Square a Deck Layout at Full Scale With the 3-4-5 Method

The Problem

A speed square slid into a corner confirms the first 7 inches are square. Over a 20-foot deck run, an error too small to see at the corner grows into inches of racking at the far end — every joist after that inherits the error. Squaring has to be checked at the scale you're building.

The Technique

The method is a right triangle: any corner whose sides measure 3 and 4 with a diagonal of exactly 5 is a true 90 degrees.

1. From the corner, measure 4 feet along one leg (one board or string line) and mark it. Measure consistently — inside face or outside face, the same on both legs.
2. Measure 3 feet along the other leg and mark it.
3. Pull a tape between the two marks. Move the free leg until the diagonal reads exactly 5 feet.
4. Tack the board down so it can't drift, then pull your layout for the whole beam off the squared line.