Gazebos and other outdoor structures rest on foundations or footings that support framing lumber and distribute structural loads to the ground. Because local building codes often set foundation size and type requirements for outdoor structures, check with building authorities or a contractor prior to construction. In areas that experience regular freezing, footings must extend below the frost line to prevent heaving and settling. Additionally, footings that raise gazebo posts above surrounding soil will protect the post's base from deterioration.
Arguably the most permanent base for gazebos and other outdoor structures, concrete slabs are essentially broad, thick blocks of concrete. Builders set gazebo post framing directly into wet concrete or anchor framing to cured concrete with brackets and concrete anchors. If properly tooled during a pour, concrete slabs are smooth, suitable for use as a finished flooring material and readily accept both paint and stain. Alternatively, masons can cut or stamp concrete slab surfaces to create unique patterns or imitate the appearance of tile or stone.
As with wooden fences, builders often anchor gazebo post framing to holes filled with concrete. Unlike a slab, which spreads structural loads across a large area, posthole footings concentrate the load on a buried block of concrete. To construct a posthole footing, builders dig a narrow, deep hole and fill the hole with mixed concrete. Prior to curing, builders embed brackets, rebar or anchor bolts in the concrete. Although quick and easy to build, most posthole footings leave gazebo posts close to the ground, exposed to moisture and pests.
Manufactured to precise dimensions and ready to use, tubular forms are essentially large cardboard tubes. Typically cut to length with snips or a utility knife, builders set the forms upright within post holes and fill their hollow centres with concrete. Similar to a posthole footing, tubular forms create a narrow, deep column of concrete below grade. However, the top of a tubular form easily extends several inches above grade to protect the post's bottom from premature deterioration.
Cast Concrete Pier
Cast concrete piers are solid, trapezoidal blocks of concrete. Gazebo floor or post framing rests directly on the pier's flat top and the pier sits directly on grade or atop a finished posthole footing. Framing attaches to piers via several methods, including wooden blocks glued to a pier's top, straps embedded within the pier's body and keyhole-like depressions that accommodate posts. However, piers are relatively unstable and subject to frost heaving; check local codes before placing a gazebo structure on a concrete pier base.