Square the Deck Frame and Grade Lumber

What happens here

After all posts and beams are set, verify that the entire perimeter of the deck frame is square. Use a 'collapsible square' for a rough check, then perform a precise diagonal measurement (Pythagorean theorem or construction calculator) from opposing corners. Adjust the frame until the diagonals match, indicating perfect squareness. Once square, secure the perimeter joints. For optimal flatness with composite decking, ensure all lumber is graded and sorted by thickness (smallest to biggest) to avoid high spots.

Goal: Ensure the entire deck frame is perfectly square and prepare joists for a flat deck surface.