Determining how many deck boards for a 12 x 20 deck is the critical first step in any DIY building project, transforming a simple sketch into a tangible material list. A 12 by 20 foot structure provides a generous 240 square feet of outdoor living space, perfect for everything from morning coffee to evening entertaining. However, the difference between a seamless installation and a frustrating do-over often comes down to precise calculations before you ever drive to the hardware store. This guide breaks down the math and considerations needed to get your board count exactly right.
Understanding the Core Calculation
At the heart of the question "how many deck boards for a 12 x 20 deck" is a straightforward area calculation. Your deck surface measures 240 square feet, but you cannot simply divide this by the square footage of a single board. The layout of the boards, the direction of the joists, and the necessary overlap for the fascia boards all influence the final number. To find the answer, you must decide which direction the boards will run and calculate the linear footage required for each row.
Scenario One: Boards Running Along the 12-Foot Side
If you choose to run the boards parallel to the 12-foot sides, you will be installing rows that are 12 feet long. To cover the 20-foot width, you will need a specific number of these rows, accounting for the board width and the necessary gap for drainage. For instance, if you are using a common 5.5-inch wide deck board, you would need approximately 44 rows to span the 20-foot distance. Multiplying the 44 rows by the 12-foot length of each row results in 528 linear feet of decking material.
Scenario Two: Boards Running Along the 20-Foot Side
Alternatively, running the boards along the 20-foot side means each row will be 20 feet long to cover the 12-foot width. Using the same 5.5-inch wide board, you would need approximately 26 rows to span the 12-foot dimension. This calculation results in 520 linear feet of decking (26 rows x 20 feet). While the linear footage is similar to the first scenario, the waste factor and cutting requirements might differ based on your specific layout and the location of the house framing.
Accounting for Waste and Fascia
Professional installers always factor in waste, which typically ranges from 5% to 10% of the total linear footage. This buffer accounts for cuts around posts, uneven corners, and any damaged boards during handling. For a 240-square-foot deck, adding a 7% waste factor to your calculation is a safe industry standard. Furthermore, the perimeter "fascia" boards, which cover the edge of the rim joists, require additional material. You will need two extra boards for the 20-foot sides and two extra for the 12-foot sides, cut to fit precisely.
Material Choices and Their Impact
The species and profile of the wood you select will change the final count. Traditional southern yellow pine comes in nominal 2-inch by 6-inch dimensions, which actually measures about 1.5 inches by 5.5 inches. If you opt for a thinner material, such as 5/4 stock, which measures roughly 1 inch by 4 inches, you will need more boards to achieve the same coverage due to the reduced width. Always verify the actual dimensions of the lumber at your supplier, as labels like "2x6" are often nominal sizes rather than true measurements.
Step-by-Step Guide to Calculate Your Specific Needs
To ensure accuracy for your specific project, follow these steps. First, measure the exact finished length of the deck where the boards will terminate. Next, determine the true width of the board you plan to use. Then, divide the total deck width by the board width to find the number of rows. Add the lengths of all rows together to get the total linear feet. Finally, apply a 7% waste factor and add the linear footage for the perimeter fascia cuts.
Summary Table for Common Deck Board Widths
| Board Width (Nominal) | Actual Width (inches) | Rows Needed (20 ft span) | Linear Feet (Base) | With 7% Waste |
|---|---|---|---|---|
| 5.5" (2x6) | 5.5 | 44 | 528 ft | ~565 ft |
| 4.5" (5/4 x 6) | 4.5 | 54 | 1,080 ft | ~1,160 ft |
| 3.5" (2x4) | 3.5 | 68 | 1,360 ft | ~1,460 ft |
By carefully reviewing this breakdown of "how many deck boards for a 12 x 20 deck," you can confidently purchase the right amount of material. This precision saves you time, money, and the hassle of running back to the store mid-project. With the correct quantity of boards in hand, you are ready to begin building the deck of your dreams.
