Parts of a Deck: Exploring Its Anatomy

When I don’t know what to call something, it’s a thingamajig, but a friend prefers whatchamacallit, and there are doohickey people, too. Words like that aren’t much use when you’re specifying decking materials or discussing deck construction details.

You’re better off learning the correct terminology for a deck’s anatomy. Below, I’ve listed and defined several basic terms related to deck building to get you started.

Don’t worry about mastering every deck-building term—I know professionals who still confuse baluster, spindle, and stile. Instead, let your vocabulary grow naturally as your deck project takes shape and you get closer to enjoying your new outdoor living space.

Parts of a Deck

The deck components that carry the load—the weight of the deck itself, furniture, and people—are called substructure parts. The parts that give the deck its finished appearance—deck boards, stairs, and railings—are called the surface parts. Every deck installation needs both.

Substructure Parts

1. Footings

Like your feet, footings provide a solid base to support a large, elevated structure. Concrete footings are typically sunk (poured) into holes. In cold climates, the holes must be deep enough to prevent heaving, which can occur when the footings aren’t below the frost line. In warm climates, low-rise decks can sometimes use precast footings that rest on the ground. The size of the footings depends on the load that must be supported and the soil conditions.

2. Posts

Deck support posts, usually 6×6 pressure-treated lumber, sit atop the footings and anchor them using special metal brackets. The posts must be above grade to prevent them from absorbing moisture, which can lead to rot. Beams are fastened at the top of the posts, transferring their load to the posts and footings below. If you build on a slope that requires tall posts, you should add bracing so that the posts resist lateral loading and stay plumb.

3. Beams

Beams, also called girders, are horizontal supports that rest on and span the posts. They typically run parallel to the side of the house. Conventional decks often use two beams, one near the far end of the deck and a second approximately midway between the outside beam and the house. The deck beams support the joists that comprise the deck frame. Larger than joists, beams often comprise two (or more) boards nailed or bolted together. To cover large spans or to meet heavy load requirements, builders might use laminated veneer lumber (LVL), glulam beams, or even steel I-beams.

4. Joists

Joists rest on the beams and provide the base for the deck boards. They are essential to the deck’s strength, stability, and structural integrity. The size and length of the boards depend on the span and load requirements. Joists are spaced 12 or 16 inches on-center (the distance measured from the center of one joist to the center of the next). The tighter the spacing, the sturdier the deck. When installing pattern deck boards, tighter spacing is used, such as at a 45-degree angle to the joists.

5. Ledger Board

The ledger board is where the deck connects to your house. It transfers vertical loads (weight of the deck, furniture, pergolas, people, etc.) and lateral loads (forces pushing the deck away from the house) to the house’s structure. It is critical to the safety and longevity of your deck and must be anchored using heavy-duty fasteners. Very wide decks require more than one ledger. Only free-standing decks don’t need a ledger.

6. Hardware

These parts connect the deck’s substructure and surface components. They’re usually made of galvanized or stainless steel to give them durability in an outdoor space.

Deck screws

These screws secure the deck boards or hidden deck board fasteners to the joists to keep the deck boards in place. They are often self-tapping, meaning you don’t have to predrill holes for them.

Lag bolts

Sometimes called lag screws, these are large, coarse-threaded screws with a hexagonal head. They secure the ledger board to the house and the railing posts to the joists. In fact, these high-strength fasteners are used anywhere a connection must support heavy loading.

Joist hangers

These galvanized steel brackets are U-shaped to accommodate wood joists and have predrilled holes for securing them with nails or screws. Joist hangers make it easier to attach the joists to the ledger board and beams. In hurricane- or earthquake-prone regions, other styles of metal brackets are used to reinforce critical joints.

Post anchors

Also called post bases, these metal brackets secure the base of the deck posts to the footings so that the posts don’t shift. They also keep the post ends off the concrete so that they don’t absorb moisture.

 7. Flashing

Flashing is installed in areas where water is likely to seep in or accumulate, such as around the edges of the deck, where the deck meets the house, and where the deck meets other structures. Flashing is essential to prevent water damage and extend the deck’s lifespan.

Flashing Tape

This waterproof, adhesive-backed flexible membrane is applied over the tops of joists, rim joists, and ledger boards to prevent water infiltration. It’s superior to metal flashing because it adheres to uneven surfaces and doesn’t use fasteners, which leave holes that could allow water to enter.

Metal Flashing

Traditional metal flashing is inexpensive and widely available. It comes in a variety of shapes and sizes to suit the areas it protects. Examples include L-shaped flashing and Z-shaped flashing. It can also be bent into custom shapes to suit special applications.

Ledger board flashing

Protecting the ledger board is critical to preventing water damage to the ledger and to your house’s structure. You can use either flashing tape or metal flashing at the ledger. Naturally, I’m partial to our Trex Seal ledger flashing tape, which is sized to cover the entire area and includes an aluminum core that makes it easier to install.

Surface Parts

These parts are what’s visible after the deck is finished.

1. Decking

Deck boards create the deck “floor” and come in a variety of materials, textures, and colors.

Wood

Pressure-treated (PT) fir is inexpensive but must be refinished about every two years. Plus, it’s subject to cracks and splinters, and there are few design options. Cedar and redwood are more attractive than PT wood, but also more expensive and need a good deal of maintenance. Tropical hardwoods like teak, mahogany, and ipe are very durable but difficult to install for a novice.

Composite

Made from recycled plastic and wood fibers, composite deck boards are uniform and weather the elements better than most wood. They also require only minimal maintenance. Composite boards cost more than PT wood but about the same as tropical hardwoods. They come in various colors and finishes and carry warranties of 30 years or more.

PVC

This synthetic decking is another low-maintenance option. It costs about the same as composite decking and lasts about as long but is more prone to thermal expansion and contraction and offers fewer design choices.

2. Railings

are the most prominent part of your deck and include a top rail, bottom rail, and infill. Ideally, railings complement both the deck design and your home’s architectural style. Many composite decking manufacturers offer railing systems that match or accent their decking. These prefabricated kits simplify the installation for DIY projects. Custom railings might require professional help.

3. Stairs

When selecting deck stair designs, focus on safety, functionality, and coherence with the deck design. There are several options beyond straight stairs, including L-shaped (quarter-turn) stairs, U-shaped (half-turn) stairs, winder stairs (triangular steps without a landing), and spiral (circular) stairs that radiate from a central post. Make sure to check your local building codes. They govern riser height, tread depth, handrail heights, and stringer construction.

4. Deck Skirt

Also called a deck apron, deck skirting refers to the section surrounding the base of the deck, covering the space between the deck surface and the ground. The skirt serves both aesthetic and functional purposes, including hiding structural elements and preventing animals and pests from nesting under the deck. Some homeowners use deck skirting to enclose a storage area under their deck.