How to Enlarge an Existing Deck

Unscrew the decking slats from the top of the deck using a drill with a screwdriver drill bit. If the decking is laid in stages rather than right across the deck, you can instead unscrew only those slats that are around the outside of the deck. Lay the decking to one side for potential later use.

Dig out the topsoil around the outside of the deck to a depth of 8 inches, using a spade. The length and width of the area you dig out should be sufficient to make the deck as long and as wide as you want to extend it to. For example, if the existing deck is 6 by 4 feet, dig a 2-foot-wide, 8-inch-deep trench around three sides of your deck to make a 10-by-6-foot deck.

Compact the ground in the trench using a plate compactor. Dig a 4-inch-deep hole in each corner of the trench and every 12 inches along the perimeter. Measure the height from the ground to the existing deck, using a tape measure. Use a circular saw to cut enough 2-by-4 boards that are as long as the deck height plus 12 inches so you can stand one 2-by-4 in each hole.

Mix a batch of concrete from one part cement, three parts aggregate and one part water. Stand the 2-by-4 posts in the holes. Fill around the posts with concrete. Wait 72 hours for the concrete to set firmly before continuing. Pour gravel into the trench up to ground level. Rake the gravel flat.

Measure the distance between the 2-by-4 posts, both between posts on the same side of the trench and the posts on opposite sides to one another. Cut pieces of 2-by-4 to both these distances. Screw the 2-by-4s between the posts using three coutersunk 6-inch-long screws per joint, to form the deck support beams. The 2-by-4s must be placed so their top edges are flush with the top of the support posts.

Cut 6 inches of length from the original decking slats. Lay the original decking back on the top of the deck, alternating the layout of the lengths so at each end the decking looks like it has a “long” then “short” piece, then another “long” piece, and so on. Screw the decking to the frame using 2-inch-long screws.

Measure the distance from the end of each decking strip to the outer edge of the decking frame, using a tape measure. Cut strips of decking to these lengths, using the circular saw. Lay the decking on top of the frame so the new decking interlocks with the old decking sheets without leaving a clear straight line across the edge of the deck. Screw the decking to the frame using 2-inch-long screws.