How to Square a Deck Frame (Ledger, Rim Joists and Diagonals)

By the 000345 editorial team · Updated October 2026

An out-of-square deck shows up at the worst possible time: when the last decking board is a tapered sliver against the rim joist, or when the railing posts do not line up. Squaring takes ten minutes at the framing stage and saves hours later.

Start from the ledger

On an attached deck, the ledger board fastened to the house is the reference. Everything is squared to it. Make sure it is level and properly flashed and fastened according to your local code before you square anything else.

Square the first side

1. Mark the end of the ledger where the first outside joist will sit.
2. Measure 6 ft (or 9 or 12 ft) along the ledger from that point and mark it.
3. Run a string or the joist itself out from the house. Mark 8 ft (or 12 or 16 ft) along it.
4. Swing the joist until the diagonal between the marks reads 10 ft (or 15 or 20 ft). Brace it in place.

Set the rim and the other side

Measure the deck depth along the squared joist and set the rim joist. Install the opposite outside joist at the same length. Now measure both diagonals of the frame — from the ends of the ledger to the opposite outside corners. In a 12 × 16 ft deck the diagonal should be 20 ft. If one diagonal is longer, push the frame along that diagonal until both match. Temporary diagonal braces hold the frame square while posts and beams are fixed.

Freestanding decks

Without a ledger, pick the longest beam as the reference and square everything to it, or lay out post locations on batter boards exactly as you would for a foundation.

Before the first board

Re-measure the diagonals after posts are set and joists are hung — framing can rack as it is fastened. Then work out board counts with the deck board calculator and footing concrete with the footing calculator.

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