Deck Girder, Cantilevered or Drop Beam

A girder, cantilevered or drop beam is a structural member positioned below the joists to support the weight of the frame. Cantilevered beams present a reductive approach to structural design as they use the smallest number of components to accomplish the strongest connection. Cantilevered beams are stronger than flush style beams because they rely on the strength properties of the wood rather than the individual mechanical and fastener connections.

Notching posts lends itself well to cantilevered beam construction, as does the practice of chamfering the corners of the deck. In order to install a cantilevered beam you must have enough clearance below the joists to install the beam. Allow for a minimum of 6” airspace between the ground and the bottom of the beam.

Always remember to install the beam crown side up so the weight of the deck will level itself. If you install a beam crown side down the deck is more likely to sag. On the negative side cantilevered beams can create a ledge underneath the joists that can become a home to birds and insects nests as well as block views and limit headroom Cantilevered joists often vary in width by a fraction of an inch that when positioned over an even beam can lead to a slightly uneven deck surface.