Glossary
Thirty deck terms, defined
Every word that turns up on a deck quote, in plain language, with the code section attached where one applies. Useful for reading a bid line by line rather than nodding along.
- Ledger
- The board bolted to the house that carries one end of the deck joists. It is the single most safety-critical connection on an attached deck, and it must be through-bolted or lag screwed into structure with corrosion resistant flashing over the top. Nailing a ledger to sheathing is the classic cause of a deck separating from a house.
- Rim joist
- The board that runs around the outside edge of a floor frame. On the house it is what a ledger bolts through to. On the deck it is the perimeter board that closes off the joist ends and that guard posts commonly anchor to.
- Joist
- The repeating framing members that span from ledger to beam and carry the decking. Size and spacing are set by span and by the decking material on top, not by habit.
- On center
- How joist spacing is measured, center of one board to center of the next. Written as 16 in o.c. or 12 in o.c. Composite decking normally needs 16 on center perpendicular and 12 on center diagonal.
- Beam
- The horizontal member carried by the posts that supports the far ends of the joists. Usually two or three plies of dimensional lumber fastened together, sized by span and load.
- Post
- The vertical member carrying the beam down to a footing. On decks that are more than a step or two off the ground, post size and bracing become structural questions rather than cosmetic ones.
- Footing
- The concrete pad or pier under a post that spreads the load into the ground. It must bear on undisturbed soil at least 12 inches down or below the frost line, whichever is deeper.
- Frost line
- The depth to which ground can be expected to freeze in a given climate. Concrete placed above it can be lifted by frost heave. The City of Boise publishes a 24 inch minimum footing depth for the valley.
- Frost heave
- The upward movement of soil as water in it freezes and expands. It is why a shallow footing pushes a post up over a winter and why stair stringers should not bear on a slab poured above the frost line.
- Flashing
- The corrosion resistant metal or approved membrane that sits over the ledger and under the siding, directing water out and away rather than into the wall. Minimum nominal thickness for metal flashing is 0.019 inch.
- Guard
- The barrier along the open edge of a deck. Required by IRC R312.1 once the walking surface is more than 30 inches above the grade below. Detailed to 36 inches high in standard prescriptive practice.
- Baluster
- The vertical infill members between guard posts and rails. Spacing is set so that a 4 inch sphere cannot pass through the assembly.
- Handrail
- The graspable rail on a stair, distinct from a guard. Required on any stair of four or more risers, set 34 to 38 inches above the line touching the tread nosings, and returned to the guard at each end.
- Stringer
- The sawtooth-cut or solid board that carries stair treads. Minimum 2x12, with three required on any cut-stringer flight, and bearing on a footing rather than on soil.
- Riser
- The vertical face between two stair treads, and the unit stairs are counted in. Four or more risers triggers the handrail requirement.
- Nosing
- The front edge of a stair tread where it projects past the riser below. Handrail height is measured from the sloped line that touches successive nosings.
- Landing
- A flat platform in or at the end of a stair run. Minimum 36 inches in the direction of travel, and required as an intermediate landing once total stair rise passes 12 feet.
- Pressure treated lumber
- Softwood, usually southern yellow pine, infused with preservative under pressure to resist rot and insects. The default framing material for every deck regardless of what the surface is.
- KDAT
- Kiln dried after treatment. Pressure treated lumber that has been dried following treatment, so it moves less in its first season and accepts a finish sooner than standard wet treated stock.
- ACQ
- Alkaline copper quaternary, one of the common copper based wood preservatives. Its copper content is why aluminum flashing and fasteners should not sit in direct contact with treated lumber.
- Western red cedar
- A naturally rot resistant softwood used as decking and trim. Lighter and more dimensionally stable than treated pine, more pleasant underfoot in summer heat, and it takes a semi-transparent stain evenly.
- Capped composite
- Decking made from a wood flour and polymer core with a hard polymer cap bonded to the exposed faces. The cap is what resists fading, staining, and the surface degradation an uncapped board suffers.
- PVC decking
- Fully synthetic decking with no wood content. Lighter than composite, generally the best performer on heat gain and moisture, and the most expensive per square foot.
- Hidden fastener
- A clip or biscuit system that fixes grooved decking from the side rather than through the face, leaving no screw heads visible. Slower to install and standard on most composite work.
- Picture frame
- A border board running around the perimeter of a deck, with the field boards mitered or butted into it. It hides the cut ends of the field boards and reads as a finished edge.
- Fascia
- The trim board covering the outside face of the rim joist. Usually color matched to the decking, and the detail that separates a finished-looking deck from a framed one.
- Skirting
- Lattice, board, or panel infill closing the gap between the deck edge and the ground. Cosmetic, but it also keeps animals out from under the structure.
- Non-ledger deck
- A deck framed to stand free on its own posts and beams rather than attaching to the house. The standard answer where the wall construction is unknown or cannot be trusted to carry the load, common on older housing.
- Lateral load device
- A tension connector tying the deck frame back into the house floor framing, required by IRC R507 on attached decks. It resists the deck being pulled away from the house rather than pushed down.
- DCOF
- Dynamic coefficient of friction, the measure of how slippery a surface is when wet. It matters on deck surfaces and stair treads, and it is the number behind marketing terms like slip resistant.
These terms turn up throughout the build guides. The ones that carry a code number are explained in context in the railing and code guide and the footings guide.
Thinking about a deck this season?
Call (208) 314-4914 or use the quote form for a free written price on any deck project in the Treasure Valley.