How to Make a Cantilever Pergola
A pergola can be cantilevered in the same way as a wood deck. The beams should extend past the posts to provide the support for the joists on top. Since there is no covering on a pergola and no one walks on it, you don’t have to worry so much about the loading capacity for the cantilever. Even though you could extend it longer, an 18-inch cantilever makes an attractive overhang that is proportional to a standard post height of 8 feet. If the posts are taller, an increased overhang is warranted.
Prepare a plan for the pergola. Use span tables to determine the minimum size of posts, beams and joists and the spacing, or span, between them. The same span tables are also used to calculate the wood size and spacing for wood decks.
Mark the location of the posts on the ground with spray paint. The location can be determined by installing rebar into the desired location and running a string between each piece to ensure that they are squared off. Alternatively, you can build a box out of 2-by-8 wood so that the inside corners of the box match the desired location of the four posts, and attach an A-brace at each corner to secure it, ensuring that all corners are at a 90-degree angle.
Install the posts first. Drill holes, 15 inches in diameter, for each post location down to the desired depth. In locations with a freeze and thaw cycle, where the temperature drops below 32 degrees Fahrenheit, the depth of the hole should be a minimum of 4 1/2 feet. Fill the bottom of the hole with 6 inches of clear aggregate. Pour concrete, and install each post so that it is embedded into the hole for a minimum of one-third the length of the hole.
For the beams, attach the appropriate size wood girder to both sides of the post with wood screws or nails, so that the thin end of the girders and the top of the post are level. Align the beams to make sure that you achieve the desired length of the cantilever. Attach two beams between the four posts.
Attach the joists on top of the beams at the appropriate spacing, typically 16 inches on center. Drill four 3-1/2-inch wood screws at an angle to attach the joists to the beams, or you can toenail them in. Alternatively, you can use rafter ties with nails.
Things You Will Need
- Auger or drill, 15-inch diameter
- Crushed aggregate, 1-inch diameter
- Pressure treated or cedar posts, beams and joists
- Optional - Screwdriver
- Optional - Wood screws
- Optional - Rafter ties
- Optional - Rebar
- Optional - String
- Optional - Spray paint
- Consult your local building department to determine if a permit is necessary and to find out the requirements for siting, setbacks and height restrictions.