on extra trips to the lumber yard and hardware store, not to mention money on expensive, often nonreturnable lumber. To get a clear picture of the amount of decking you'll need, start with a sketch or blueprint of your deck. Figure your order correctly by starting with the total square footage.
Calculate the total square feet of your deck. For this example, a deck that is 15 feet by 15 feet is 225 square feet.
Divide the total square feet by the number of square feet per board. The typical 5/4-inch-by-6-inch treated deck board, 8 feet long, will cover 4 square feet of deck. Divide 225 by 4, which equals 56 with a one-square-foot remainder. Round up to the nearest whole board. For the example, you would need 57 boards.
Figure in any wasted footage. The example deck's length is not divisible by 8, the length of your deck boards. Multiply the length of your deck boards, 8 feet, by the number of boards per row, 2, and subtract the length of the deck: 16 feet minus 15 feet equals one wasted foot of board per row. Divide the width of your deck by the width of the boards to find out how many wasted feet of decking board you will have: 15 feet divided by 6 inches equals 30.
Round up the wasted feet to the nearest number divisible by the length of your boards. In this case that would be 32 feet, or 4 boards. Add the number of boards needed to make up for the wasted footage. We started with 57 boards; add 4, equaling 61 boards.
Calculate the decking needed for your stairs. Typical deck stairs are 12 inches deep. This requires two boards the width of the stairs for each step. Assume two steps, 48 inches wide. Add two 8-foot boards for stairs, totaling 63 boards.
Add 10 percent for mistaken cuts and damaged materials. In this case, 63 plus 10 percent equals 69.3 boards. Round up to the nearest board. For a 15-by-15-foot deck, place your order for 70 pieces of 5/4-by-6-inch decking, 8 feet long.