Any deck-building project requires plenty of screws, but figuring out just how many comes down to a bit of math. The number of screws you'll need depends on the square footage of the deck, the joist spacing, and the width of the boards. Always have more fasteners on hand than you think you'll need once you've calculated the quantity, as it's nearly impossible to complete a deck project without dropping at least a few screws along the way.
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Basic Deck Screw Math
The number of screws needed for a deck depends on the square footage of the deck and a few other factors. You'll also have to know the joist spacing since the screws go through the deck boards into the joists. The width of the deck boards also matters, as you'll have more boards per square foot with 4-inch-wide boards than with a 6-inch-wide boards since every board must be fastened to the joists.
For best results installing the decking, use two screws per joist on each board, with the screws set a consistent 3/4 inch to 1 inch from each side of the board to avoid splitting the wood. If a deck board is long enough to span five joists, it requires 10 screws. Keep the spacing uniform on all the boards so the deck looks as nice as possible. With screws installed randomly and not in straight lines, it won't look like a professional job.
On average, expect to use 350 screws per 100 square feet if using deck boards 6 inches wide with joists spaced 16 inches apart. With deck boards 4 inches wide, you'll need 480 screws for the same joist spacing. Use 3-inch corrosion-resistant deck screws for the job or screws long enough that they'll sink 1 1/2 inches into the joists below. Drilling pilot holes for the screws near the ends of the boards helps prevent the boards from splitting. Countersinking just slightly helps ensure the screw heads don't protrude from the deck.
Deck Foundation Fasteners
It's a bit trickier to figure out how many fasteners are needed to build a deck frame or foundation, as this depends on the number and thickness of the posts, the type of joist hangers used, and the general design and size of the deck. As with the boards on top of the deck, the amount of screws needed increases if the joists are closer together than 16 inches, but 16 inches is fairly standard. Consult with your local building department for specific regulations that may affect your design and therefore the number of fasteners needed. For instance, building a deck frame in an area prone to earthquakes or on a slope has different requirements than building a deck that's more like a platform and low to the ground.
Some joist hangers may require specific types of fasteners, and the screws used to build the deck frame or foundation generally aren't the same size or type as what's used for the surface deck boards. For the framing, use nails instead or structural screws designed to prevent splitting the wood.