Stand facing the structure wall where you have attached the ledger board. Place your tape measure at the left corner of the ledger board.
Measure 3 feet to the right along the ledger board and mark the spot.
Measure a line 4 feet out along the first joist board to the left of the ledger corner. Mark the spot with the marker.
Measure the distance from each marked spot to the other to form the shape of a triangle. This measure must measure 5 feet to meet the 3-4-5 method of measuring.
Adjust the joist left or right until you obtain the 5-foot measurement.
Connect the next joist board to build the deck frame. Set a layout square tool at the corner of the two boards to ensure you have right angles.
Use the 3-4-5 method again to ensure the boards are square with the house.
Square the last perimeter frame board in the same manner. Set a level on each joist to ensure an even, level deck frame.
Measure the distance from one corner of the square or rectangle frame diagonally to the opposite corner. Measure the opposite two corners.
These measurements need to be equal to ensure you square a deck floor.