What the take-off counts
- Joists: length ÷ spacing, rounded up, plus one — including the two end joists.
- Rim and ledger: one rim across the front (two for freestanding decks) and a ledger along the house.
- Beams: one near the outer edge for ledger decks, two for freestanding — each built from your chosen number of plies, spliced over posts if longer than your lumber.
- Posts and footings: evenly spaced along each beam at no more than your maximum spacing.
- Hardware: a joist hanger per joist at the ledger and a hurricane tie per joist at each beam.
Example: a 20 × 12 ft ledger deck
Joists at 16" on center, a 2-ply beam 1 ft in from the edge and posts at up to 8 ft: 16 joists, 2 beam boards, 4 posts and footings, 16 joist hangers and 16 hurricane ties.
Not included
Blocking, stair stringers, ledger fasteners and flashing, post bases, lateral load connectors and bracing all depend on your design and local code. Add them to your list after checking your span tables and permit requirements.
Frequently asked questions
How many joists do I need for my deck?
Divide the deck length along the house by the joist spacing, round up and add one. A 20 ft deck with joists at 16" on center needs 16 joists. The calculator counts them for you, along with rims, ledger and beams.
What size joists and beams should I use?
What is a deck ledger board?
The ledger is the board that attaches a deck to the house — the answer to “what is the deck board that attaches to the house called?”. It is usually the same depth as the joists, fastened to the house's rim joist with lag screws or through-bolts in a set pattern, with flashing above it to keep water out of the wall. Joists hang from it in metal joist hangers. The IRC sets the fastening details in section R507.9, and the ledger vs freestanding guide explains the options.
What is a deck fascia board?
A fascia board is a trim board fixed to the outside of the rim joists, covering the framing and the ends of the joists around the deck's edge. It isn't structural. Composite and PVC lines sell matching fascia, which usually needs its own fastening pattern and expansion gaps. Fascia isn't counted in the board calculator, so add it to your order separately.
Ledger or freestanding?
A ledger-attached deck bolts to the house rim and needs one beam at the outer edge. A freestanding deck has a beam at each end and no connection to the house, so it needs more posts and footings. Ledger attachment has strict flashing and fastener rules — get it inspected.
How many posts and footings does a deck need?
Each beam needs a post at each end and at intervals no longer than your beam span allows. The calculator places posts evenly along each beam at your maximum spacing and puts one footing under each post. Size the footings with the footing calculator.