Square a Wall or Frame by Matching the Diagonals

The Problem

Four equal sides don't make a square — they make a rhombus. A framed wall with every stud and plate cut perfectly can still be racked: the creator's 16-inch demo frame keeps all four sides at 16" while its diagonals read 25-3/4" and 24-1/8". Stand a wall up in that state and everything built on it inherits the lean.

The Technique

1. Measure corner to corner across one diagonal, then across the other. Square means the two numbers match exactly.
2. If they differ, push ("rack") the frame along its long side, toward the long diagonal, and re-measure. Keep nudging until both diagonals read the same.
3. Lock the frame before you move it, or it racks right back. Two ways:
   • Sheathing — fasten the sheet goods on while the wall is still lying flat and square; the panel holds the geometry.
   • Diagonal bracing — run a 2x4 diagonally across the wall and nail it into each stud it crosses. Pull it off after the wall is standing and tied to its neighbors.