Before beginning to build a deck, you'll need to determine how much material you need. Once you have constructed the support and structure of your deck, calculating the necessary surface material is fairly simple. However, you will need to consider not only the size of the boards used but also the spacing between them as this will also take up space. Rely on manufacturer recommendations for spacing whenever possible.

Wooden deck of family home.
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How to Calculate Deck Boards

Calculating Deck Area

First, you will need to determine the area of your deck. If your supporting structure is the same size as your deck, you can use measurements of it for this step. If the deck is to overhang the supports, refer to your original design plans to determine the measurements of each side.

To calculate the area, multiply the length of the deck by the width of the deck. Be sure to account for any uneven areas, angled edges or other non-perpendicular spaces. If your deck is largely rectangular or square and then has a side piece, calculate those two areas separately and then add them together.

For instance, if your deck is 20 feet long by 10 feet wide, its area is 200 square feet. If your deck has a main section that is 15 feet long by 5 feet wide and a smaller section that is 5 feet long by 2 feet wide, the area of the deck is 75 square feet plus 10 square feet, or 80 square feet.

Determining Deck Board Spacing

When building a deck, gaps between boards are important because they allow for drainage. This helps prevent rot and decay over time. Different materials often specify larger or smaller gaps. Always refer to the manufacturer's instructions before proceeding.

As a general rule of thumb, lumber decking should be installed with gaps of between 1/8 and 1/4 of an inch. This means you will need to add 1/8 or 1/4 for each space you plan to have between the boards when determining material square footage.

Changes in heat and temperature can cause composite deck materials, especially those that are hollow, to shrink or expand. Over time, this can lead to buckling if proper gaps were not left. As a general rule of thumb, you should leave at least 1/8-inch of space between composite boards.

Some composite boards use hidden fasteners rather than screws. Depending on the manufacturer, the spacing of these can vary, but they often lie in the range of 1/8 or 5/32.

Calculating How Much Material to Purchase

You'll need the dimensions of the deck boards you hope to use. If you are using 2-foot by 4-foot boards, for instance, the square footage of each board is 8 square feet.

For the deck size example provided above, of 200 square feet, you would need 25 boards of this size. But the gaps between your boards must be taken into consideration, too. If you were to use 25 boards, 23 of them would have a 1/8-inch gap, but the outer two pieces would not. That creates approximately another 3 inches width-wise that you wouldn't need material to cover. Since the deck is 20 feet long, the square footage of the gaps is 3 inches by 20 feet – or 5 square feet. Because this square footage is less than one board's square footage, you might still wish to purchase 25 boards. In any case, you should always purchase a bit of extra material to account for damage or errors. With an extra 15 percent, you should be prepared for potential mishaps.