How to Build a Cantilever Deck

Sketch your deck dimensions on graph paper or with deck design software available online. Note the proposed length and width of the deck in an overview sketch, called a plan view, showing it in relation to your house and any windows, doors and inside or outside corners. Sketch the proposed deck as seen from the side, for what is called an elevation sketch. Label the ledger, the support bolted to the side of your house, and the rim joist, the edge of the deck farthest from and parallel to the ledger.

Add details of the framing to the sketch. Designate the location of a beam with a double line set back 2 feet from the rim joist. Moving the beam back from the front of the deck, such that the regular joists run perpendicular to it and project into space, creates the cantilever. Draw circles under the beam, provisionally at each end and every 6 feet in between, to indicate the location of concrete footings and posts to support the beam.

Review span tables, found in deck-building books, to determine whether you need a double or triple beam, or a joist size larger than 2-by-6, to safely support your deck according to code. Match the span table to the type of framing lumber you plan to use, typically pressure-treated Southern yellow pine. Or have an architect, engineer, designer or your municipal permits department review your plans. Revise the plan as needed, including moving the beam setback, to ensure that your cantilever is a safe length.

Pour concrete footings in sonotubes at the positions noted in your deck plans, to at least the depth dictated by your local frost line. Set 6-by-6 posts in post bases on top of the footings.

Screw lag bolts into the joist above your house foundation through your ledger, typically a 2-by-6 board to match the regular joists, or larger if your joists need to be stronger to safely support your deck.

Mark your posts with a carpenter’s pencil level at the top of the ledger by using a long two-by-four with a spirit level on top of it to determine this point. Mark the depth of your beam on the face of the post, as determined by measuring the actual 2-by-10 boards or larger boards being used for the beam, and mark this span “beam.” Mark the depth of the joists under the depth of the beam, and mark this span “joist.” Carry the lowest mark around all four sides of the post with a speed square and cut the post at this point with a reciprocating saw.

Top the post with a post-beam cap. Lift the beam into place and nail the cap to the beam and the post with galvanized nails.

Run your joists from joist hangers hung every 16 inches on the ledger over the top of the beam and 2 feet past the beam, or as noted in your deck plans. Nail through the joist hangers on the ledger with galvanized joist nails. Nail hurricane ties to hold the joists to the beam. Cut a rim joist to cover the free ends of the regular joists and fasten it in place with joist hangers, as well. Complete your cantilevered deck by fastening deck boards to the joists and 4-by-4 posts, railings and balusters to the rim and end joists as called for by your plans.