How to Square Joists for a Deck
TOH general contractor Tom Silva gets joists square by using the Pythagorean theorem
One way This Old House general contractor Tom Silva ensures that mason's strings and deck framing are square when he is building a deck is by employing the 3-4-5 rule (based on the Pythagorean theorem): First, he tacks a double joist hanger to one end of the ledger and slips a side rim joist into place. Then he marks the top of the ledger 3 feet from the corner and the top of the side rim joist 4 feet out. Finally, he extends his tape measure between the two marks and moves the rim from side to side until the marks are exactly 5 feet apart. At this point, the two pieces are square. Larger triangles, with sides of 6-8-10 and 9-12-15, yield even more accurate results.