What is a Deck Joist?

Troy Holmes

A deck is a man-made platform that is normally connected to a house. Most decks are made of wood and are suspended above the ground, supported by large post and beams. A deck joist is the deck support beam that holds the deck floor boards securely onto the deck. These joists are connected to the main header board and posts beams of the deck with joist hangers.

View of deck floor joists from below.
View of deck floor joists from below.

Building a deck requires proper planning and organization. This planning process is used as the guide throughout construction. Planning a deck doesn't necessarily require an expert, but should include purchasing materials, obtaining permits and sketching out a general time line for overall construction.

A deck being built with the joists exposed.
A deck being built with the joists exposed.

Many people use deck kits to speed up the planning and design tasks of deck construction. These kits come in multiple sizes and include all the necessary material and architectural blue prints for most deck designs. A typical deck project will include deck boards, floor joists, hangers, rails, posts, and fasteners. It is important to develop a purchasing material list prior to starting any deck project. This will limit the need for repeated visits to the hardware store.

Deck joist spacing is an important element that determines the strength of a deck. Closer joist spacing typically creates a stronger sub-floor area. The deck joists are the repeated boards that are used to build a deck frame. This is the outer skeleton frame of a deck that supports the load of the deck surface. These joists ensure the shape and strength of the deck are permanently maintained.

A deck joist runs perpendicular to the house. It is supported between the header board, which is attached to the house and the post beams which are planted into the ground with concrete. Joist hangers are the attachment device used to connect the ends of joists to the face of a header board. The joists are installed parallel to one another. This has the effect of evenly distributing the load on the entire deck area.

Deck boards and deck joists come in many sizes depending on the desired look and load capacity required for the deck. The deck joist is the base support for the deck. As the size of the deck increases larger, decks joists are necessary for added support. The required deck spacing between the deck joist is determined by the type of wood used on the deck and the spacing between each post beam.

When the deck is finally complete, an inspection will be necessary by a qualified permit inspector. He will ensure construction was completed based on safety code requirements. During this inspection he will validate the required deck joist spacing based on the size and beam span of the deck.

Deck joists hold deck boards to the deck structure.
Deck joists hold deck boards to the deck structure.

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