To cover the unsightly area under a deck, you can install skirting. The skirting is attached to horizontal backer boards, which are pieces of 2-by-4 lumber that attach directly to the deck post. You'll need two courses of backing, one at the top of the post and one at the opposite end near the ground. Once the backing is installed, you can attach vertical boards also known as skirting. Skirting may consist of any size lumber, preferably 1-inch thick, but the width depends on your preference. The wider the board, the less you'll need to cut and fasten.
Measure one side of the deck to determine how long the 2-by-4 lumber needs to be. If you'll need more than one piece of lumber, measure the longest span possible, making sure the board stops at the center of a deck post. Cut the pieces of 2-by-4 lumber down with a circular saw so that the butted joints are centered on the deck post.
Fasten the first course of horizontal 2-by-4 lumber near the top of the deck post, starting from one end and working towards the other. Place two 3-inch wood screws at each fastening point directly through the 2-by-4 lumber and into each deck post using a screw gun that is fitted with a Phillips bit. Repeat this process with each piece of 2-by-4 lumber until the full course of skirt backing is installed.
Measure 6 to 8 inches from the ground at the first deck post and place a mark. Hold a 2-by-4 board on this mark and secure with a 3-inch wood screw. Move to the opposite end of the board and set a 2- or 4-foot level on the top edge. Lift the board until its level and secure it with a 3-inch screw. Place an additional 3-inch screw at that post and work down the entire board, setting a pair of screws at each deck post until it is fully secure. Repeat the process until the lower horizontal backing board is fully installed the entire length of the deck.
Measure from the bottom of the deck to the ground to determine what length of skirt board you'll need. Subtract an inch or 2 to leave a slight gap between the board and ground to prevent wood rot. Transfer the measurement to the skirt board lumber and cut it with a circular saw.
Starting at one end of the deck, butt the top edge of the skirt board to the bottom of the deck. Secure it into place by placing two 2-inch wood screws where the skirt board makes contact with the horizontal 2-by-4 backing.
Cut an additional skirt board to length and butt it to the first installed piece. Check that the fit is clean and repeat the fastening process with the 2-inch wood screws. Repeat the skirt board cutting and installing process, taking additional measurements if the ground is uneven.
Repeat the above steps for each side of the deck; deck corners consist of two skirt boards butted perpendicular to each other and secured with the two 2-inch wood screws.