How do you square up a deck?

As long as the deck is a square or rectangle, the measurement diagonally should be equal in both directions. If you are trying to square off the house, use the 3-4-5 method. 3' along the foundation from a selected point, 4' away from the house from the same point, Diagonal or hypotenuse should measure 5'. Adjust the location of the 4' point until the measurement equals 5'. This point from the point selected on the house foundation will be 90 degrees or square.