How to Build Round Edges on a Deck

Install concrete footings and posts held by post anchors as indicated by your architectural plans. Your design will have open circles labeled, for example, “8-inch diameter footing” to support one or more beams that, in turn, support the joists. The design will indicate the distance between footings and their distance from fixed objects, such as your back door.

Double 2-inch dimensional lumber, typically 2-by-10s but possibly 2-by-12s, depending on your engineering plans, to create beams per the plan specifications. Bolt the boards together with galvanized lag bolts then nail them into place on post-beam caps on top of the deck posts. You can anticipate that two beams will meet at a 90-degree angle to create a support platform for joists to be shaped into a round edge that cantilevers past the beams.

Install joists to run from the ledger -- typically a 2-by-6 attached to your house foundation under the patio or back door -- on top and past the beam to create a cantilevered frame. Run a final regular joist to a spot 16 inches from the beginning of the round corner.

Double 2-inch dimensional lumber, usually 2-by-6s but possibly 2-by-8s, to create a doubled joist by face-nailing one board to the other. Bevel the doubled joist at 45 degrees at one end with a circular saw. Use galvanized nails to attach it to the final regular joist with a joist hanger specially designed for a 45-degree angle. Run the doubled joist about 4 inches past the corner where the beams meet to the point where your plans indicate the midpoint of the round edge.

Install cripple joists -- so-called because they are shorter than full-length joists -- at 45-degree angles from the doubled joist to run slightly “long,” meaning past the point indicated on the plans for the edge of the deck. Cut 2-by-6 blocking to fit between the cripple joists to provide extra support and toe-nail the blocking in place.

Nail a length of 1-by-2 board to the doubled joist equidistant from the points where the straight rim joists at the front and sides of the deck meet the beginning of the round edge. This board serves as a homemade tool called a trammel. Drill a hole at the point indicated as the radius in the trammel, minus 1-1/2 inches for the rim joist, and insert a pencil into the hole. Scribe the shape of the round edge onto the joists with the pencil in the trammel. Cut the joists to the angle indicated by the scribed marks.

Bend a rim joist, typically a 2-by-6 or a size to match the joists, by cutting numerous kerfs an inch apart on its hidden side, three-quarters of the depth of the lumber. Or use a composite fascia board, which is flexible, as a rim joist. Install the rim joist by bending and clamping it to the regular joist ends then driving galvanized deck screws through the surface and into the regular joist ends or blocking.