Time and weather can wear down wooden decks fairly quickly. A deck that seems sturdy initially may start to sag or wobble after a few years. The primary cause of instability in an older deck is a lack of support columns and beams. Reinforcing the deck requires solidifying existing columns or adding columns while installing additional support beams along the underside of the deck. The project is two stages but can be accomplished in one to two days with the proper tools and supplies.
Wobbly and Sagging Decks
Measure and cut a 4-by-4 beam to the height of the deck at the point where the deck sags. Make the cut 6 to 10 inches less than the height. Place the beam vertically on top of a jack and raise it up to meet the deck. Raise the jack so that the deck's surface is level. Skip to Step 3 if the deck is simply wobbly but does not sag.
Dig two holes in front of and behind the location of the jack. Dig six holes for larger decks with two holes to each side of the first two holes. The holes need to be centered beneath existing support beams. The support columns will attach to the sides of the support beams.
Dig a hole 2-foot deep and 1-foot wide centered between existing columns along the exterior of the deck. Place one hole between each pair of columns on both sides and front of the deck. For example, a deck with three support columns on one side needs two holes on that side.
Tie a weighted string -- any string long enough to reach the ground with a heavy weight attached to it -- to the support beam directly above each hole. Tie the string so that it points straight to the ground from the inside edge of the exterior support beam. The string will mark where the new support column will meet the beam.
Mix cement with water according to the directions on the bag of cement and fill each hole. Place a column support brace on top of each pile of cement with the anchor bolts pressed into the cement. A column support brace is a C-shaped metal foot that attaches to the base of a 4-by-4 wood beam and is secured to the cement through cement anchors or bolts. The edge of the brace needs to be flush with the weighted string and level. Allow the cement to dry for 24 hours.
Measure from the column brace to the underside of the deck. Cut a 4-by-4 treated beam to the size using a miter saw. Measure and cut a beam for each location.
Set the beam onto the column brace and drill 1-1/2-inch wood screws through the brace into the support column. Drill 3-inch wood screws through the exterior support beam into the wood column. Place two screws near the top of the support column and two near the lower edge of the support beam. Repeat for each location.
Waves in the Deck's Surface
Mark a vertical line on the support beams beneath any section of deck surface material that dips down, causing a wave on the deck's surface.
Pry up any boards that sag beneath the top edge of the exterior support beams. Remove any nails or screws with a pry bar or drill with screwdriver attachment.
Measure and cut a 2-by-6 treated beam to fit the location. Measure from the mark on the exterior support beam to the opposite edge.
Screw a beam support brace into the exterior support beam. The support brace should be centered on the mark made in Step 1. Use 1-1/2-inch wood screws to secure the brace to the beam.
Slide the 2-by-6 support beam into place. Make sure the top edge is level with the top edge of the exterior support beams. Drive 1-1/2-inch wood nails through the brace into the beams.
Anchor the surface boards to the support beams, including the new support beam, with 3-inch wood screws. Use a minimum of two screws per support beam.
Repeat the whole process for each location on the surface of the deck that sags.