How to Build the Foundation of a Curved Deck

Consult your design plans for the location of the support posts and their concrete footings. The plans will describe the diameter and depth necessary for the footings, based on local frost-line depths and codes. The footing locations will be indicated by circles that appear under pairs of double lines, indicating the beam location. Hammer layout stakes in the ground at the indicated footing locations. Remove the stakes and dig larger holes for the footings centered on the stake holes with a post digger or power auger.

Pour 3 inches of gravel in the bottom of each hole. Cut cardboard sonotubes with a reciprocating saw to the depth of the hole plus 2 inches. Place the sonotubes in the footing hole, checking that the top is horizontal with a spirit level and that the top projects 2 inches from the ground. Mix concrete in a wheelbarrow and shovel it into the sonotube. Level the concrete with a 2-by-4 and insert a J bolt in the wet material. Let the concrete cure and cut the exposed sonotube away with a utility knife.

Bolt nuts to affix the post anchors to the J bolts with a socket bit attached to a drill. Cut posts with a chop saw and set them on the post anchors. Brace the posts with 1-by-4s running from the ground to the post at a 45-degree angle, held to the post by a drywall screw. Check that the post is plumb with a spirit level and adjust the support brace as needed.

Bolt a ledger, typically a 2-by-6 or larger length of pressure-treated Southern pine, to the side of your house with lag bolts long enough to penetrate the house rim joist and mud joist above your foundation and under your back or patio door. Mark the level of the top of the ledger on the posts by extending a length of 2-by-4, topped by a spirit level, from the ledger to the posts. Place scraps of beam and joist lumber below this mark and make a new lower mark. Scribe all four sides of the post with a speed square at the height of the lower mark and cut the post top off with a circular or reciprocating saw.

Face-nail two pieces of 2-inch dimensional lumber, typically 2-by-10 or 2-by-12 as dictated by your plans, to create a beam. Nail post-beam connectors to the top of the posts. With one or more helpers, lift the beam on top of the posts and hammer galvanized nails through the connector into the beam.

Nail one end of each joist, typically 2-by-6 or larger pressure-treated pine, to joist hangers hammered into the ledger; the other end lies on top of the beam. This creates a cantilever, essential for a curved deck foundation. Let the joists “run wild,” meaning run longer than the eventual arc of the deck curve. Space the joists on the ledger and the beam according to your plans, typically 16 inches on center. Nail the joists to the beam with hurricane ties -- typically metal straps with nail holes and a 90-degree twist to enable nailing foundation elements that run perpendicularly to each other.

Nail a 1-by-3 board at the spot indicated on your plans as the center of your curved arc to create a tool called a trammel. Drill a hole in the end of the trammel at the length of your curved deck’s radius and insert a pencil in the hole. Scribe an arc on the tops of the run-wild joists. Extend the top marks down the sides of the joists with a speed square and cut the resulting bevels with a circular saw. Nail blocking -- scrap pieces of joists -- between the joists that extend the farthest past the beam to create framing stability. Your deck foundation is complete and awaits the installation of platform elements, including deck boards, balusters and railings.