How to Calculate Spacing Between Deck Boards
All installed decking should have a gap between each course of material to promote adequate drainage. The purpose of drainage is to allow all standing water to fall to the ground below and any moisture that is left to dry.
When deck boards are too tight, water will not be able to drain, causing the wood to rot over time. The most common spacing measurement between deck boards is 1/8 inch. A convenient way to get consistent spacing is by using a 6-penny or 8-penny nail between each course of decking; the width of the nail is 1/8 inch.
Install the first course of decking material. Prior to the installation process, inspect the deck boards to make sure they are straight.
Place the second course of decking next to the first course. Make sure to stagger the butt ends of each course throughout the deck.
Slip the pointed end of a 6-penny or 8-penny nail between the two courses so that the head of the nail rests on the surface of the decking material; this will create a 1/8-inch gap between the boards.
Place a chisel behind the board and hit it with a hammer to dig the sharp end of the chisel into the joist where the board is bowed out. Pull back on the chisel to pry the bowed board in place so that the gap between the boards equals that of the nail width.
Screw or nail the board into place. Use the 6-penny or 8-penny nail continuously to make sure that the spacing is 1/8 of an inch across the deck.